PR target/110598: Fix rega = 0; rega ^= rega regression in i386.md

This patch fixes the regression PR target/110598 caused by my recent
addition of a peephole2.  The intention of that optimization was to
simplify zeroing a register, followed by an IOR, XOR or PLUS operation
on it into a move, or as described in the comment:
;; Peephole2 rega = 0; rega op= regb into rega = regb.

The issue is that I'd failed to consider the (rare and unusual) case,
where regb is rega, where the transformation leads to the incorrect
"rega = rega", when it should be "rega = 0".  The minimal fix is to
add a !reg_mentioned_p check to the recent peephole2.

In addition to resolving the regression, I've added a second peephole2
to optimize the problematic case above, which contains a false
dependency and is therefore tricky to optimize elsewhere.  This is an
improvement over GCC 13, for example, that generates the redundant:

        xorl    %edx, %edx
        xorq    %rdx, %rdx

2023-07-12  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/110598
	* config/i386/i386.md (peephole2): Check !reg_mentioned_p when
	optimizing rega = 0; rega op= regb for op in [XOR,IOR,PLUS].
	(peephole2): Simplify rega = 0; rega op= rega cases.

gcc/testsuite/ChangeLog
	PR target/110598
	* gcc.target/i386/pr110598.c: New test case.

diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index ef9683491bd54ed888b7aaf3b86c171619f41eae..d4a948df46ee2fbce3c073a26ef5dd2cb40bcd48 100644 (file)
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -12325,9 +12325,21 @@
                   (any_or_plus:SWI (match_dup 0)
                                    (match_operand:SWI 1 "<general_operand>")))
              (clobber (reg:CC FLAGS_REG))])]
-  ""
+  "!reg_mentioned_p (operands[0], operands[1])"
   [(set (match_dup 0) (match_dup 1))])
-               
+
+;; Peephole2 dead instruction in rega = 0; rega op= rega.
+(define_peephole2
+  [(parallel [(set (match_operand:SWI 0 "general_reg_operand")
+                  (const_int 0))
+             (clobber (reg:CC FLAGS_REG))])
+   (parallel [(set (match_dup 0)
+                  (any_or_plus:SWI (match_dup 0) (match_dup 0)))
+             (clobber (reg:CC FLAGS_REG))])]
+  ""
+  [(parallel [(set (match_dup 0) (const_int 0))
+             (clobber (reg:CC FLAGS_REG))])])
+
 ;; Split DST = (HI<<32)|LO early to minimize register usage.
 (define_insn_and_split "*concat<mode><dwi>3_1"
   [(set (match_operand:<DWI> 0 "nonimmediate_operand" "=ro,r")

diff --git a/gcc/testsuite/gcc.target/i386/pr110598.c b/gcc/testsuite/gcc.target/i386/pr110598.c
new file mode 100644 (file)
index 0000000..1c88031
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr110598.c
@@ -0,0 +1,46 @@
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+typedef unsigned long long u64;
+
+#define MAX_SUBTARGET_WORDS 4
+
+int notequal(const void *a, const void *b)
+{
+  return __builtin_memcmp(a,b,MAX_SUBTARGET_WORDS*sizeof(u64)) != 0;
+}
+typedef struct FeatureBitset {
+  u64 Bits[MAX_SUBTARGET_WORDS];
+}FeatureBitset;
+
+__attribute__((noipa))
+_Bool is_eq_buggy (const FeatureBitset * lf, const FeatureBitset * rf) {
+  u64 Bits_l[MAX_SUBTARGET_WORDS];
+  Bits_l[0] = lf->Bits[0]&1;
+  Bits_l[1] = 0;
+  Bits_l[2] = 0;
+  Bits_l[3] = 0;
+  u64 Bits_r[MAX_SUBTARGET_WORDS];
+  Bits_r[0] = rf->Bits[0]&1;
+  Bits_r[1] = 0;
+  Bits_r[2] = 0;
+  Bits_r[3] = 0;
+  return !notequal(Bits_l, Bits_r);
+}
+
+__attribute__((noipa))
+void bug(void) {
+  FeatureBitset lf, rf;
+  lf.Bits[0] = rf.Bits[0] = 1;
+  lf.Bits[1] = rf.Bits[1] = 1;
+  lf.Bits[2] = rf.Bits[2] = 1;
+  lf.Bits[3] = rf.Bits[3] = 1;
+
+  _Bool r = is_eq_buggy (&lf, &rf);
+  if (!r) __builtin_trap();
+}
+
+__attribute__((noipa))
+int main(void) {
+  bug();
+}