From: Kyrylo Tkachov <ktkachov@nvidia.com>
Date: Thu, 30 Jul 2026 17:36:40 +0000 (-0700)
Subject: aarch64: Add missing CC clobber to max/min-of-add/sub patterns [PR116815]
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=c62176c0ea0d8139161ebb82fabfa0a02e493e2e;p=thirdparty%2Fgcc.git

aarch64: Add missing CC clobber to max/min-of-add/sub patterns [PR116815]

*aarch64_plus_within_<optab><mode>3_<ovf_commutate> and
*aarch64_minus_within_<optab><mode>3 split into a flag-setting ADDS or SUBS
followed by a CSEL, but their insn patterns do not say that they write the
condition codes.

For

  unsigned f (unsigned a, unsigned b, unsigned c, unsigned d)
  {
    unsigned s = a + b;
    unsigned m = s > a ? s : a;
    return (c < d && a < b) ? m : d;
  }

combine produces

  (parallel [(set (reg:SI 0 x0)
                  (umax:SI (plus:SI (reg:SI 107) (reg:SI 108))
                           (reg:SI 107)))
             (clobber (scratch:SI))])

which claims to leave the flags alone.  The compare feeding the enclosing
CCMP chain is therefore treated as still live and is removed, and split1
then emits an ADDS that overwrites the flags the outer CSEL reads:

  adds  w1, w0, w1
  csel  w1, w1, w0, cc
  csel  w0, w1, w3, cc

so f (5, 7, 9, 2) returns 12 rather than 2.

Add the (clobber (reg:CC CC_REGNUM)) that the neighbouring
*aarch64_minmax_plus pattern already carries.  The comparison is then kept:

  cmp   w2, w3
  ccmp  w0, w1, 2, cc
  bcs   .L2
  adds  w1, w0, w1
  csel  w3, w1, w0, cc

Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	PR middle-end/116815
	* config/aarch64/aarch64.md
	(*aarch64_plus_within_<optab><mode>3_<ovf_commutate>): Add a
	clobber of CC_REGNUM.
	(*aarch64_minus_within_<optab><mode>3): Likewise.

gcc/testsuite/ChangeLog:

	PR middle-end/116815
	* gcc.target/aarch64/pr116815-4.c: New test.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>
---

diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
index b2185c63819..59af2bd2088 100644
--- a/gcc/config/aarch64/aarch64.md
+++ b/gcc/config/aarch64/aarch64.md
@@ -4554,7 +4554,8 @@
 	  (plus:GPI (match_operand:GPI 1 "register_operand" "r")
 		    (match_operand:GPI 2 "register_operand" "r"))
 	  (match_dup ovf_commutate)))
-   (clobber (match_scratch:GPI 3 "=r"))]
+   (clobber (match_scratch:GPI 3 "=r"))
+   (clobber (reg:CC CC_REGNUM))]
   "!TARGET_CSSC"
   "#"
   "&& 1"
@@ -4584,7 +4585,8 @@
 	  (minus:GPI (match_operand:GPI 1 "register_operand" "r")
 		     (match_operand:GPI 2 "register_operand" "r"))
 	  (match_dup 1)))
-   (clobber (match_scratch:GPI 3 "=r"))]
+   (clobber (match_scratch:GPI 3 "=r"))
+   (clobber (reg:CC CC_REGNUM))]
   "!TARGET_CSSC"
   "#"
   "&& 1"
diff --git a/gcc/testsuite/gcc.target/aarch64/pr116815-4.c b/gcc/testsuite/gcc.target/aarch64/pr116815-4.c
new file mode 100644
index 00000000000..6c640f58542
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/pr116815-4.c
@@ -0,0 +1,94 @@
+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+/* PR middle-end/116815 */
+
+/* The max/min-of-add/sub patterns split into a flag-setting ADDS/SUBS
+   followed by a CSEL, so they have to declare that they clobber the
+   condition codes.  Without that clobber the compare feeding an enclosing
+   CCMP chain is treated as still live across the insn and gets deleted,
+   so the wrong value is selected.  */
+
+#pragma GCC target "+nocssc"
+
+__attribute__ ((noipa)) unsigned
+umax_plus (unsigned a, unsigned b, unsigned c, unsigned d)
+{
+  unsigned s = a + b;
+  unsigned m = s > a ? s : a;
+  return (c < d && a < b) ? m : d;
+}
+
+__attribute__ ((noipa)) unsigned
+umin_plus (unsigned a, unsigned b, unsigned c, unsigned d)
+{
+  unsigned s = a + b;
+  unsigned m = s < a ? s : a;
+  return (c < d && a < b) ? m : d;
+}
+
+__attribute__ ((noipa)) unsigned
+umax_minus (unsigned a, unsigned b, unsigned c, unsigned d)
+{
+  unsigned s = a - b;
+  unsigned m = s > a ? s : a;
+  return (c < d && a < b) ? m : d;
+}
+
+__attribute__ ((noipa)) unsigned
+umin_minus (unsigned a, unsigned b, unsigned c, unsigned d)
+{
+  unsigned s = a - b;
+  unsigned m = s < a ? s : a;
+  return (c < d && a < b) ? m : d;
+}
+
+__attribute__ ((noipa)) unsigned long long
+umax_plus_di (unsigned long long a, unsigned long long b,
+	      unsigned long long c, unsigned long long d)
+{
+  unsigned long long s = a + b;
+  unsigned long long m = s > a ? s : a;
+  return (c < d && a < b) ? m : d;
+}
+
+__attribute__ ((noipa)) unsigned long long
+umin_minus_di (unsigned long long a, unsigned long long b,
+	       unsigned long long c, unsigned long long d)
+{
+  unsigned long long s = a - b;
+  unsigned long long m = s < a ? s : a;
+  return (c < d && a < b) ? m : d;
+}
+
+int
+main (void)
+{
+  /* c < d is false, so every call must return d.  */
+  if (umax_plus (5, 7, 9, 2) != 2)
+    __builtin_abort ();
+  if (umin_plus (5, 7, 9, 2) != 2)
+    __builtin_abort ();
+  if (umax_minus (5, 7, 9, 2) != 2)
+    __builtin_abort ();
+  if (umin_minus (5, 7, 9, 2) != 2)
+    __builtin_abort ();
+  if (umax_plus_di (5, 7, 9, 2) != 2)
+    __builtin_abort ();
+  if (umin_minus_di (5, 7, 9, 2) != 2)
+    __builtin_abort ();
+
+  /* a < b is false, so these must return d too.  */
+  if (umax_plus (7, 5, 1, 2) != 2)
+    __builtin_abort ();
+  if (umax_minus (7, 5, 1, 2) != 2)
+    __builtin_abort ();
+
+  /* Both true: the max/min result is selected.  */
+  if (umax_plus (5, 7, 1, 2) != 12)
+    __builtin_abort ();
+  if (umin_plus (5, 7, 1, 2) != 5)
+    __builtin_abort ();
+
+  return 0;
+}