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* fix PR46029: reimplement if conversion of loads and stores
@ 2015-06-12 21:05 Abe Skolnik
  2015-06-22 16:31 ` Alan Lawrence
  2015-06-24 16:51 ` Ramana Radhakrishnan
  0 siblings, 2 replies; 17+ messages in thread
From: Abe Skolnik @ 2015-06-12 21:05 UTC (permalink / raw)
  To: gcc-patches, alalaw01, richard.guenther, sebpop

Hi everybody!

In the current implementation of if conversion, loads and stores are
if-converted in a thread-unsafe way:

  * loads were always executed, even when they should have not been.
    Some source code could be rendered invalid due to null pointers
    that were OK in the original program because they were never
    dereferenced.

  * writes were if-converted via load/maybe-modify/store, which
    renders some code multithreading-unsafe.

This patch reimplements if-conversion of loads and stores in a safe
way using a scratchpad allocated by the compiler on the stack:

  * loads are done through an indirection, reading either the correct
    data from the correct source [if the condition is true] or reading
    from the scratchpad and later ignoring this read result [if the
    condition is false].

  * writes are also done through an indirection, writing either to the
    correct destination [if the condition is true] or to the
    scratchpad [if the condition is false].

Vectorization of "if-cvt-stores-vect-ifcvt-18.c" disabled because the
old if-conversion resulted in unsafe code that could fail under
multithreading even though the as-written code _was_ thread-safe.

Passed regression testing and bootstrap on amd64-linux.
Is this OK to commit to trunk?

Regards,

Abe




2015-06-12  Sebastian Pop  <s.pop@samsung.com>
            Abe Skolnik  <a.skolnik@samsung.com>

	PR tree-optimization/46029
	* tree-data-ref.c (struct data_ref_loc_d): Moved...
	(get_references_in_stmt): Exported.
	* tree-data-ref.h (struct data_ref_loc_d): ... here.
	(get_references_in_stmt): Declared.

	* doc/invoke.texi (-ftree-loop-if-convert-stores): Update description.
	* tree-if-conv.c (struct ifc_dr): Removed.
	(IFC_DR): Removed.
	(DR_WRITTEN_AT_LEAST_ONCE): Removed.
	(DR_RW_UNCONDITIONALLY): Removed.
	(memrefs_read_or_written_unconditionally): Removed.
	(write_memrefs_written_at_least_once): Removed.
	(ifcvt_could_trap_p): Does not take refs parameter anymore.
	(ifcvt_memrefs_wont_trap): Removed.
	(has_non_addressable_refs): New.
	(if_convertible_gimple_assign_stmt_p): Call has_non_addressable_refs.
	Removed use of refs.
	(if_convertible_stmt_p): Removed use of refs.
	(if_convertible_gimple_assign_stmt_p): Same.
	(if_convertible_loop_p_1): Removed use of refs.  Remove initialization
	of dr->aux, DR_WRITTEN_AT_LEAST_ONCE, and DR_RW_UNCONDITIONALLY.
	(insert_address_of): New.
	(create_scratchpad): New.
	(create_indirect_cond_expr): New.
	(predicate_mem_writes): Call create_indirect_cond_expr.  Take an extra
	parameter for scratch_pad.
	(combine_blocks): Same.
	(tree_if_conversion): Same.

	testsuite/
	* g++.dg/tree-ssa/ifc-pr46029.C: New.
	* gcc.dg/tree-ssa/ifc-5.c: Make it exactly like the FFmpeg kernel.
	* gcc.dg/tree-ssa/ifc-8.c: New.
	* gcc.dg/tree-ssa/ifc-9.c: New.
	* gcc.dg/tree-ssa/ifc-10.c: New.
	* gcc.dg/tree-ssa/ifc-11.c: New.
	* gcc.dg/tree-ssa/ifc-12.c: New.
	* gcc.dg/vect/if-cvt-stores-vect-ifcvt-18.c: Disabled.
	* gcc.dg/vect/if-cvt-stores-vect-ifcvt-19.c: New.
---
 gcc/ChangeLog                                      |  28 ++
 gcc/doc/invoke.texi                                |  18 +-
 gcc/testsuite/g++.dg/tree-ssa/ifc-pr46029.C        |  76 ++++
 gcc/testsuite/gcc.dg/tree-ssa/ifc-10.c             |  17 +
 gcc/testsuite/gcc.dg/tree-ssa/ifc-11.c             |  16 +
 gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c             |  13 +
 gcc/testsuite/gcc.dg/tree-ssa/ifc-5.c              |  19 +-
 gcc/testsuite/gcc.dg/tree-ssa/ifc-8.c              |  29 ++
 gcc/testsuite/gcc.dg/tree-ssa/ifc-9.c              |  17 +
 .../gcc.dg/vect/if-cvt-stores-vect-ifcvt-18.c      |  10 +-
 .../gcc.dg/vect/if-cvt-stores-vect-ifcvt-19.c      |  46 +++
 gcc/tree-data-ref.c                                |  13 +-
 gcc/tree-data-ref.h                                |  14 +
 gcc/tree-if-conv.c                                 | 392 +++++++++------------
 14 files changed, 460 insertions(+), 248 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/tree-ssa/ifc-pr46029.C
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ifc-10.c
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ifc-11.c
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ifc-8.c
 create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/ifc-9.c
 create mode 100644 gcc/testsuite/gcc.dg/vect/if-cvt-stores-vect-ifcvt-19.c

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 3dec6b1..70af07c 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,31 @@
+2015-05-18  Sebastian Pop  <s.pop@samsung.com>
+
+	PR tree-optimization/46029
+	* doc/invoke.texi (-ftree-loop-if-convert-stores): Update description.
+	* tree-if-conv.c (has_unaligned_memory_refs): New.
+	(if_convertible_gimple_assign_stmt_p): Call has_unaligned_memory_refs.
+	(create_scratchpad): New.
+	(create_indirect_cond_expr): New.
+	(predicate_mem_writes): Call create_indirect_cond_expr.  Take an extra
+	parameter for scratch_pad.
+	(combine_blocks): Same.
+	(tree_if_conversion): Same.
+	(main_tree_if_conversion): Pass to tree_if_conversion a pointer to
+	scratch_pad.
+	(struct ifc_dr): Removed.
+	(IFC_DR): Removed.
+	(DR_WRITTEN_AT_LEAST_ONCE): Removed.
+	(DR_RW_UNCONDITIONALLY): Removed.
+	(memrefs_read_or_written_unconditionally): Removed.
+	(write_memrefs_written_at_least_once): Removed.
+	(ifcvt_memrefs_wont_trap): Removed.
+	(ifcvt_could_trap_p): Does not take refs parameter anymore.
+	(if_convertible_gimple_assign_stmt_p): Same.
+	(if_convertible_stmt_p): Same.
+	(if_convertible_loop_p_1): Remove initialization of dr->aux,
+	DR_WRITTEN_AT_LEAST_ONCE, and DR_RW_UNCONDITIONALLY.
+	(if_convertible_loop_p): Remove deallocation of the same.
+
 2015-05-15  Marc Glisse  <marc.glisse@inria.fr>
 
