On Fri, Jul 25, 2025 at 10:53:38AM +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;
> }
Are you going to add this as a testcase with the BEXTC framework?
>
> 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
> with a widening sign-extension.
> ---
> gcc/gimple-lower-bitint.cc | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
> index 631a7844790..d80191c6761 100644
> --- a/gcc/gimple-lower-bitint.cc
> +++ b/gcc/gimple-lower-bitint.cc
> @@ -3161,6 +3161,14 @@ bitint_large_huge::lower_mergeable_stmt (gimple *stmt,
> tree_code &cmp_code,
> {
> tree l = limb_access (nlhs ? NULL_TREE : lhs_type,
> nlhs ? nlhs : lhs, idx, true);
> +
> + if (bitint_extended && sext && TYPE_UNSIGNED (lhs_type)
> + && tree_fits_uhwi_p (idx) && !nlhs)
If condition doesn't fit on a single line, then every toplevel && or ||
should be on a separate line. So
if (bitint_extended
&& sext
&& TYPE_UNSIGNED (lhs_type)
&& tree_fits_uhwi_p (idx)
&& !nlhs)
> + {
> + rhs1 = add_cast (limb_access_type (lhs_type, idx), rhs1);
> + rhs1 = add_cast (TREE_TYPE (l), rhs1);
> + }
> +
> g = gimple_build_assign (l, rhs1);
> }
> insert_before (g);
> --
> 2.46.0
Jakub
