[PATCH V7] Remove empty loop with assumed finiteness (PR tree-optimization/89713)

[Feng Xue OS <fxue@os.amperecomputing.com>]

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 37aab79..87cc125 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,16 @@
+2019-06-04  Feng Xue  <fxue@os.amperecomputing.com>
+
+	PR tree-optimization/89713
+	* doc/invoke.texi (-ffinite-loops): Document new option.
+	* common.opt (-ffinite-loops): New option.
+	* tree-ssa-dce.c (mark_stmt_if_obviously_necessary): Mark
+	IFN_GOACC_LOOP calls as necessary.
+	* tree-ssa-loop-niter.c (finite_loop_p): Assume loop with an exit
+	is finite.
+	* omp-offload.c (oacc_xform_loop): Skip lowering if return value of
+	IFN_GOACC_LOOP call is not used.
+	* opts.c (default_options_table): Enable -ffinite-loops at -O2+.
+
 2019-06-04  Alan Modra  <amodra@gmail.com>
 
 	PR target/90689
diff --git a/gcc/common.opt b/gcc/common.opt
index 0e72fd0..8b0e6ad 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1437,6 +1437,10 @@ ffinite-math-only
 Common Report Var(flag_finite_math_only) Optimization SetByCombined
 Assume no NaNs or infinities are generated.
 
+ffinite-loops
+Common Report Var(flag_finite_loops) Optimization
+Assume that loops with an exit will terminate and not loop indefinitely.
+
 ffixed-
 Common Joined RejectNegative Var(common_deferred_options) Defer
 -ffixed-<register>	Mark <register> as being unavailable to the compiler.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 91c9bb8..0fe4c52 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -412,6 +412,7 @@ Objective-C and Objective-C++ Dialects}.
 -fdevirtualize-at-ltrans  -fdse @gol
 -fearly-inlining  -fipa-sra  -fexpensive-optimizations  -ffat-lto-objects @gol
 -ffast-math  -ffinite-math-only  -ffloat-store  -fexcess-precision=@var{style} @gol
+-ffinite-loops @gol
 -fforward-propagate  -ffp-contract=@var{style}  -ffunction-sections @gol
 -fgcse  -fgcse-after-reload  -fgcse-las  -fgcse-lm  -fgraphite-identity @gol
 -fgcse-sm  -fhoist-adjacent-loads  -fif-conversion @gol
@@ -8282,6 +8283,7 @@ also turns on the following optimization flags:
 -fdelete-null-pointer-checks @gol
 -fdevirtualize  -fdevirtualize-speculatively @gol
 -fexpensive-optimizations @gol
+-ffinite-loops @gol 
 -fgcse  -fgcse-lm  @gol
 -fhoist-adjacent-loads @gol
 -finline-small-functions @gol
@@ -9503,6 +9505,15 @@ that may set @code{errno} but are otherwise free of side effects.  This flag is
 enabled by default at @option{-O2} and higher if @option{-Os} is not also
 specified.
 
+@item -ffinite-loops
+@opindex ffinite-loops
+@opindex fno-finite-loops
+Assume that a loop with an exit will eventually take the exit and not loop
+indefinitely.  This allows the compiler to remove loops that otherwise have
+no side-effects, not considering eventual endless looping as such.
+
+This option is enabled by default at @option{-O2}.
+
 @item -ftree-dominator-opts
 @opindex ftree-dominator-opts
 Perform a variety of simple scalar cleanups (constant/copy
diff --git a/gcc/omp-offload.c b/gcc/omp-offload.c
index 97ae47b..369122f 100644
--- a/gcc/omp-offload.c
+++ b/gcc/omp-offload.c
@@ -300,7 +300,7 @@ oacc_xform_loop (gcall *call)
   tree chunk_size = NULL_TREE;
   unsigned mask = (unsigned) TREE_INT_CST_LOW (gimple_call_arg (call, 5));
   tree lhs = gimple_call_lhs (call);
-  tree type = TREE_TYPE (lhs);
+  tree type = NULL_TREE;
   tree diff_type = TREE_TYPE (range);
   tree r = NULL_TREE;
   gimple_seq seq = NULL;
@@ -308,6 +308,15 @@ oacc_xform_loop (gcall *call)
   unsigned outer_mask = mask & (~mask + 1); // Outermost partitioning
   unsigned inner_mask = mask & ~outer_mask; // Inner partitioning (if any)
 
