[gcc r13-6690] builtins: Move the character difference into result instead of reassigning result [PR109086]

[Xi Ruoyao <xry111@gcc.gnu.org>]

https://gcc.gnu.org/g:45641f3a99281bb0a429649741a29c2aace4c63e

commit r13-6690-g45641f3a99281bb0a429649741a29c2aace4c63e
Author: Xi Ruoyao <xry111@xry111.site>
Date:   Wed Mar 15 15:34:52 2023 +0800

    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.

Diff:
---
 gcc/builtins.cc | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/gcc/builtins.cc b/gcc/builtins.cc
index 305c65c29be..90246e214d6 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);