[gcc r9-8895] combine: Don't replace SET_SRC with REG_EQUAL note content if SET_SRC has side-effects [PR94873]

[gcc r9-8895] combine: Don't replace SET_SRC with REG_EQUAL note content if SET_SRC has side-effects [PR94873]

[Jakub Jelinek]

https://gcc.gnu.org/g:0f717ba5975ab42e1176db4cd2384f1862872519

commit r9-8895-g0f717ba5975ab42e1176db4cd2384f1862872519
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed May 6 09:31:19 2020 +0200

    combine: Don't replace SET_SRC with REG_EQUAL note content if SET_SRC has side-effects [PR94873]
    
    There were some discussions about whether REG_EQUAL notes are valid on insns with a single
    set which contains auto-inc-dec side-effects in the SET_SRC and the majority thinks that
    it should be valid.  So, this patch fixes the combiner to punt in that case, because otherwise
    the auto-inc-dec side-effects from the SET_SRC are lost.
    
    2020-05-06  Jakub Jelinek  <jakub@redhat.com>
    
            PR rtl-optimization/94873
            * combine.c (combine_instructions): Don't optimize using REG_EQUAL
            note if SET_SRC (set) has side-effects.
    
            * gcc.dg/pr94873.c: New test.
    
    (cherry picked from commit 8982e39b46b1e4a4b09022ddebd758b77ab73bac)

Diff:
---
 gcc/combine.c                  |  1 +
 gcc/testsuite/gcc.dg/pr94873.c | 27 +++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/gcc/combine.c b/gcc/combine.c
index 48a60eb0206..ab8463bf27b 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -1488,6 +1488,7 @@ combine_instructions (rtx_insn *f, unsigned int nregs)
 	      if ((set = single_set (temp)) != 0
 		  && (note = find_reg_equal_equiv_note (temp)) != 0
 		  && (note = XEXP (note, 0), GET_CODE (note)) != EXPR_LIST
+		  && ! side_effects_p (SET_SRC (set))
 		  /* Avoid using a register that may already been marked
 		     dead by an earlier instruction.  */
 		  && ! unmentioned_reg_p (note, SET_SRC (set))
diff --git a/gcc/testsuite/gcc.dg/pr94873.c b/gcc/testsuite/gcc.dg/pr94873.c
new file mode 100644
index 00000000000..36152176fbf
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr94873.c
@@ -0,0 +1,27 @@
+/* PR rtl-optimization/94873 */
+/* { dg-do run { target int128 } } */
+/* { dg-options "-O -fno-merge-constants -fno-split-wide-types -fno-tree-fre" } */
+
+__attribute__((noipa)) void
+foo (const char *p, int q)
+{
+  if (p[0] != '%' || p[1] != '0' || p[2] != '2' || p[3] != 'x' || p[4] != '\0')
+    __builtin_abort ();
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+  if ((unsigned char) q != 0x95)
+    __builtin_abort ();
+#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+  if ((unsigned char) q != 0)
+    __builtin_abort ();
+#endif
+}
+
+int
+main ()
+{
+  union U { __int128 a; char b[sizeof (__int128)]; };
+  char x = ((union U){ .a = 0xF4409395252B9560ULL}).b[1];
+  for (unsigned i = 0; i < sizeof (x); i++)
+    foo ("%02x", i[(volatile unsigned char *) &x]);
+  return 0;
+}