vect: Add missing skip-vector check for peeling with versioning [PR121020]

This fixes a miscompilation issue introduced by the enablement of
combined loop peeling and versioning. A test case that reproduces the
issue is included in the patch.

When performing loop peeling, GCC usually inserts a skip-vector check.
This ensures that after peeling, there are enough remaining iterations
to enter the main vectorized loop. Previously, the check was omitted if
loop versioning for alignment was applied. It was safe before because
versioning and peeling for alignment were mutually exclusive.

However, with combined peeling and versioning enabled, this is not safe
any more. A loop may be peeled and versioned at the same time. Without
the skip-vector check, the main vectorized loop can be entered even if
its iteration count is zero. This can cause the loop running many more
iterations than needed, resulting in incorrect results.

To fix this, the patch updates the condition of omitting the skip-vector
check to when versioning is performed alone without peeling.

gcc/ChangeLog:

	PR tree-optimization/121020
	* tree-vect-loop-manip.cc (vect_do_peeling): Update the
	condition of omitting the skip-vector check.
	* tree-vectorizer.h (LOOP_VINFO_USE_VERSIONING_WITHOUT_PEELING):
	Add a helper macro.

gcc/testsuite/ChangeLog:

	PR tree-optimization/121020
	* gcc.dg/vect/vect-early-break_138-pr121020.c: New test.

diff --git a/gcc/testsuite/gcc.dg/vect/vect-early-break_138-pr121020.c b/gcc/testsuite/gcc.dg/vect/vect-early-break_138-pr121020.c
new file mode 100644 (file)
index 0000000..8cb62bf
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/vect/vect-early-break_138-pr121020.c
@@ -0,0 +1,54 @@
+/* PR tree-optimization/121020 */
+/* { dg-options "-O3 --vect-cost-model=unlimited" } */
+/* { dg-additional-options "-march=znver2" { target x86_64-*-* i?86-*-* } } */
+/* { dg-require-effective-target mmap } */
+/* { dg-require-effective-target vect_early_break } */
+
+#include <stdint.h>
+#include <stdio.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include "tree-vect.h"
+
+__attribute__((noipa))
+bool equal (uint64_t *restrict p, uint64_t *restrict q, int length)
+{
+  for (int i = 0; i < length; i++) {
+    if (*(p + i) != *(q + i))
+      return false;
+  }
+  return true;
+}
+
+int main ()
+{
+  check_vect ();
+
+  long pgsz = sysconf (_SC_PAGESIZE);
+  if (pgsz == -1) {
+    fprintf (stderr, "sysconf failed\n");
+    return 0;
+  }
+
+  /* Allocate a whole page of memory.  */
+  void *mem = mmap (NULL, pgsz, PROT_READ | PROT_WRITE,
+                   MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  if (mem == MAP_FAILED) {
+    fprintf (stderr, "mmap failed\n");
+    return 0;
+  }
+  uint64_t *p1 = (uint64_t *) mem;
+  uint64_t *p2 = (uint64_t *) mem + 32;
+
+  /* The first 16 elements pointed to by p1 and p2 are the same.  */
+  for (int i = 0; i < 32; i++) {
+    *(p1 + i) = 0;
+    *(p2 + i) = (i < 16 ? 0 : -1);
+  }
+
+  /* All calls to equal should return true.  */
+  for (int len = 0; len < 16; len++) {
+    if (!equal (p1 + 1, p2 + 1, len))
+      __builtin_abort();
+  }
+}

diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 2d01a4b0ed1c8431510ad5e6e2175dbe89371618..7fcbc1ad2eb8dda6012640ddbe9c82478d7dd6a6 100644 (file)
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -3295,7 +3295,7 @@ vect_do_peeling (loop_vec_info loop_vinfo, tree niters, tree nitersm1,
   bool skip_vector = (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
                      ? maybe_lt (LOOP_VINFO_INT_NITERS (loop_vinfo),
                                  bound_prolog + bound_epilog)
-                     : (!LOOP_REQUIRES_VERSIONING_FOR_ALIGNMENT (loop_vinfo)
+                     : (!LOOP_VINFO_USE_VERSIONING_WITHOUT_PEELING (loop_vinfo)
                         || vect_epilogues));
 
   /* Epilog loop must be executed if the number of iterations for epilog

diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
index 203e5ad964a688f655fecf5a2b6d36c76e62f891..e190027943240ac7b04a2a1816fc2f490e7e7ef1 100644 (file)
--- a/gcc/tree-vectorizer.h
+++ b/gcc/tree-vectorizer.h
@@ -1197,6 +1197,10 @@ public:
    || LOOP_REQUIRES_VERSIONING_FOR_NITERS (L)          \
    || LOOP_REQUIRES_VERSIONING_FOR_SIMD_IF_COND (L))
 
+#define LOOP_VINFO_USE_VERSIONING_WITHOUT_PEELING(L)   \
+  ((L)->may_misalign_stmts.length () > 0               \
+   && !LOOP_VINFO_ALLOW_MUTUAL_ALIGNMENT (L))
+
 #define LOOP_VINFO_NITERS_KNOWN_P(L)          \
   (tree_fits_shwi_p ((L)->num_iters) && tree_to_shwi ((L)->num_iters) > 0)