+  /* Skip lowering if return value of IFN_GOACC_LOOP call is not used. */
+  if (!lhs)
+    {
+      gsi_replace_with_seq (&gsi, seq, true);
+      return;
+    }
+
+  type = TREE_TYPE (lhs);
+ 
 #ifdef ACCEL_COMPILER
   chunk_size = gimple_call_arg (call, 4);
   if (integer_minus_onep (chunk_size)  /* Force static allocation.  */
diff --git a/gcc/opts.c b/gcc/opts.c
index 64f94ac..b38bfb1 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -494,6 +494,7 @@ static const struct default_options default_options_table[] =
     { OPT_LEVELS_2_PLUS, OPT_fdevirtualize, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fdevirtualize_speculatively, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fexpensive_optimizations, NULL, 1 },
+    { OPT_LEVELS_2_PLUS, OPT_ffinite_loops, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fgcse, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_fhoist_adjacent_loads, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_findirect_inlining, NULL, 1 },
diff --git a/gcc/testsuite/g++.dg/tree-ssa/empty-loop.C b/gcc/testsuite/g++.dg/tree-ssa/empty-loop.C
new file mode 100644
index 0000000..6b1e879
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/empty-loop.C
@@ -0,0 +1,33 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-cddce2 -ffinite-loops" } */
+
+#include <string>
+#include <vector>
+#include <list>
+#include <set>
+#include <map>
+
+using namespace std;
+
+int foo (vector<string> &v, list<string> &l, set<string> &s, map<int, string> &m)
+{
+  for (vector<string>::iterator it = v.begin (); it != v.end (); ++it)
+    it->length();
+
+  for (list<string>::iterator it = l.begin (); it != l.end (); ++it)
+    it->length();
+
+  for (map<int, string>::iterator it = m.begin (); it != m.end (); ++it)
+    it->first + it->second.length();
+
+  for (set<string>::iterator it0 = s.begin (); it0 != s.end(); ++it0)
+    for (vector<string>::reverse_iterator it1 = v.rbegin(); it1 != v.rend(); ++it1)
+      {
+        it0->length();
+        it1->length();
+      }  
+
+  return 0;
+}
+/* { dg-final { scan-tree-dump-not "if" "cddce2"} } */
+
diff --git a/gcc/testsuite/gcc.dg/const-1.c b/gcc/testsuite/gcc.dg/const-1.c
index a5b2b16..2e95bd8 100644
--- a/gcc/testsuite/gcc.dg/const-1.c
+++ b/gcc/testsuite/gcc.dg/const-1.c
@@ -1,5 +1,5 @@
 /* { dg-do compile { target nonpic } } */
-/* { dg-options "-O2 -Wsuggest-attribute=const" } */
+/* { dg-options "-O2 -Wsuggest-attribute=const -fno-finite-loops" } */
 
 extern int extern_const(int a) __attribute__ ((const));
 
diff --git a/gcc/testsuite/gcc.dg/graphite/graphite.exp b/gcc/testsuite/gcc.dg/graphite/graphite.exp
index ea61446..523a955 100644
--- a/gcc/testsuite/gcc.dg/graphite/graphite.exp
+++ b/gcc/testsuite/gcc.dg/graphite/graphite.exp
@@ -56,7 +56,7 @@ set vect_files        [lsort [glob -nocomplain $srcdir/$subdir/vect-*.c ] ]
 
 # Tests to be compiled.
 set dg-do-what-default compile
-dg-runtest $scop_files        "" "-O2 -fgraphite -fdump-tree-graphite-all"
+dg-runtest $scop_files        "" "-O2 -fgraphite -fdump-tree-graphite-all -fno-finite-loops"
 dg-runtest $id_files          "" "-O2 -fgraphite-identity -ffast-math -fdump-tree-graphite-details"
 
