[gcc/devel/c++-modules] tree-optimization/94103 avoid CSE of loads with padding

[gcc/devel/c++-modules] tree-optimization/94103 avoid CSE of loads with padding

[Nathan Sidwell]

https://gcc.gnu.org/g:1dc00a8ec9aeba86b74b16bff6f171824bb7b4a1

commit 1dc00a8ec9aeba86b74b16bff6f171824bb7b4a1
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Mar 12 14:18:35 2020 +0100

    tree-optimization/94103 avoid CSE of loads with padding
    
    VN currently replaces a load of a 16 byte entity 128 bits of precision
    (TImode) with the result of a load of a 16 byte entity with 80 bits of
    mode precision (XFmode).  That will go downhill since if the padding
    bits are not actually filled with memory contents those bits are
    missing.
    
    2020-03-12  Richard Biener  <rguenther@suse.de>
    
            PR tree-optimization/94103
            * tree-ssa-sccvn.c (visit_reference_op_load): Avoid type
            punning when the mode precision is not sufficient.
    
            * gcc.target/i386/pr94103.c: New testcase.

Diff:
---
 gcc/ChangeLog                           |  6 ++++++
 gcc/testsuite/ChangeLog                 |  5 +++++
 gcc/testsuite/gcc.target/i386/pr94103.c | 17 +++++++++++++++++
 gcc/tree-ssa-sccvn.c                    | 23 ++++++++++++++++-------
 4 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 22ba5ed36e6..a43b453178d 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,9 @@
+2020-03-12  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/94103
+	* tree-ssa-sccvn.c (visit_reference_op_load): Avoid type
+	punning when the mode precision is not sufficient.
+
 2020-03-12  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR target/89229
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index a1f0e3a18c5..a5730ebbbc0 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-03-12  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/94103
+	* gcc.target/i386/pr94103.c: New testcase.
+
 2020-03-12  Tobias Burnus  <tobias@codesourcery.com>
 
 	PR middle-end/94120
diff --git a/gcc/testsuite/gcc.target/i386/pr94103.c b/gcc/testsuite/gcc.target/i386/pr94103.c
new file mode 100644
index 00000000000..91b5fc64b5e
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr94103.c
@@ -0,0 +1,17 @@
+/* { dg-do run { target lp64 } } */
+/* { dg-options "-O3" } */
+
+int main()
+{
+  long double x;
+  unsigned long u[2] = {0xEEEEEEEEEEEEEEEEUL, 0xEEEEEEEEEEEEEEEEUL};
+  __builtin_memcpy(&x, &u, sizeof x);
+  __builtin_memcpy(&u, &x, sizeof u);
+  ++*(unsigned char *)&x;
+  (void)-x;
+  __builtin_memcpy(&u, &x, sizeof u);
+  if (u[1] != 0xEEEEEEEEEEEEEEEEUL
+      || u[0] != 0xEEEEEEEEEEEEEEEFUL)
+    __builtin_abort ();
+  return 0;
+}
diff --git a/gcc/tree-ssa-sccvn.c b/gcc/tree-ssa-sccvn.c
index b7174cd603f..150ddad3e69 100644
--- a/gcc/tree-ssa-sccvn.c
+++ b/gcc/tree-ssa-sccvn.c
@@ -4899,13 +4899,22 @@ visit_reference_op_load (tree lhs, tree op, gimple *stmt)
   if (result
       && !useless_type_conversion_p (TREE_TYPE (result), TREE_TYPE (op)))
     {
-      /* We will be setting the value number of lhs to the value number
-	 of VIEW_CONVERT_EXPR <TREE_TYPE (result)> (result).
-	 So first simplify and lookup this expression to see if it
-	 is already available.  */
-      gimple_match_op res_op (gimple_match_cond::UNCOND,
-			      VIEW_CONVERT_EXPR, TREE_TYPE (op), result);
-      result = vn_nary_build_or_lookup (&res_op);
+      /* Avoid the type punning in case the result mode has padding where
+	 the op we lookup has not.  */
+      if (maybe_lt (GET_MODE_PRECISION (TYPE_MODE (TREE_TYPE (result))),
+		    GET_MODE_PRECISION (TYPE_MODE (TREE_TYPE (op)))))
+	result = NULL_TREE;
+      else
+	{
+	  /* We will be setting the value number of lhs to the value number
+	     of VIEW_CONVERT_EXPR <TREE_TYPE (result)> (result).
+	     So first simplify and lookup this expression to see if it
+	     is already available.  */
+	  gimple_match_op res_op (gimple_match_cond::UNCOND,
+				  VIEW_CONVERT_EXPR, TREE_TYPE (op), result);
+	  result = vn_nary_build_or_lookup (&res_op);
+	}
+
       /* When building the conversion fails avoid inserting the reference
          again.  */
       if (!result)