 	PR tree-optimization/64454
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 117b5d9..c0e27e8 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -8815,20 +8815,26 @@ if vectorization is enabled.
 @item -ftree-loop-if-convert-stores
 @opindex ftree-loop-if-convert-stores
 Attempt to also if-convert conditional jumps containing memory writes.
-This transformation can be unsafe for multi-threaded programs as it
-transforms conditional memory writes into unconditional memory writes.
 For example,
 @smallexample
 for (i = 0; i < N; i++)
   if (cond)
-    A[i] = expr;
+    A[i] = B[i] + 2;
 @end smallexample
 is transformed to
 @smallexample
-for (i = 0; i < N; i++)
-  A[i] = cond ? expr : A[i];
+void *scratchpad = alloca (64);
+for (i = 0; i < N; i++) @{
+  a = cond ? &A[i] : scratchpad;
+  b = cond ? &B[i] : scratchpad;
+  *a = *b + 2;
+@}
 @end smallexample
-potentially producing data races.
+The compiler allocates a scratchpad memory on the stack for each
+function in which the if-conversion of memory stores or reads
+happened.  This scratchpad memory is used during the part of the
+computation that is discarded, i.e., when the condition is evaluated
+to false.
 
 @item -ftree-loop-distribution
 @opindex ftree-loop-distribution
diff --git a/gcc/testsuite/g++.dg/tree-ssa/ifc-pr46029.C b/gcc/testsuite/g++.dg/tree-ssa/ifc-pr46029.C
new file mode 100644
index 0000000..2a54bdb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/ifc-pr46029.C
@@ -0,0 +1,76 @@
+// { dg-do run }
+/* { dg-options "-O -ftree-loop-if-convert-stores" } */
+
+namespace
+{
+  struct rb_tree_node_
+  {
+    rb_tree_node_ ():m_p_left (0), m_p_parent (0), m_metadata (0)
+    {
+    }
+    unsigned &get_metadata ()
+    {
+      return m_metadata;
+    }
+    rb_tree_node_ *m_p_left;
+    rb_tree_node_ *m_p_parent;
+    unsigned m_metadata;
+  };
+
+  struct bin_search_tree_const_node_it_
+  {
+    bin_search_tree_const_node_it_ (rb_tree_node_ * p_nd):m_p_nd (p_nd)
+    {
+    }
+    unsigned &get_metadata ()
+    {
+      return m_p_nd->get_metadata ();
+    }
+    bin_search_tree_const_node_it_ get_l_child ()
+    {
+      return bin_search_tree_const_node_it_ (m_p_nd->m_p_left);
+    }
+
+    rb_tree_node_ *m_p_nd;
+  };
+
+  struct bin_search_tree_no_data_
+  {
+    typedef rb_tree_node_ *node_pointer;
+      bin_search_tree_no_data_ ():m_p_head (new rb_tree_node_ ())
+    {
+    }
+    void insert_imp_empty (int r_value)
+    {
+      rb_tree_node_ *p_new_node = new rb_tree_node_ ();
+      m_p_head->m_p_parent = p_new_node;
+      p_new_node->m_p_parent = m_p_head;
+      update_to_top (m_p_head->m_p_parent);
+    }
+    void apply_update (bin_search_tree_const_node_it_ nd_it)
+    {
+      unsigned
+	l_max_endpoint
+	=
+	(nd_it.get_l_child ().m_p_nd ==
+	 0) ? 0 : nd_it.get_l_child ().get_metadata ();
+      nd_it.get_metadata () = l_max_endpoint;
+    }
+    void update_to_top (node_pointer p_nd)
+    {
+      while (p_nd != m_p_head)
+	{
+	  apply_update (p_nd);
+	  p_nd = p_nd->m_p_parent;
+	}
+    }
+
+    rb_tree_node_ * m_p_head;
+  };
+}
+
+int main ()
+{
+  bin_search_tree_no_data_ ().insert_imp_empty (0);
+  return 0;
+}
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ifc-10.c b/gcc/testsuite/gcc.dg/tree-ssa/ifc-10.c
new file mode 100644
index 0000000..3dcc202
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ifc-10.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-c -O3 -ftree-loop-if-convert-stores -fdump-tree-ifcvt-details -fdump-tree-ifcvt-stats" { target *-*-* } } */
+
+/* middle-end/PR65947 */
+
+int a[32];
+
+int main(int argc, char **argv)
+{
+  int res = 3;
+  for (int i = 0; i < 32; i++)
+    if (a[i]) res = 7;
+  return res;
+}
+
+/* { dg-final { scan-tree-dump-times "Applying if-conversion" 1 "ifcvt" } } */
+/* { dg-final { cleanup-tree-dump "ifcvt" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ifc-11.c b/gcc/testsuite/gcc.dg/tree-ssa/ifc-11.c
new file mode 100644
index 0000000..2943c04
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ifc-11.c
@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-options "-c -O3 -ftree-loop-if-convert-stores -fdump-tree-ifcvt-details -fdump-tree-ifcvt-stats" { target *-*-* } } */
+
+int read(int *r, int *c)
+{
+  int res = 42;
+  int i;
+  for (i = 0; i < 32; i++)
+    if (c[i])
+      res = *r;
+
+  return res;
+}
+
+/* { dg-final { scan-tree-dump-times "Applying if-conversion" 1 "ifcvt" } } */
+/* { dg-final { cleanup-tree-dump "ifcvt" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c b/gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c
new file mode 100644
index 0000000..01acf0b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ifc-12.c
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-c -O3 -ftree-loop-if-convert-stores -fdump-tree-ifcvt-details -fdump-tree-ifcvt-stats" { target *-*-* } } */
+
+void write(int *r, int *c)
+{
+  int i;
+  for (i = 0; i < 32; i++)
+    if (c[i])
+      *r = c[i];
+}
+
+/* { dg-final { scan-tree-dump-times "Applying if-conversion" 1 "ifcvt" } } */
+/* { dg-final { cleanup-tree-dump "ifcvt" } } */
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ifc-5.c b/gcc/testsuite/gcc.dg/tree-ssa/ifc-5.c
index a9cc816..4be2cdb 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/ifc-5.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ifc-5.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-c -O2 -ftree-vectorize -fdump-tree-ifcvt-stats" { target *-*-* } } */
+/* { dg-options "-c -O2 -ftree-vectorize -ftree-loop-if-convert-stores -fdump-tree-ifcvt-stats" { target *-*-* } } */
 