 # Tests to be run.
diff --git a/gcc/testsuite/gcc.dg/loop-unswitch-1.c b/gcc/testsuite/gcc.dg/loop-unswitch-1.c
index f6fc41d..de2fb2c 100644
--- a/gcc/testsuite/gcc.dg/loop-unswitch-1.c
+++ b/gcc/testsuite/gcc.dg/loop-unswitch-1.c
@@ -1,6 +1,6 @@
 /* For PR rtl-optimization/27735  */
 /* { dg-do compile } */
-/* { dg-options "-O2 -funswitch-loops -fdump-tree-unswitch-details" } */
+/* { dg-options "-O2 -funswitch-loops -fdump-tree-unswitch-details -fno-finite-loops" } */
 
 void set_color(void);
 void xml_colorize_line(unsigned int *p, int state)
diff --git a/gcc/testsuite/gcc.dg/predict-9.c b/gcc/testsuite/gcc.dg/predict-9.c
index 7e5ba08..f491c51 100644
--- a/gcc/testsuite/gcc.dg/predict-9.c
+++ b/gcc/testsuite/gcc.dg/predict-9.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdisable-tree-evrp -fdump-tree-profile_estimate" } */
+/* { dg-options "-O2 -fdisable-tree-evrp -fdump-tree-profile_estimate -fno-finite-loops" } */
 
 extern int global;
 extern int global2;
diff --git a/gcc/testsuite/gcc.dg/pure-2.c b/gcc/testsuite/gcc.dg/pure-2.c
index fe6e2bc..318cfd1 100644
--- a/gcc/testsuite/gcc.dg/pure-2.c
+++ b/gcc/testsuite/gcc.dg/pure-2.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -Wsuggest-attribute=pure" } */
+/* { dg-options "-O2 -Wsuggest-attribute=pure -fno-finite-loops" } */
 /* { dg-add-options bind_pic_locally } */
 
 extern int extern_const(int a) __attribute__ ((pure));
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/20040211-1.c b/gcc/testsuite/gcc.dg/tree-ssa/20040211-1.c
index d289e5d..a9bdf26 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/20040211-1.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/20040211-1.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-cddce2" } */
+/* { dg-options "-O2 -fdump-tree-cddce2 -fno-finite-loops" } */
 
 struct rtx_def;
 typedef struct rtx_def *rtx;
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/dce-2.c b/gcc/testsuite/gcc.dg/tree-ssa/dce-2.c
new file mode 100644
index 0000000..18c1ddb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/dce-2.c
@@ -0,0 +1,37 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-cddce1 -ffinite-loops" } */
+
+typedef struct list {
+    char pad[15];
+    struct list *next;
+} list;
+
+int data;
+
+list *head, *tail;
+
+int __attribute__((pure)) pfn (int);
+
+int foo (unsigned u, int s)
+{
+  unsigned i;
+  list *p;
+  int j;
+
+  for (i = 0; i < u; i += 2)
+    ;
+
+  for (p = head; p; p = p->next)
+    ;
+
+  for (j = data; j & s; j = pfn (j + 3))
+    ;
+
+  for (p = head; p != tail; p = p->next)
+    for (j = data + 1; j > s; j = pfn (j + 2))
+      ;
+
+  return 0;
+}
+/* { dg-final { scan-tree-dump-not "if" "cddce1"} } */
+
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/loop-10.c b/gcc/testsuite/gcc.dg/tree-ssa/loop-10.c
index a29c9fb..3d05ad2 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/loop-10.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/loop-10.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-options "-O2 -fdump-tree-optimized -fno-finite-loops" } */
 /* { dg-require-effective-target int32plus } */
 
 int bar (void);
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c b/gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c
index e9b4f26..187c084 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fsplit-paths -fno-tree-cselim -fdump-tree-split-paths-details -w" } */
+/* { dg-options "-O2 -fsplit-paths -fno-tree-cselim -fdump-tree-split-paths-details -w -fno-finite-loops" } */
 
