On Mon, 15 Jan 2024, Jakub Jelinek wrote:
> Hi!
>
> The INTEGER_CST code uses the remainder bits in computations whether to use
> whole constant or just part of it and extend it at runtime, and furthermore
> uses it to avoid using all bits even when using the (almost) whole constant.
> The problem is that the prec % (2 * limb_prec) computation it uses is
> appropriate only for the normal lowering of mergeable operations (where
> we process 2 limbs at a time in a loop starting with least significant
> limbs and process the remaining 0-2 limbs after the loop (there with
> constant indexes). For that case it is ok not to emit the upper
> prec % (2 * limb_prec) bits into the constant, because those bits will be
> extracted using INTEGER_CST idx and so will be used directly in the
> statements as INTEGER_CSTs.
> For other cases, where we either process just a single limb in a loop,
> process it downwards (e.g. non-equality comparisons) or with some runtime
> addends (some shifts), there is either just at most one limb lowered with
> INTEGER_CST idx after the loop (e.g. for right shift) or before the loop
> (e.g. non-equality comparisons), or all limbs are processed with
> non-INTEGER_CST indexes (e.g. for left shift, when m_var_msb is set).
> Now, the m_var_msb case is already handled through
> if (m_var_msb)
> type = TREE_TYPE (op);
> else
> /* If we have a guarantee the most significant partial limb
> (if any) will be only accessed through handle_operand
> with INTEGER_CST idx, we don't need to include the partial
> limb in .rodata. */
> type = build_bitint_type (prec - rem, 1);
> but for the right shifts or comparisons the prec - rem when rem was
> prec % (2 * limb_prec) was incorrect, so the following patch fixes it
> to use remainder for 2 limbs only if m_upwards_2limb and remainder for
> 1 limb otherwise.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
> 2024-01-15 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/113370
> * gimple-lower-bitint.cc (bitint_large_huge::handle_operand): Only
> set rem to prec % (2 * limb_prec) if m_upwards_2limb, otherwise
> set it to just prec % limb_prec.
>
> * gcc.dg/torture/bitint-48.c: New test.
>
> --- gcc/gimple-lower-bitint.cc.jj 2024-01-13 11:29:08.005574338 +0100
> +++ gcc/gimple-lower-bitint.cc 2024-01-13 11:33:55.284646856 +0100
> @@ -869,7 +869,7 @@ bitint_large_huge::handle_operand (tree
> && m_data[m_data_cnt + 1] == NULL_TREE))
> {
> unsigned int prec = TYPE_PRECISION (TREE_TYPE (op));
> - unsigned int rem = prec % (2 * limb_prec);
> + unsigned int rem = prec % ((m_upwards_2limb ? 2 : 1) * limb_prec);
> int ext;
> unsigned min_prec = bitint_min_cst_precision (op, ext);
> if (m_first)
> @@ -996,7 +996,7 @@ bitint_large_huge::handle_operand (tree
> if (m_data[m_data_cnt + 1] == integer_type_node)
> {
> unsigned int prec = TYPE_PRECISION (TREE_TYPE (op));
> - unsigned rem = prec % (2 * limb_prec);
> + unsigned rem = prec % ((m_upwards_2limb ? 2 : 1) * limb_prec);
> int ext = wi::neg_p (wi::to_wide (op)) ? -1 : 0;
> tree c = m_data[m_data_cnt];
> unsigned min_prec = TYPE_PRECISION (TREE_TYPE (c));
> --- gcc/testsuite/gcc.dg/torture/bitint-48.c.jj 2024-01-13
> 11:46:30.707309245 +0100
> +++ gcc/testsuite/gcc.dg/torture/bitint-48.c 2024-01-13 11:46:25.493380637
> +0100
> @@ -0,0 +1,23 @@
> +/* PR tree-optimization/113370 */
> +/* { 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" } { "" } } */
> +
> +#if __BITINT_MAXWIDTH__ >= 255
> +_BitInt(255)
> +foo (int s)
> +{
> + return -(_BitInt(255)) 3 >> s;
> +}
> +#endif
> +
> +int
> +main ()
> +{
> +#if __BITINT_MAXWIDTH__ >= 255
> + if (foo (51) != -1)
> + __builtin_abort ();
> +#endif
> + return 0;
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Ivo Totev, Andrew McDonald, Werner Knoblich; (HRB 36809, AG Nuernberg)
