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

[Jakub Jelinek via Gcc-patches]

Hi!

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.

Bootstrapped/regtested on {x86_64,i686,aarch64,powerpc64{,le}}-linux,
preapproved in the PR by Segher, committed to trunk so far.

2020-05-06  Jakub Jelinek  <ja...@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.

--- gcc/combine.c.jj    2020-04-24 14:40:21.087840193 +0200
+++ gcc/combine.c       2020-05-05 12:15:36.793466471 +0200
@@ -1485,6 +1485,7 @@ combine_instructions (rtx_insn *f, unsig
              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))
--- gcc/testsuite/gcc.dg/pr94873.c.jj   2020-05-05 12:18:35.056778567 +0200
+++ gcc/testsuite/gcc.dg/pr94873.c      2020-05-05 12:14:27.437512245 +0200
@@ -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;
+}


        Jakub