[PATCH] [i386] Support avx512 vector shift with vector [PR98434]

Hongtao Liu crazylht@gmail.com
Wed Jun 23 07:23:44 GMT 2021

Here's the patch I'm going to check in.

The patch will regress pr91838.C with extra options: -march=cascadelake

using T = unsigned char; // or ushort, or uint
using V [[gnu::vector_size(8)]] = T;
V f(V x) { return x >> 8 * sizeof(T); }

Basically, the testcase is UB with logic right shift more than 8 bits
for unsigned char, but however w/o lashr_optab,  vector operation will
be lowered to scalar and be optimized by pass_ccp4 in gimple.
But w/ lashr_optab, it's not optimized and left to the backend, in the
backend we don't optimize such UB(just like what we did in
So I guess maybe gimple should handle such situations to avoid
"nonoptimal codegen".

On Mon, May 24, 2021 at 5:49 PM Hongtao Liu <crazylht@gmail.com> wrote:
> Hi:
>   This patch is about to add expanders for vashl<VI12_AVX512BW>,
> vlshr<VI12_AVX512BW>,
> vashr<VI1_AVX512BW> and vashr<v32hi,v16hi,v4di,v8di>.
> Besides there's some assumption in expand_mult_const that mul and
> add must be available at the same time, but for i386, addv8qi is
> restricted under TARGET_64BIT, but mulv8qi not, that could cause ICE.
> So restrict mulv8qi and shiftv8qi under TARGET_64BIT.
>   Bootstrap and regtested on x86_64-linux-gnu{-m32,} and
> x86_64-linux-gnu{-m32\ -march=cascadelake,-march=cascadelake}
> gcc/ChangeLog:
>         PR target/98434
>         * config/i386/i386-expand.c (ix86_expand_vec_interleave):
>         Adjust comments for ix86_expand_vecop_qihi2.
>         (ix86_expand_vecmul_qihi): Renamed to ..
>         (ix86_expand_vecop_qihi2): Adjust function prototype to
>         support shift operation, add static to definition.
>         (ix86_expand_vec_shift_qihi_constant): Add static to definition.
>         (ix86_expand_vecop_qihi): Call ix86_expand_vecop_qihi2 and
>         ix86_expand_vec_shift_qihi_constant.
>         * config/i386/i386-protos.h (ix86_expand_vecmul_qihi): Deleted.
>         (ix86_expand_vec_shift_qihi_constant): Deleted.
>         * config/i386/sse.md (mulv8qi3): Call ix86_expand_vecop_qihi
>         directly, add condition TARGET_64BIT.
>         (mul<mode>3): Ditto.
>         (<insn><mode>3): Ditto.
>         (vlshr<mode>3): Extend to support avx512 vlshr.
>         (v<insn><mode>3): New expander for
>         vashr/vlshr/vashl.
>         (v<insn>v8qi3): Ditto.
>         (vashrv8hi3<mask_name>): Renamed to ..
>         (vashr<mode>3): And extend to support V16QImode for avx512.
>         (vashrv16qi3): Deleted.
>         (vashrv2di3<mask_name>): Extend expander to support avx512
>         instruction.
> gcc/testsuite/ChangeLog:
>         PR target/98434
>         * gcc.target/i386/pr98434-1.c: New test.
>         * gcc.target/i386/pr98434-2.c: New test.
>         * gcc.target/i386/avx512vl-pr95488-1.c: Adjust testcase.

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