 void
 dct_unquantize_h263_inter_c (short *block, int n, int qscale, int nCoeffs)
@@ -12,11 +12,18 @@ dct_unquantize_h263_inter_c (short *block, int n, int qscale, int nCoeffs)
   for (i = 0; i <= nCoeffs; i++)
     {
       level = block[i];
-      if (level < 0)
-	level = level * qmul - qadd;
-      else
-	level = level * qmul + qadd;
-      block[i] = level;
+      if (level)
+        {
+          if (level < 0)
+            {
+              level = level * qmul - qadd;
+            }
+          else
+            {
+              level = level * qmul + qadd;
+            }
+          block[i] = level;
+        }
     }
 }
 
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ifc-8.c b/gcc/testsuite/gcc.dg/tree-ssa/ifc-8.c
new file mode 100644
index 0000000..d7cf279
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ifc-8.c
@@ -0,0 +1,29 @@
+/* { dg-do compile } */
+/* { dg-options "-c -O2 -ftree-vectorize" { target *-*-* } } */
+
+typedef union tree_node *tree;
+struct tree_common
+{
+  unsigned volatile_flag : 1;
+  unsigned unsigned_flag : 1;
+};
+struct tree_type
+{
+  tree next_variant;
+  tree main_variant;
+};
+union tree_node
+{
+  struct tree_common common;
+  struct tree_type type;
+};
+void finish_enum (tree enumtype)
+{
+  tree tem;
+  for (tem = ((enumtype)->type.main_variant); tem; tem = ((tem)->type.next_variant))
+    {
+      if (tem == enumtype)
+	continue;
+      ((tem)->common.unsigned_flag) = ((enumtype)->common.unsigned_flag);
+    }
+}
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ifc-9.c b/gcc/testsuite/gcc.dg/tree-ssa/ifc-9.c
new file mode 100644
index 0000000..6d82bb4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ifc-9.c
@@ -0,0 +1,17 @@
+/* { dg-do compile } */
+/* { dg-options "-c -O2 -ftree-vectorize -ftree-loop-if-convert-stores -fdump-tree-ifcvt-stats" { target *-*-* } } */
+
+/* middle-end/PR65947 */
+
+int a[32];
+
+int main(int argc, char **argv)
+{
+  int res = 3;
+  for (int i = 0; i < 32; i++)
+    if (a[i]) res = 7;
+  return res;
+}
+
+/* { dg-final { scan-tree-dump-times "Applying if-conversion" 1 "ifcvt" } } */
+/* { dg-final { cleanup-tree-dump "ifcvt" } } */
diff --git a/gcc/testsuite/gcc.dg/vect/if-cvt-stores-vect-ifcvt-18.c b/gcc/testsuite/gcc.dg/vect/if-cvt-stores-vect-ifcvt-18.c
index cdf687a..7bc652b 100644
--- a/gcc/testsuite/gcc.dg/vect/if-cvt-stores-vect-ifcvt-18.c
+++ b/gcc/testsuite/gcc.dg/vect/if-cvt-stores-vect-ifcvt-18.c
@@ -65,5 +65,13 @@ main (void)
   return 0;
 }
 
-/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect"  { xfail { { vect_no_align && { ! vect_hw_misalign } } || { ! vect_strided2 } } } } } */
+/* "foo()" is not vectorized FOR NOW because gather-load
+   cannot handle conditional gather loads as of June 2015.
+   
+   The old way of if-converting loads was unsafe because
+   it resulted in thread-unsafe code where the as-written code was OK.
+   The old if conversion did not contain indirection in the loads,
+   so it was simpler, therefor the vectorizer was able to pick it up.  */
+
+/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 0 "vect"  { xfail { { vect_no_align && { ! vect_hw_misalign } } || { ! vect_strided2 } } } } } */
 /* { dg-final { cleanup-tree-dump "vect" } } */
diff --git a/gcc/testsuite/gcc.dg/vect/if-cvt-stores-vect-ifcvt-19.c b/gcc/testsuite/gcc.dg/vect/if-cvt-stores-vect-ifcvt-19.c
new file mode 100644
index 0000000..6331d5a
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/if-cvt-stores-vect-ifcvt-19.c
@@ -0,0 +1,46 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target vect_int } */
+
+#include <stdarg.h>
+#include "tree-vect.h"
+
+#define N 50
+
+typedef struct {
+  short a;
+  short b;
+} data;
+
+data in1[N], in2[N], out[N];
+
+__attribute__ ((noinline)) void
+foo ()
+{
+  int i;
+  short c, d;
+
+  for (i = 0; i < N; i++)
+    {
+      c = in1[i].b;
+      d = in2[i].b;
+
+      if (c >= d)
+        {
+          out[i].b = 11;
+          out[i].a = d + 5;
+        }
+      else
+        {
+          out[i].b = d - 12;
+          out[i].a = d;
+        }
+    }
+}
+
+/* It is safe to if-convert and vectorize the above loop because
+   out[i].a and out[i].b are always written to.
+   This is the same as "if-cvt-stores-vect-ifcvt-18.c"
+   except there are no array loads inside the "if" here.  */
+
+/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 1 "vect"  { xfail { { vect_no_align && { ! vect_hw_misalign } } || { ! vect_strided2 } } } } } */
+/* { dg-final { cleanup-tree-dump "vect" } } */
diff --git a/gcc/tree-data-ref.c b/gcc/tree-data-ref.c
index 3d1d5f8..eb1c559 100644
--- a/gcc/tree-data-ref.c
+++ b/gcc/tree-data-ref.c
@@ -4393,22 +4393,11 @@ compute_all_dependences (vec<data_reference_p> datarefs,
   return true;
 }
 
