[PR86438] compare-elim: cope with set of in_b

When in_a resolves to a register set in the prev_clobber insn, we may
use the SET_SRC for the compare instead.  However, when in_b so
resolves, we proceed to use the reg with its earlier value.  When both
resolve to the same register and prev_clobber is an insn that modifies
the register, this arrangement may cause the compare to match (when it
shouldn't) and the elimination of the compare to incorrectly succeed.

(set (reg 1) (plus (reg 1) (const_int N)))
(set (reg 2) (reg 1))
(set (reg flags) (compare (reg 1) (reg 2)))

in_a: (reg 1)            --> (plus (reg 1) (const_int N))
in_b: (reg 2) -> (reg 1) -/> oops

(parallel [
 (set (reg flags) (compare (plus (reg 1) (const_int N))
                           (reg 1))) ;; should be (plus...)
 (set (reg 1) (plus (reg 1) (const_int N)))])
(set (reg 2) (reg 1))

This patch arranges for in_b to also undergo SET_SRC substitution
when appropriate, with a shortcut for when in_a and in_b are the same
rtx.

for  gcc/ChangeLog

	PR rtl-optimization/86438
	* compare-elim.c (try_eliminate_compare): Use SET_SRC instead
	of in_b for the compare if in_b is SET_DEST.

for  gcc/testsuite/ChangeLog

	PR rtl-optimization/86438
	* gcc.dg/torture/pr86438.c: New.

From-SVN: r265957

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 21d9ae799a655caa7c4f98c078dc9ffe88f2976e..46ab4e955b9d1c84762bbbcbbc086ceeef535b06 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,9 @@
 2018-11-09  Alexandre Oliva <aoliva@redhat.com>
 
+       PR rtl-optimization/86438
+       * compare-elim.c (try_eliminate_compare): Use SET_SRC instead
+       of in_b for the compare if in_b is SET_DEST.
+
        PR target/87793
        * config/i386/i386.c (ix86_const_not_ok_for_debug_p): Reject
        non-toplevel UNSPEC.

diff --git a/gcc/compare-elim.c b/gcc/compare-elim.c
index 50bbaa84b6dfbbf30e1ba654592e2139f5a249cf..8afbe76c502bdd0f3cd87c7191425f6229e6ba50 100644 (file)
--- a/gcc/compare-elim.c
+++ b/gcc/compare-elim.c
@@ -734,7 +734,7 @@ try_merge_compare (struct comparison *cmp)
 static bool
 try_eliminate_compare (struct comparison *cmp)
 {
-  rtx flags, in_a, in_b, cmp_src;
+  rtx flags, in_a, in_b, cmp_a, cmp_b;
 
   if (try_merge_compare (cmp))
     return true;
@@ -786,7 +786,7 @@ try_eliminate_compare (struct comparison *cmp)
 
   rtx x = XVECEXP (PATTERN (insn), 0, 0);
   if (rtx_equal_p (SET_DEST (x), in_a))
-    cmp_src = SET_SRC (x);
+    cmp_a = SET_SRC (x);
 
   /* Also check operations with implicit extensions, e.g.:
      [(set (reg:DI)
@@ -800,7 +800,7 @@ try_eliminate_compare (struct comparison *cmp)
           && (GET_CODE (SET_SRC (x)) == ZERO_EXTEND
               || GET_CODE (SET_SRC (x)) == SIGN_EXTEND)
           && GET_MODE (XEXP (SET_SRC (x), 0)) == GET_MODE (in_a))
-    cmp_src = XEXP (SET_SRC (x), 0);
+    cmp_a = XEXP (SET_SRC (x), 0);
 
   /* Also check fully redundant comparisons, e.g.:
      [(set (reg:SI)
@@ -811,7 +811,7 @@ try_eliminate_compare (struct comparison *cmp)
           && GET_CODE (SET_SRC (x)) == MINUS
           && rtx_equal_p (XEXP (SET_SRC (x), 0), in_a)
           && rtx_equal_p (XEXP (SET_SRC (x), 1), in_b))
-    cmp_src = in_a;
+    cmp_a = in_a;
 
   else
     return false;
@@ -819,17 +819,26 @@ try_eliminate_compare (struct comparison *cmp)
   /* If the source uses addressing modes with side effects, we can't
      do the merge because we'd end up with a PARALLEL that has two
      instances of that side effect in it.  */
-  if (side_effects_p (cmp_src))
+  if (side_effects_p (cmp_a))
+    return false;
+
+  if (in_a == in_b)
+    cmp_b = cmp_a;
+  else if (rtx_equal_p (SET_DEST (x), in_b))
+    cmp_b = SET_SRC (x);
+  else
+    cmp_b = in_b;
+  if (side_effects_p (cmp_b))
     return false;
 
   /* Determine if we ought to use a different CC_MODE here.  */
-  flags = maybe_select_cc_mode (cmp, cmp_src, in_b);
+  flags = maybe_select_cc_mode (cmp, cmp_a, cmp_b);
   if (flags == NULL)
     flags = gen_rtx_REG (cmp->orig_mode, targetm.flags_regnum);
 
   /* Generate a new comparison for installation in the setter.  */
-  rtx y = copy_rtx (cmp_src);
-  y = gen_rtx_COMPARE (GET_MODE (flags), y, in_b);
+  rtx y = copy_rtx (cmp_a);
+  y = gen_rtx_COMPARE (GET_MODE (flags), y, copy_rtx (cmp_b));
   y = gen_rtx_SET (flags, y);
 
   /* Canonicalize instruction to:

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 9a7dfc26dfa3b65c99a03aaa7248fecae8950eae..6434be07cfa7a4135d5cc616fbdc9630d9303ae6 100644 (file)
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,8 @@
 2018-11-09  Alexandre Oliva <aoliva@redhat.com>
 
+       PR rtl-optimization/86438
+       * gcc.dg/torture/pr86438.c: New.
+
        PR target/87793
        * gcc.dg/pr87793.c: New.
 

diff --git a/gcc/testsuite/gcc.dg/torture/pr86438.c b/gcc/testsuite/gcc.dg/torture/pr86438.c
new file mode 100644 (file)
index 0000000..3e95515
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr86438.c
@@ -0,0 +1,29 @@
+/* { dg-do run } */
+
+typedef unsigned int u32;
+#if __SIZEOF_INT128__
+typedef unsigned long long u64;
+typedef unsigned __int128 u128;
+#else
+typedef unsigned long u64;
+typedef unsigned long long u128;
+#endif
+
+u128 g;
+
+static __attribute__ ((noinline, noclone))
+void check (u64 a, u64 b)
+{
+  if (a != 0 || b != 4)
+    __builtin_abort ();
+}
+
+int
+main (void)
+{
+  u64 d = (g ? 5 : 4);
+  u32 f = __builtin_sub_overflow_p (d, (u128) d, (u64) 0);
+  u128 x = g + f + d;
+  check (x >> (sizeof (u64) * __CHAR_BIT__), x);
+  return 0;
+}