 struct __sFILE
 {
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-12.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-12.c
index d829b04..6752676 100644
--- a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-12.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-12.c
@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-thread2-details -fdump-tree-thread3-details -fdump-tree-thread4-details" } */
+/* { dg-options "-O2 -fdump-tree-thread2-details -fdump-tree-thread3-details -fdump-tree-thread4-details -fno-finite-loops" } */
 /* { dg-final { scan-tree-dump "FSM" "thread2" } } */
 /* { dg-final { scan-tree-dump "FSM" "thread3" } } */
 /* { dg-final { scan-tree-dump "FSM" "thread4" { xfail *-*-* } } } */
diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c
index 2478219..179605e 100644
--- a/gcc/tree-ssa-dce.c
+++ b/gcc/tree-ssa-dce.c
@@ -245,6 +245,17 @@ mark_stmt_if_obviously_necessary (gimple *stmt, bool aggressive)
 	    mark_stmt_necessary (stmt, true);
 	    return;
 	  }
+        /* IFN_GOACC_LOOP calls are necessary in that they are used to
+           represent parameter (i.e. step, bound) of a lowered OpenACC
+           partitioned loop. But this kind of partitioned loop might not
+           survive from aggressive loop removal for it has loop exit and
+           is assumed to be finite. Therefore, we need to explicitly mark
+           these calls. (An example is libgomp.oacc-c-c++-common/pr84955.c) */
+        if (gimple_call_internal_p (stmt, IFN_GOACC_LOOP))
+          {
+            mark_stmt_necessary (stmt, true);
+            return;
+          }
 	if (!gimple_call_lhs (stmt))
 	  return;
 	break;
diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
index 470b6a2..25c2a59 100644
--- a/gcc/tree-ssa-loop-niter.c
+++ b/gcc/tree-ssa-loop-niter.c
@@ -2798,6 +2798,27 @@ finite_loop_p (struct loop *loop)
 		 loop->num);
       return true;
     }
+
+  if (flag_finite_loops)
+    {
+      unsigned i;
+      vec<edge> exits = get_loop_exit_edges (loop);
+      edge ex;
+
+      /* If the loop has a normal exit, we can assume it will terminate. */
+      FOR_EACH_VEC_ELT (exits, i, ex)
+	if (!(ex->flags & (EDGE_EH | EDGE_ABNORMAL | EDGE_FAKE)))
+	  {
+	    exits.release ();
+	    if (dump_file)
+	      fprintf (dump_file, "Assume loop %i to be finite: it has an exit "
+		       "and -ffinite-loop is on.\n", loop->num);
+	    return true;
+          }
+
+      exits.release ();
+    }
+
   return false;
 }
 
diff --git a/libgomp/testsuite/libgomp.oacc-c-c++-common/pr84955-1.c b/libgomp/testsuite/libgomp.oacc-c-c++-common/pr84955-1.c
new file mode 100644
index 0000000..44767cd
--- /dev/null
+++ b/libgomp/testsuite/libgomp.oacc-c-c++-common/pr84955-1.c
@@ -0,0 +1,31 @@
+/* { dg-do compile }  */
+/* { dg-options "-O2 -fdump-tree-cddce2 -ffinite-loops" } */
+
+int
+f1 (void)
+{
+  int i, j;
+
+#pragma acc parallel loop tile(2,3)
+  for (i = 1; i < 10; i++)
+    for (j = 1; j < 10; j++)
+      for (;;)
+	;
+
+  return i + j;
+}
+
+int
+f2 (void)
+{
+  int i, j, k;
+
+#pragma acc parallel loop tile(2,3)
+  for (i = 1; i < 10; i++)
+    for (j = 1; j < 10; j++)
+      for (k = 1; k < 10; k++)
+	;
+
+  return i + j;
+}
+/* { dg-final { scan-tree-dump-not "if" "cddce2"} } */