-/* Describes a location of a memory reference.  */
-
-typedef struct data_ref_loc_d
-{
-  /* The memory reference.  */
-  tree ref;
-
-  /* True if the memory reference is read.  */
-  bool is_read;
-} data_ref_loc;
-
 
 /* Stores the locations of memory references in STMT to REFERENCES.  Returns
    true if STMT clobbers memory, false otherwise.  */
 
-static bool
+bool
 get_references_in_stmt (gimple stmt, vec<data_ref_loc, va_heap> *references)
 {
   bool clobbers_memory = false;
diff --git a/gcc/tree-data-ref.h b/gcc/tree-data-ref.h
index edb3b56..06a0f1a 100644
--- a/gcc/tree-data-ref.h
+++ b/gcc/tree-data-ref.h
@@ -557,4 +557,18 @@ lambda_matrix_new (int m, int n, struct obstack *lambda_obstack)
   return mat;
 }
 
+/* Describes a location of a memory reference.  */
+
+typedef struct data_ref_loc_d
+{
+  /* The memory reference.  */
+  tree ref;
+
+  /* True if the memory reference is read.  */
+  bool is_read;
+} data_ref_loc;
+
+bool
+get_references_in_stmt (gimple /* stmt */, vec<data_ref_loc, va_heap>* /* references */);
+
 #endif  /* GCC_TREE_DATA_REF_H  */
diff --git a/gcc/tree-if-conv.c b/gcc/tree-if-conv.c
index 49ff458..1272a01 100644
--- a/gcc/tree-if-conv.c
+++ b/gcc/tree-if-conv.c
@@ -19,7 +19,7 @@ along with GCC; see the file COPYING3.  If not see
 <http://www.gnu.org/licenses/>.  */
 
 /* This pass implements a tree level if-conversion of loops.  Its
-   initial goal is to help the vectorizer to vectorize loops with
+   initial goal is to help the vectorizer to vectorize loops with 
    conditions.
 
    A short description of if-conversion:
@@ -606,188 +606,39 @@ if_convertible_phi_p (struct loop *loop, basic_block bb, gphi *phi,
   return true;
 }
 
-/* Records the status of a data reference.  This struct is attached to
-   each DR->aux field.  */
-
-struct ifc_dr {
-  /* -1 when not initialized, 0 when false, 1 when true.  */
-  int written_at_least_once;
-
-  /* -1 when not initialized, 0 when false, 1 when true.  */
-  int rw_unconditionally;
-};
-
-#define IFC_DR(DR) ((struct ifc_dr *) (DR)->aux)
-#define DR_WRITTEN_AT_LEAST_ONCE(DR) (IFC_DR (DR)->written_at_least_once)
-#define DR_RW_UNCONDITIONALLY(DR) (IFC_DR (DR)->rw_unconditionally)
-
-/* Returns true when the memory references of STMT are read or written
-   unconditionally.  In other words, this function returns true when
-   for every data reference A in STMT there exist other accesses to
-   a data reference with the same base with predicates that add up (OR-up) to
-   the true predicate: this ensures that the data reference A is touched
-   (read or written) on every iteration of the if-converted loop.  */
+/* Wrapper around gimple_could_trap_p refined for the needs of the
+   if-conversion.  */
 
 static bool
-memrefs_read_or_written_unconditionally (gimple stmt,
-					 vec<data_reference_p> drs)
+ifcvt_could_trap_p (gimple stmt)
 {
-  int i, j;
-  data_reference_p a, b;
-  tree ca = bb_predicate (gimple_bb (stmt));
-
-  for (i = 0; drs.iterate (i, &a); i++)
-    if (DR_STMT (a) == stmt)
-      {
-	bool found = false;
-	int x = DR_RW_UNCONDITIONALLY (a);
-
-	if (x == 0)
-	  return false;
-
-	if (x == 1)
-	  continue;
-
-	for (j = 0; drs.iterate (j, &b); j++)
-          {
-            tree ref_base_a = DR_REF (a);
-            tree ref_base_b = DR_REF (b);
-
-            if (DR_STMT (b) == stmt)
-              continue;
-
-            while (TREE_CODE (ref_base_a) == COMPONENT_REF
-                   || TREE_CODE (ref_base_a) == IMAGPART_EXPR
-                   || TREE_CODE (ref_base_a) == REALPART_EXPR)
-              ref_base_a = TREE_OPERAND (ref_base_a, 0);
-
-            while (TREE_CODE (ref_base_b) == COMPONENT_REF
-                   || TREE_CODE (ref_base_b) == IMAGPART_EXPR
-                   || TREE_CODE (ref_base_b) == REALPART_EXPR)
-              ref_base_b = TREE_OPERAND (ref_base_b, 0);
-
-  	    if (!operand_equal_p (ref_base_a, ref_base_b, 0))
-	      {
-	        tree cb = bb_predicate (gimple_bb (DR_STMT (b)));
-
-	        if (DR_RW_UNCONDITIONALLY (b) == 1
-		    || is_true_predicate (cb)
-		    || is_true_predicate (ca
-                        = fold_or_predicates (EXPR_LOCATION (cb), ca, cb)))
-		  {
-		    DR_RW_UNCONDITIONALLY (a) = 1;
-  		    DR_RW_UNCONDITIONALLY (b) = 1;
-		    found = true;
-		    break;
-		  }
-               }
-	    }
-
-	if (!found)
-	  {
-	    DR_RW_UNCONDITIONALLY (a) = 0;
-	    return false;
-	  }
-      }
+  if (gimple_vuse (stmt)
+      && !gimple_could_trap_p_1 (stmt, false, false))
+    return false;
 
