[PATCH] allow nul-over-nul elimination only for single-byte stores (PR 93213)

[Martin Sebor]

The multi-byte store enhancement to the strlen optimization checked
sometime last summer didn't take care to prevent the nul-over-nul
store elimination of multi-byte assignments.  This made it possible
for subsequent multi-byte stores of fewer nuls to cause prior larger
stores to be eliminated.  The latent bug was exposed by a small
unrelated change last December that had masked it until then.

The attached patch makes sure that only single-byte nul stores are
considered as candidates for this elimination.  I will commit it
next week if there are no concerns or suggestions.

Martin

PR tree-optimization/93213 - wrong code with -Og -foptimize-strlen

gcc/testsuite/ChangeLog:

	PR tree-optimization/93213
	* gcc.c-torture/execute/pr93213.c: New test.

gcc/ChangeLog:

	PR tree-optimization/93213
	* tree-ssa-strlen.c (handle_store): Only allow single-byte nul-over-nul
	stores to be eliminated.

Index: gcc/testsuite/gcc.c-torture/execute/pr93213.c
===================================================================
--- gcc/testsuite/gcc.c-torture/execute/pr93213.c	(nonexistent)
+++ gcc/testsuite/gcc.c-torture/execute/pr93213.c	(working copy)
@@ -0,0 +1,45 @@
+/* PR tree-optimization/93213 - wrong code on a multibyte store with
+   -Og -foptimize-strlen
+   { dg-additional-options "-Og -foptimize-strlen" } */
+
+typedef unsigned short u16;
+typedef unsigned int u32;
+typedef unsigned __int128 u128;
+
+static inline u128
+foo (u16 u16_1, u32 u32_1, u128 u128_1)
+{
+  u128 u128_0 = 0;
+  u128_1 -= __builtin_mul_overflow (u32_1, u16_1, &u32_1);
+  __builtin_memmove (&u16_1, &u128_0, 2);
+  __builtin_memmove (&u16_1, &u128_1, 1);
+  return u16_1;
+}
+
+__attribute__ ((noipa)) void
+bar (void)
+{
+  char a[] = { 1, 2 };
+  const char b[] = { 0, 0 };
+  const char c[] = { 2 };
+  __builtin_memcpy (a, b, 2);
+  // The above is transformed into
+  //   MEM <short unsigned int> [(char * {ref-all})&a] = 0;
+  // which was then dropped because of the non-nul store below.
+  __builtin_memcpy (a, c, 1);
+
+  volatile char *p = a;
+  if (p[0] != 2 || p[1] != 0)
+    __builtin_abort ();
+}
+
+int
+main (void)
+{
+  u16 x = foo (-1, -1, 0);
+  if (x != 0xff)
+    __builtin_abort ();
+
+  bar ();
+  return 0;
+}
Index: gcc/tree-ssa-strlen.c
===================================================================
--- gcc/tree-ssa-strlen.c	(revision 280100)
+++ gcc/tree-ssa-strlen.c	(working copy)
@@ -5239,10 +5239,10 @@ handle_store (gimple_stmt_iterator *gsi, bool *zer
 	}
     }
 
-  if (si != NULL && offset == 0 && storing_all_zeros_p)
+  if (si != NULL && offset == 0 && storing_all_zeros_p && lenrange[2] == 1)
     {
-      /* Allow adjust_last_stmt to remove it if the stored '\0'
-	 is immediately overwritten.  */
+      /* For single-byte stores only, allow adjust_last_stmt to remove
+	 the statement if the stored '\0' is immediately overwritten.  */
       laststmt.stmt = stmt;
       laststmt.len = build_int_cst (size_type_node, 1);
       laststmt.stridx = si->idx;