On Sat, Aug 02, 2025 at 05:14:23PM +0800, Yang Yujie wrote:
> A widening cast from a signed _BitInt operand to an unsigned _BitInt
> type involves filling the extra limb(s) with sign extension.
> On a target that wants _BitInts extended in memory, if this unsigned
> type has a partial limb, the unused part of it should be zeroed.
>
> e.g. Assuming limb_mode == E_DImode, at the end of
>
> void
> test (unsigned _BitInt(519) *t, _BitInt(512) x)
> {
> *t = -x;
> }
>
> the most significant limb of *t should be masked with 0x7f.
>
> This patch also fixes gcc.dg/torture/bitint-16.c, which aborts at -O2
> when the extension on load is optimized away.
>
> gcc/ChangeLog:
>
> * gimple-lower-bitint.cc (bitint_large_huge::lower_mergeable_stmt):
> zero-extend the partial limb of any unsigned _BitInt LHS assigned
ChangeLog entry should start with capital letter, so Zero-extend
> new file mode 100644
> index 00000000000..b106dd36ef1
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/torture/bitint-83.c
> @@ -0,0 +1,52 @@
> +/* Derived from a test in gcc.dg/torture/bitint-16.c */
> +/* { dg-do run { target bitint } } */
> +/* { dg-options "-std=c23 -pedantic-errors" } */
> +/* { dg-skip-if "" { ! run_expensive_tests } { "*" } { "-O0" "-O2" } } */
> +/* { dg-skip-if "" { ! run_expensive_tests } { "-flto" } { "" } } */
> +
> +#include "../bitintext.h"
> +
> +#define BASIC_TESTS \
> + TEST(8) \
> + TEST(16) \
> + TEST(32) \
The above 3 are certainly ok always, C23 requires that BITINT_MAXWIDTH
is at least ULLONG_WIDTH and ULLONG_WIDTH has minimum 64.
> + TEST(64) \
> + TEST(128)
The above aren't, 64 + 7 is more than 64 and 128 + 7 even more.
> +
> +#if __BITINT_MAXWIDTH__ >= 512
You're using _BitInt(N + 7), so this should
be #if __BITINT_MAXWIDTH__ >= 519
> +#define __ALL_TESTS__ \
Why __ around the macro name? Just use ALL_TESTS
> + BASIC_TESTS \
> + TEST(256) \
> + TEST(512)
> +#elif __BITINT_MAXWIDTH__ >= 256
I wouldn't bother with >= 256 (256 + 7) case, perhaps there will be targets
with only <= 64 or <= 128 support (though hopefully not), others should
have maximum around 64K. Just I want to make sure the tests are compilable
also with other compilers etc. and clearly state their needs.
So, I'd go with
#if __BITINT_MAXWIDTH__ >= 519
#define ALL_TESTS \
BASIC_TESTS \
TEST(64) \
TEST(128) \
TEST(256) \
TEST(512)
#else
#define ALL_TESTS BASIC_TESTS
#endif
Jakub