-  return true;
+  return gimple_could_trap_p (stmt);
 }
 
-/* Returns true when the memory references of STMT are unconditionally
-   written.  In other words, this function returns true when for every
-   data reference A written in STMT, there exist other writes to the
-   same data reference with predicates that add up (OR-up) to the true
-   predicate: this ensures that the data reference A is written on
-   every iteration of the if-converted loop.  */
-
+/* Returns true when stmt contains a data reference.  */
 static bool
-write_memrefs_written_at_least_once (gimple stmt,
-				     vec<data_reference_p> drs)
+has_non_addressable_refs (gimple stmt)
 {
-  int i, j;
-  data_reference_p a, b;
-  tree ca = bb_predicate (gimple_bb (stmt));
-
-  for (i = 0; drs.iterate (i, &a); i++)
-    if (DR_STMT (a) == stmt
-	&& DR_IS_WRITE (a))
-      {
-	bool found = false;
-	int x = DR_WRITTEN_AT_LEAST_ONCE (a);
-
-	if (x == 0)
-	  return false;
-
-	if (x == 1)
-	  continue;
-
-	for (j = 0; drs.iterate (j, &b); j++)
-	  if (DR_STMT (b) != stmt
-	      && DR_IS_WRITE (b)
-	      && same_data_refs_base_objects (a, b))
-	    {
-	      tree cb = bb_predicate (gimple_bb (DR_STMT (b)));
+  vec<data_ref_loc, va_heap>* refs;
+  vec_alloc (refs, 3);
 
-	      if (DR_WRITTEN_AT_LEAST_ONCE (b) == 1
-		  || is_true_predicate (cb)
-		  || is_true_predicate (ca = fold_or_predicates (EXPR_LOCATION (cb),
-								 ca, cb)))
-		{
-		  DR_WRITTEN_AT_LEAST_ONCE (a) = 1;
-		  DR_WRITTEN_AT_LEAST_ONCE (b) = 1;
-		  found = true;
-		  break;
-		}
-	    }
+  bool res = get_references_in_stmt (stmt, refs);
+  unsigned i;
+  data_ref_loc *ref;
 
-	if (!found)
-	  {
-	    DR_WRITTEN_AT_LEAST_ONCE (a) = 0;
-	    return false;
-	  }
+  FOR_EACH_VEC_ELT (*refs, i, ref)
+    if (may_be_nonaddressable_p (ref->ref))
+      {
+	res = true;
+	break;
       }
 
-  return true;
-}
-
-/* Return true when the memory references of STMT won't trap in the
-   if-converted code.  There are two things that we have to check for:
-
-   - writes to memory occur to writable memory: if-conversion of
-   memory writes transforms the conditional memory writes into
-   unconditional writes, i.e. "if (cond) A[i] = foo" is transformed
-   into "A[i] = cond ? foo : A[i]", and as the write to memory may not
-   be executed at all in the original code, it may be a readonly
-   memory.  To check that A is not const-qualified, we check that
-   there exists at least an unconditional write to A in the current
-   function.
-
-   - reads or writes to memory are valid memory accesses for every
-   iteration.  To check that the memory accesses are correctly formed
-   and that we are allowed to read and write in these locations, we
-   check that the memory accesses to be if-converted occur at every
-   iteration unconditionally.  */
-
-static bool
-ifcvt_memrefs_wont_trap (gimple stmt, vec<data_reference_p> refs)
-{
-  return write_memrefs_written_at_least_once (stmt, refs)
-    && memrefs_read_or_written_unconditionally (stmt, refs);
-}
-
-/* Wrapper around gimple_could_trap_p refined for the needs of the
-   if-conversion.  Try to prove that the memory accesses of STMT could
-   not trap in the innermost loop containing STMT.  */
-
-static bool
-ifcvt_could_trap_p (gimple stmt, vec<data_reference_p> refs)
-{
-  if (gimple_vuse (stmt)
-      && !gimple_could_trap_p_1 (stmt, false, false)
-      && ifcvt_memrefs_wont_trap (stmt, refs))
-    return false;
-
-  return gimple_could_trap_p (stmt);
+  vec_free (refs);
+  return res;
 }
 
 /* Return true if STMT could be converted into a masked load or store
@@ -849,7 +700,6 @@ ifcvt_can_use_mask_load_store (gimple stmt)
 
 static bool
 if_convertible_gimple_assign_stmt_p (gimple stmt,
-				     vec<data_reference_p> refs,
 				     bool *any_mask_load_store)
 {
   tree lhs = gimple_assign_lhs (stmt);
@@ -883,7 +733,7 @@ if_convertible_gimple_assign_stmt_p (gimple stmt,
 
   if (flag_tree_loop_if_convert_stores)
     {
-      if (ifcvt_could_trap_p (stmt, refs))
+      if (ifcvt_could_trap_p (stmt))
 	{
 	  if (ifcvt_can_use_mask_load_store (stmt))
 	    {
@@ -892,9 +742,17 @@ if_convertible_gimple_assign_stmt_p (gimple stmt,
 	      return true;
 	    }
 	  if (dump_file && (dump_flags & TDF_DETAILS))
-	    fprintf (dump_file, "tree could trap...\n");
+	    fprintf (dump_file, "tree could trap\n");
 	  return false;
 	}
+
+      if (has_non_addressable_refs (stmt))
+	{
+	  if (dump_file && (dump_flags & TDF_DETAILS))
+	    fprintf (dump_file, "has non-addressable memory references\n");
+	  return false;
+	}
+
       return true;
     }
 
@@ -942,7 +800,7 @@ if_convertible_gimple_assign_stmt_p (gimple stmt,
    - it is builtins call.  */
 
 static bool
-if_convertible_stmt_p (gimple stmt, vec<data_reference_p> refs,
+if_convertible_stmt_p (gimple stmt, 
 		       bool *any_mask_load_store)
 {
   switch (gimple_code (stmt))
@@ -953,7 +811,7 @@ if_convertible_stmt_p (gimple stmt, vec<data_reference_p> refs,
       return true;
 
     case GIMPLE_ASSIGN:
-      return if_convertible_gimple_assign_stmt_p (stmt, refs,
+      return if_convertible_gimple_assign_stmt_p (stmt,
 						  any_mask_load_store);
 
     case GIMPLE_CALL:
@@ -1173,8 +1031,8 @@ get_loop_body_in_if_conv_order (const struct loop *loop)
    predicate_bbs first allocates the predicates of the basic blocks.
    These fields are then initialized with the tree expressions
    representing the predicates under which a basic block is executed
-   in the LOOP.  As the loop->header is executed at each iteration, it
-   has the "true" predicate.  Other statements executed under a
+   in the LOOP.  As the loop->header is executed at each iteration,
+   it has the "true" predicate.  Other statements executed under a
    condition are predicated with that condition, for example
 
