Hi!
The following testcase is miscompiled on aarch64-linux at -O3 since the
introduction of WIDEN_MINUS_EXPR.
The problem is if the inner type (half_type) is unsigned and the result
type in which the subtraction is performed (type) has precision more than
twice as larger as the inner type's precision.
For other widening operations like WIDEN_{PLUS,MULT}_EXPR, if half_type
is unsigned, the addition/multiplication result in itype is also unsigned
and needs to be zero-extended to type.
But subtraction is special, even when half_type is unsigned, the subtraction
behaves as signed (also regardless of whether the result type is signed or
unsigned), 0xfeU - 0xffU is -1 or 0xffffffffU, not 0x0000ffff.
I think it is better not to use mixed signedness of types in
WIDEN_MINUS_EXPR (have unsigned vector of operands and signed result
vector), so this patch instead adds another cast to make sure we always
sign-extend the result from itype to type if type is wider than itype.
Bootstrapped/regtested on aarch64-linux, x86_64-linux and i686-linux, ok
for trunk/11.3?
2021-08-31 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/102124
* tree-vect-patterns.c (vect_recog_widen_op_pattern): For ORIG_CODE
MINUS_EXPR, if itype is unsigned with smaller precision than type,
add an extra cast to signed variant of itype to ensure sign-extension.
* gcc.dg/torture/pr102124.c: New test.
--- gcc/tree-vect-patterns.c.jj 2021-08-17 21:05:07.000000000 +0200
+++ gcc/tree-vect-patterns.c 2021-08-30 11:54:03.651474845 +0200
@@ -1268,11 +1268,31 @@ vect_recog_widen_op_pattern (vec_info *v
/* Check target support */
tree vectype = get_vectype_for_scalar_type (vinfo, half_type);
tree vecitype = get_vectype_for_scalar_type (vinfo, itype);
+ tree ctype = itype;
+ tree vecctype = vecitype;
+ if (orig_code == MINUS_EXPR
+ && TYPE_UNSIGNED (itype)
+ && TYPE_PRECISION (type) > TYPE_PRECISION (itype))
+ {
+ /* Subtraction is special, even if half_type is unsigned and no matter
+ whether type is signed or unsigned, if type is wider than itype,
+ we need to sign-extend from the widening operation result to the
+ result type.
+ Consider half_type unsigned char, operand 1 0xfe, operand 2 0xff,
+ itype unsigned short and type either int or unsigned int.
+ Widened (unsigned short) 0xfe - (unsigned short) 0xff is
+ (unsigned short) 0xffff, but for type int we want the result -1
+ and for type unsigned int 0xffffffff rather than 0xffff. */
+ ctype = build_nonstandard_integer_type (TYPE_PRECISION (itype), 0);
+ vecctype = get_vectype_for_scalar_type (vinfo, ctype);
+ }
+
enum tree_code dummy_code;
int dummy_int;
auto_vec<tree> dummy_vec;
if (!vectype
|| !vecitype
+ || !vecctype
|| !supportable_widening_operation (vinfo, wide_code, last_stmt_info,
vecitype, vectype,
&dummy_code, &dummy_code,
@@ -1291,8 +1311,12 @@ vect_recog_widen_op_pattern (vec_info *v
gimple *pattern_stmt = gimple_build_assign (var, wide_code,
oprnd[0], oprnd[1]);
+ if (vecctype != vecitype)
+ pattern_stmt = vect_convert_output (vinfo, last_stmt_info, ctype,
+ pattern_stmt, vecitype);
+
return vect_convert_output (vinfo, last_stmt_info,
- type, pattern_stmt, vecitype);
+ type, pattern_stmt, vecctype);
}
/* Try to detect multiplication on widened inputs, converting MULT_EXPR
--- gcc/testsuite/gcc.dg/torture/pr102124.c.jj 2021-08-30 12:08:05.838649133
+0200
+++ gcc/testsuite/gcc.dg/torture/pr102124.c 2021-08-30 12:07:52.669834031
+0200
@@ -0,0 +1,27 @@
+/* PR tree-optimization/102124 */
+
+int
+foo (const unsigned char *a, const unsigned char *b, unsigned long len)
+{
+ int ab, ba;
+ unsigned long i;
+ for (i = 0, ab = 0, ba = 0; i < len; i++)
+ {
+ ab |= a[i] - b[i];
+ ba |= b[i] - a[i];
+ }
+ return (ab | ba) >= 0;
+}
+
+int
+main ()
+{
+ unsigned char a[32] = { 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', 'a', 'a', 'a', 'a' };
+ unsigned char b[32] = { 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a', 'a',
'a', 'a', 'a', 'a', 'a', 'a' };
+ unsigned char c[32] = { 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b', 'b',
'b', 'b', 'b', 'b', 'b', 'b' };
+ if (!foo (a, b, 16))
+ __builtin_abort ();
+ if (foo (a, c, 16))
+ __builtin_abort ();
+ return 0;
+}
Jakub
