builtins: Move the character difference into result instead of reassigning result [PR109086]

expand_simple_binop() is allowed to allocate a new pseudo-register and
return it, instead of forcing the result into the provided
pseudo-register.  This can cause a problem when we expand the unrolled
loop for __builtin_strcmp: the compiler always generates code for all n
iterations of the loop, so "result" will be an alias of the
pseudo-register allocated and used in the last iteration; but at runtime
the loop can break early, causing this pseudo-register uninitialized.

Emit a move instruction in the iteration to force the difference into
one register which has been allocated before the loop, to avoid this
issue.

gcc/ChangeLog:

	PR other/109086
	* builtins.cc (inline_string_cmp): Force the character
	difference into "result" pseudo-register, instead of reassign
	the pseudo-register.

1 files changed, 10 insertions, 2 deletions

diff --git a/gcc/builtins.cc b/gcc/builtins.cc
index 305c65c..90246e2 100644
--- a/gcc/builtins.cc
+++ b/gcc/builtins.cc
@@ -7142,8 +7142,16 @@ inline_string_cmp (rtx target, tree var_str, const char *const_str,
 
       op0 = convert_modes (mode, unit_mode, op0, 1);
       op1 = convert_modes (mode, unit_mode, op1, 1);
-      result = expand_simple_binop (mode, MINUS, op0, op1,
-				    result, 1, OPTAB_WIDEN);
+      rtx diff = expand_simple_binop (mode, MINUS, op0, op1,
+				      result, 1, OPTAB_WIDEN);
+
+      /* Force the difference into result register.  We cannot reassign
+	 result here ("result = diff") or we may end up returning
+	 uninitialized result when expand_simple_binop allocates a new
+	 pseudo-register for returning.  */
+      if (diff != result)
+	emit_move_insn (result, diff);
+
       if (i < length - 1)
 	emit_cmp_and_jump_insns (result, CONST0_RTX (mode), NE, NULL_RTX,
 	    			 mode, true, ne_label);