    | if (x)
@@ -1322,17 +1180,7 @@ if_convertible_loop_p_1 (struct loop *loop,
     }
 
   if (flag_tree_loop_if_convert_stores)
-    {
-      data_reference_p dr;
-
-      for (i = 0; refs->iterate (i, &dr); i++)
-	{
-	  dr->aux = XNEW (struct ifc_dr);
-	  DR_WRITTEN_AT_LEAST_ONCE (dr) = -1;
-	  DR_RW_UNCONDITIONALLY (dr) = -1;
-	}
-      predicate_bbs (loop);
-    }
+    predicate_bbs (loop);
 
   for (i = 0; i < loop->num_nodes; i++)
     {
@@ -1342,7 +1190,7 @@ if_convertible_loop_p_1 (struct loop *loop,
       /* Check the if-convertibility of statements in predicated BBs.  */
       if (!dominated_by_p (CDI_DOMINATORS, loop->latch, bb))
 	for (itr = gsi_start_bb (bb); !gsi_end_p (itr); gsi_next (&itr))
-	  if (!if_convertible_stmt_p (gsi_stmt (itr), *refs,
+	  if (!if_convertible_stmt_p (gsi_stmt (itr), 
 				      any_mask_load_store))
 	    return false;
     }
@@ -1988,6 +1836,82 @@ mask_exists (int size, vec<int> vec)
   return -1;
 }
 
+/* Inserts at position GSI a statement "ADDRESS_OF_AI = &AI;" and
+   returns the ADDRESS_OF_AI.  */
+
+static tree
+insert_address_of (tree ai, gimple_stmt_iterator *gsi)
+{
+  tree addr_expr = build_fold_addr_expr (ai);
+  tree address_of_ai = create_tmp_reg (TREE_TYPE (addr_expr), "_ifc_");
+  gimple addr_stmt = gimple_build_assign (address_of_ai, addr_expr);
+
+  address_of_ai = make_ssa_name (address_of_ai, addr_stmt);
+  gimple_assign_set_lhs (addr_stmt, address_of_ai);
+  SSA_NAME_DEF_STMT (address_of_ai) = addr_stmt;
+  update_stmt (addr_stmt);
+  gsi_insert_before (gsi, addr_stmt, GSI_SAME_STMT);
+
+  return address_of_ai;
+}
+
+/* Insert at the beginning of the first basic block of the current
+   function the allocation on the stack of N_BYTES of memory and
+   return a pointer to this scratchpad memory.  */
+
+static tree
+create_scratchpad (unsigned int n_bytes)
+{
+  basic_block bb = single_succ ( ENTRY_BLOCK_PTR_FOR_FN(cfun) );
+  gimple_stmt_iterator gsi = gsi_after_labels (bb);
+  tree x = build_int_cst (integer_type_node, n_bytes - 1);
+  tree elt_type = char_type_node;
+  tree array_type = build_array_type (elt_type, build_index_type (x));
+  tree base = create_tmp_reg (array_type, "scratch_pad");
+  tree a0 = build4 (ARRAY_REF, elt_type, base, integer_zero_node, NULL_TREE,
+		    NULL_TREE);
+
+  return insert_address_of (a0, &gsi);
+}
+
+/* Returns a memory reference to the pointer defined by the
+   conditional expression: pointer = cond ? &A[i] : scratch_pad; and
+   inserts this code at GSI.  */
+
+static tree
+create_indirect_cond_expr (tree ai, tree cond, tree *scratch_pad,
+			   gimple_stmt_iterator *gsi, bool swap)
+{
+  tree cond_expr;
+  tree pointer;
+  gimple pointer_stmt;
+
+  /* address_of_ai = &A[i];  */
+  tree address_of_ai = insert_address_of (ai, gsi);
+
+  /* Allocate the scratch pad only once per function.  */
+  if (!*scratch_pad)
+    *scratch_pad = create_scratchpad (64);
+
+  /* pointer = cond ? address_of_ai : scratch_pad;  */
+  pointer = create_tmp_reg (TREE_TYPE (address_of_ai), "_ifc_");
+
+  cond_expr = build3 (COND_EXPR, TREE_TYPE (address_of_ai),
+		      unshare_expr (cond),
+		      swap ? *scratch_pad  : address_of_ai,
+		      swap ? address_of_ai : *scratch_pad);
+
+  pointer_stmt = gimple_build_assign (pointer, cond_expr);
+  pointer = make_ssa_name (pointer, pointer_stmt);
+  gimple_assign_set_lhs (pointer_stmt, pointer);
+  SSA_NAME_DEF_STMT (pointer) = pointer_stmt;
+  update_stmt (pointer_stmt);
+  gsi_insert_before (gsi, pointer_stmt, GSI_SAME_STMT);
+
+  return build2 (MEM_REF, TREE_TYPE (ai), pointer,
+		 build_int_cst (reference_alias_ptr_type (ai), 0));
+}
+
 /* Predicate each write to memory in LOOP.
 
    This function transforms control flow constructs containing memory
@@ -1997,12 +1921,21 @@ mask_exists (int size, vec<int> vec)
    |   if (cond)
    |     A[i] = expr;
 
-   into the following form that does not contain control flow:
+   into the following form:
 
-   | for (i = 0; i < N; i++)
-   |   A[i] = cond ? expr : A[i];
+   | void *scratch_pad = alloca (MAX_TYPE_SIZE_IN_BYTES);
+   |
+   | for (i = 0; i < N; i++) {
+   |   p = cond ? &A[i] : scratch_pad;
+   |   *p = expr;
+   | }
 
-   The original CFG looks like this:
+   SCRATCH_PAD is allocated on the stack for each function once and it is
+   large enough to contain any kind of scalar assignment or read.  All
+   values read or written to SCRATCH_PAD are not used in the computation.
+
+   In a more detailed way, the if-conversion of memory writes works
+   like this, supposing that the original CFG looks like this:
 
    | bb_0
    |   i = 0
@@ -2052,10 +1985,12 @@ mask_exists (int size, vec<int> vec)
    |   goto bb_1
    | end_bb_4
 
