On Thu, Jun 13, 2024 at 4:20 AM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> This patch makes more use of m32bcst and m64bcst addressing modes in
> ix86_expand_ternlog. Previously, the i386 backend would only consider
> using a m32bcst if the inner mode of the vector was 32-bits, or using
> m64bcst if the inner mode was 64-bits. For ternlog (and other logic
> operations) this is a strange restriction, as how the same constant
> is materialized is dependent upon the mode it is used/operated on.
> Hence, the V16QI constant {2,2,2,2,2,2,2,2,2,2,2,2,2,2,2,2} wouldn't
> use m??bcst, but (V4SI){0x02020202,0x02020202,0x02020202,0x02020202}
> which has the same bit pattern would. This can optimized by (re)checking
> whether a CONST_VECTOR can be broadcast from memory after casting it
> to VxSI (or for m64bst to VxDI) where x has the appropriate vector size.
>
>
> Taking the test case from pr115407:
>
> __attribute__((__vector_size__(64))) char v;
> void foo() {
> v = v | v << 7;
> }
>
> Compiled with -O2 -mcmodel=large -mavx512bw
> GCC 14 generates a 64-byte (512-bit) load from the constant pool:
>
> foo: movabsq $v, %rax // 10
> movabsq $.LC0, %rdx // 10
> vpsllw $7, (%rax), %zmm1 // 7
> vmovdqa64 (%rax), %zmm0 // 6
> vpternlogd $248, (%rdx), %zmm1, %zmm0 // 7
> vmovdqa64 %zmm0, (%rax) // 6
> vzeroupper // 3
> ret // 1
> .LC0: .byte -12 // 64 = 114 bytes
> .byte -128
> ;; repeated another 62 times
>
> mainline currently generates two instructions, using interunit broadcast:
>
> foo: movabsq $v, %rdx // 10
> movl $-2139062144, %eax // 5
> vmovdqa64 (%rdx), %zmm2 // 6
> vpbroadcastd %eax, %zmm0 // 6
> vpsllw $7, %zmm2, %zmm1 // 7
> vpternlogd $236, %zmm0, %zmm2, %zmm1 // 7
> vmovdqa64 %zmm1, (%rdx) // 6
> vzeroupper // 3
> ret // 1 = 51 bytes
>
> With this patch, we now generate a broadcast addressing mode:
>
> foo: movabsq $v, %rax // 10
> movabsq $.LC1, %rdx // 10
> vmovdqa64 (%rax), %zmm1 // 6
> vpsllw $7, %zmm1, %zmm0 // 7
> vpternlogd $236, (%rdx){1to16}, %zmm1, %zmm0 // 7
> vmovdqa64 %zmm0, (%rax) // 6
> vzeroupper // 3
> ret // 1 = 50 total
>
> Without -mcmodel=large, the benefit is two instructions:
>
> foo: vmovdqa64 v(%rip), %zmm1 // 10
> vpsllw $7, %zmm1, %zmm0 // 7
> vpternlogd $236, .LC2(%rip){1to16}, %zmm1, %zmm0 // 11
> vmovdqa64 %zmm0, v(%rip) // 10
> vzeroupper // 3
> ret // 1 = 42
> total
>
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32}
> with no new failures. Ok for mainline?
Ok.
>
>
> 2024-06-12 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386-expand.cc (ix86_expand_ternlog): Try performing
> logic operation in a different vector mode if that enables use of
> a 32-bit or 64-bit broadcast addressing mode.
>
> gcc/testsuite/ChangeLog
> * gcc.target/i386/pr115407.c: New test case.
>
>
> Thanks in advance,
> Roger
> --
>
--
BR,
Hongtao
