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.

gcc/ChangeLog

+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.
+
 2020-05-05  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/94942

gcc/combine.c

@@ -1485,6 +1485,7 @@ 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))

gcc/testsuite/ChangeLog

+2020-05-06  Jakub Jelinek  <jakub@redhat.com>
+
+	PR rtl-optimization/94873
+	* gcc.dg/pr94873.c: New test.
+
 2020-05-05  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/94942

gcc/testsuite/gcc.dg/pr94873.c

+/* 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;
+}