-   predicate_mem_writes is then predicating the memory write as follows:
+   predicate_mem_writes is then allocating SCRATCH_PAD in the basic block
+   preceding the loop header, and is predicating the memory write:
 
    | bb_0
    |   i = 0
+   |   scratch_pad = alloca (MAX_TYPE_SIZE_IN_BYTES);
    | end_bb_0
    |
    | bb_1
@@ -2063,12 +1998,14 @@ mask_exists (int size, vec<int> vec)
    | end_bb_1
    |
    | bb_2
+   |   cond = some_computation;
    |   if (cond) goto bb_3 else goto bb_4
    | end_bb_2
    |
    | bb_3
    |   cond = some_computation;
-   |   A[i] = cond ? expr : A[i];
+   |   p = cond ? &A[i] : scratch_pad;
+   |   *p = expr;
    |   goto bb_4
    | end_bb_3
    |
@@ -2081,12 +2018,14 @@ mask_exists (int size, vec<int> vec)
 
    | bb_0
    |   i = 0
+   |   scratch_pad = alloca (MAX_TYPE_SIZE_IN_BYTES);
    |   if (i < N) goto bb_5 else goto bb_1
    | end_bb_0
    |
    | bb_1
    |   cond = some_computation;
-   |   A[i] = cond ? expr : A[i];
+   |   p = cond ? &A[i] : scratch_pad;
+   |   *p = expr;
    |   if (i < N) goto bb_5 else goto bb_4
    | end_bb_1
    |
@@ -2096,7 +2035,7 @@ mask_exists (int size, vec<int> vec)
 */
 
 static void
-predicate_mem_writes (loop_p loop)
+predicate_mem_writes (loop_p loop, tree *scratch_pad)
 {
   unsigned int i, orig_loop_num_nodes = loop->num_nodes;
   auto_vec<int, 1> vect_sizes;
@@ -2179,24 +2118,29 @@ predicate_mem_writes (loop_p loop)
 	  }
 	else if (gimple_vdef (stmt))
 	  {
-	    tree lhs = gimple_assign_lhs (stmt);
-	    tree rhs = gimple_assign_rhs1 (stmt);
-	    tree type = TREE_TYPE (lhs);
 
-	    lhs = ifc_temp_var (type, unshare_expr (lhs), &gsi);
-	    rhs = ifc_temp_var (type, unshare_expr (rhs), &gsi);
-	    if (swap)
-	      {
-		tree tem = lhs;
-		lhs = rhs;
-		rhs = tem;
-	      }
-	    cond = force_gimple_operand_gsi_1 (&gsi, unshare_expr (cond),
-					       is_gimple_condexpr, NULL_TREE,
-					       true, GSI_SAME_STMT);
-	    rhs = fold_build_cond_expr (type, unshare_expr (cond), rhs, lhs);
-	    gimple_assign_set_rhs1 (stmt, ifc_temp_var (type, rhs, &gsi));
+  	    /* A[i] = x;  */
+	    tree ai = gimple_assign_lhs (stmt);
+
+	    /* pointer = cond ? &A[i] : scratch_pad;  */
+	    tree star_pointer = create_indirect_cond_expr (ai, cond,
+			  scratch_pad, &gsi, swap);
+	    /* *pointer = x;  */
+	    gimple_assign_set_lhs (stmt, star_pointer);
 	    update_stmt (stmt);
+
+	  }
+	else if (gimple_vuse (stmt))
+	  {
+	      /* x = A[i];  */
+	      tree ai = gimple_assign_rhs1 (stmt);
+
+	      /* pointer = cond ? &A[i] : scratch_pad;  */
+	      tree star_pointer = create_indirect_cond_expr (ai, cond,
+					     scratch_pad, &gsi, swap);
+	      /* x = *pointer;  */
+	      gimple_assign_set_rhs1 (stmt, star_pointer);
+	      update_stmt (stmt);
 	  }
     }
 }
@@ -2247,7 +2191,7 @@ remove_conditions_and_labels (loop_p loop)
    blocks.  Replace PHI nodes with conditional modify expressions.  */
 
 static void
-combine_blocks (struct loop *loop, bool any_mask_load_store)
+combine_blocks (struct loop *loop, bool any_mask_load_store, tree *scratch_pad)
 {
   basic_block bb, exit_bb, merge_target_bb;
   unsigned int orig_loop_num_nodes = loop->num_nodes;
@@ -2261,7 +2205,7 @@ combine_blocks (struct loop *loop, bool any_mask_load_store)
   predicate_all_scalar_phis (loop);
 
   if (flag_tree_loop_if_convert_stores || any_mask_load_store)
-    predicate_mem_writes (loop);
+    predicate_mem_writes (loop, scratch_pad);
 
   /* Merge basic blocks: first remove all the edges in the loop,
      except for those from the exit block.  */
@@ -2523,7 +2467,7 @@ ifcvt_walk_pattern_tree (tree var, vec<gimple> *defuse_list,
   return;
 }
 
-/* Returns true if STMT can be a root of bool pattern apllied
+/* Returns true if STMT can be a root of bool pattern applied
    by vectorizer.  */
 
 static bool
@@ -2553,7 +2497,7 @@ stmt_is_root_of_bool_pattern (gimple stmt)
   return false;
 }
 
-/*  Traverse all statements in BB which correspondent to loop header to
+/*  Traverse all statements in BB which correspond to loop header to
     find out all statements which can start bool pattern applied by
     vectorizer and convert multiple uses in it to conform pattern
     restrictions.  Such case can occur if the same predicate is used both
@@ -2634,7 +2578,7 @@ ifcvt_local_dce (basic_block bb)
       gimple_set_plf (phi, GF_PLF_2, true);
       worklist.safe_push (phi);
     }
-  /* Consider load/store statemnts, CALL and COND as live.  */
+  /* Consider load/store statements, CALL and COND as live.  */
   for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
     {
       stmt = gsi_stmt (gsi);
@@ -2716,13 +2660,13 @@ ifcvt_local_dce (basic_block bb)
    changed.  */
 
 static unsigned int
-tree_if_conversion (struct loop *loop)
+tree_if_conversion (struct loop *loop, tree *scratch_pad)
 {
   unsigned int todo = 0;
   ifc_bbs = NULL;
   bool any_mask_load_store = false;
 
-  /* Set-up aggressive if-conversion for loops marked with simd pragma.  */
+  /* Set up aggressive if-conversion for loops marked with simd pragma.  */
   aggressive_if_conv = loop->force_vectorize;
   /* Check either outer loop was marked with simd pragma.  */
   if (!aggressive_if_conv)
@@ -2751,7 +2695,7 @@ tree_if_conversion (struct loop *loop)
   /* Now all statements are if-convertible.  Combine all the basic
      blocks into one huge basic block doing the if-conversion
      on-the-fly.  */
-  combine_blocks (loop, any_mask_load_store);
+  combine_blocks (loop, any_mask_load_store, scratch_pad);
 
   /* Delete dead predicate computations and repair tree correspondent
      to bool pattern to delete multiple uses of preidcates.  */
@@ -2832,12 +2776,14 @@ pass_if_conversion::execute (function *fun)
   if (number_of_loops (fun) <= 1)
     return 0;
 
+  tree scratch_pad = NULL_TREE;
+
   FOR_EACH_LOOP (loop, 0)
     if (flag_tree_loop_if_convert == 1
 	|| flag_tree_loop_if_convert_stores == 1
 	|| ((flag_tree_loop_vectorize || loop->force_vectorize)
 	    && !loop->dont_vectorize))
-      todo |= tree_if_conversion (loop);
+      todo |= tree_if_conversion (loop, &scratch_pad);
 
 #ifdef ENABLE_CHECKING
   {
-- 
2.1.0.243.g30d45f7

^ permalink raw reply	[flat|nested] 17+ messages in thread
[parent not found: <001301d0ae99$0f015800$2d040800$@samsung.com>]
* RE: fix PR46029: reimplement if conversion of loads and stores
@ 2015-07-07 21:23 Abe
  2015-07-08  9:14 ` Alan Lawrence
  2015-07-08  9:56 ` Richard Biener
  0 siblings, 2 replies; 17+ messages in thread
From: Abe @ 2015-07-07 21:23 UTC (permalink / raw)
  To: Jeff Law, Richard Biener, Alan Lawrence,
	sebpop@gmail.com >> Sebastian Pop
  Cc: gcc-patches

> (if-conversion could directly generate masked load/stores
>  of course and not use a scratch-pad at all in that case).

IMO that`s a great idea, but I don`t know how to do it.  Hints would be welcome.  In particular, how does one "generate masked load/stores" at the GIMPLE level?


> But are we correctly handling these cases in the current if conversion code?

I`m uncertain to what that is intended to refer, but I believe Sebastian would agree that the new if converter is safer than the old one in terms of correctness at the time of running the code being compiled.


> Abe's changes would seem like a step forward from a correctness standpoint

Not to argue, but as a point of humility: Sebastian did by far the most work on this patch.  I just modernized it and helped move it along.  Even _that_ was done with Sebastian`s help.


> even if they take us a step backwards from a performance standpoint.

For now, we have a few performance regressions, and so far we have found that it`s non-trivial to remove all of those regressions.
We may be better off pushing the current patch to trunk and having the performance regressions currently introduced by the new if converter be fixed by later patches.
Pushing to trunk gives us excellent "visibility" amongst GCC hackers, so the code will get more "eyeballs" than if it lingers in an uncommitted patch or in a branch.
I, for one, would love some help in fixing these performance regressions. ;-)

If fixing the performance regressions winds up taking too long, perhaps the current imperfect patch could be undone on trunk just before a release is tagged,
and then we`ll push it in again when trunk goes back to being allowed to be unstable?  According to my analysis of the data near the end of the page at
<https://gcc.gnu.org/develop.html>, we have until roughly April of 2016 to work on not-yet-perfect patches in trunk.


>> So the question is whether we get more non-vectorized if-converted
>> code out of this (and thus whether we want to use --param
>> allow-store-data-races to get the old code back which is nicer to less
>> capable CPUs and probably faster than using scatter/gather or masked loads/stores).

> I do think conditionalizing some of this on the allow-store-data-races makes sense.

I think having both the old if-converter and the new one "live on" in GCC is nontrivial, but not impossible.  I also don`t think it`s the best long-term goal,
but only a short-term workaround.  In the long run, IMO there should be only one if converter, albeit perhaps with tweaking flags [e.g. "-fallow-unsafe-if-conversion"].


> I also wonder if we should really care about load data races (not sure your patch does).

According to a recent long discussion I had with Sebastian, our current patch does not have the flaw I was concerned it might have in terms of loads because:

   [1] the scratchpad is only being used to if-convert assignments to thread-local scalars, never to globals/statics, and because...

   [2] the gimplifier is supposed to detect "the address of this scalar has been taken" and when such is detected in the code being compiled,
       it causes the scalar to no longer look like a scalar in GIMPLE so that we are also safe from stale-data problems that could come from
       corner-case code that takes the address of a thread-local variable and gives that address to another thread [which then proceeds to
       overwrite the value in the supposedly-thread-local scalar that belongs to a different thread from the one doing the writing]


Regards,

Abe


^ permalink raw reply	[flat|nested] 17+ messages in thread
end of thread, other threads:[~2015-07-08 16:27 UTC | newest]

Thread overview: 17+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2015-06-12 21:05 fix PR46029: reimplement if conversion of loads and stores Abe Skolnik
2015-06-22 16:31 ` Alan Lawrence
2015-06-24 17:11   ` Jeff Law
2015-06-25  9:48     ` Richard Biener
2015-06-25 14:28       ` Sebastian Pop
2015-06-26 12:35         ` Alan Lawrence
2015-06-26 15:10           ` Richard Biener
2015-06-26 21:29             ` Jeff Law
2015-07-06 20:46       ` Jeff Law
2015-06-24 16:51 ` Ramana Radhakrishnan
2015-06-24 17:02   ` Jeff Law
     [not found] <001301d0ae99$0f015800$2d040800$@samsung.com>
     [not found] ` <682387955.453848.1435166632115.JavaMail.yahoo@mail.yahoo.com>
     [not found]   ` <680180205.479081.1435168382362.JavaMail.yahoo@mail.yahoo.com>
2015-06-30  9:36     ` Alan Lawrence
2015-07-07 21:23 Abe
2015-07-08  9:14 ` Alan Lawrence
2015-07-08 15:52   ` Abe
2015-07-08  9:56 ` Richard Biener
2015-07-08 16:27   ` Abe

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