[PATCH] x86_64: Improved V1TImode rotations by non-constant amounts.

Roger Sayle roger@nextmovesoftware.com
Sun Nov 28 14:02:37 GMT 2021


This patch builds on the recent improvements to TImode rotations (and
Jakub's fixes to shldq/shrdq patterns).  Now that expanding a TImode
rotation can never fail, it is safe to allow general_operand constraints
on the QImode shift amounts in rotlv1ti3 and rotrv1ti3 patterns.
I've also made an additional tweak to ix86_expand_v1ti_to_ti to use
vec_extract via V2DImode, which avoid using memory and takes advantage
vpextrq on recent hardware.

For the following test case:

typedef unsigned __int128 uv1ti __attribute__ ((__vector_size__ (16)));
uv1ti rotr(uv1ti x, unsigned int i) { return (x >> i) | (x << (128-i)); }

GCC with -O2 -mavx2 would previously generate:

rotr:   vmovdqa %xmm0, -24(%rsp)
        movq    -16(%rsp), %rdx
        movl    %edi, %ecx
        xorl    %esi, %esi
        movq    -24(%rsp), %rax
        shrdq   %rdx, %rax
        shrq    %cl, %rdx
        testb   $64, %dil
        cmovne  %rdx, %rax
        cmovne  %rsi, %rdx
        negl    %ecx
        xorl    %edi, %edi
        andl    $127, %ecx
        vmovq   %rax, %xmm2
        movq    -24(%rsp), %rax
        vpinsrq $1, %rdx, %xmm2, %xmm1
        movq    -16(%rsp), %rdx
        shldq   %rax, %rdx
        salq    %cl, %rax
        testb   $64, %cl
        cmovne  %rax, %rdx
        cmovne  %rdi, %rax
        vmovq   %rax, %xmm3
        vpinsrq $1, %rdx, %xmm3, %xmm0
        vpor    %xmm1, %xmm0, %xmm0
        ret

with this patch, we now generate:

rotr:   movl    %edi, %ecx
        vpextrq $1, %xmm0, %rax
        vmovq   %xmm0, %rdx
        shrdq   %rax, %rdx
        vmovq   %xmm0, %rsi
        shrdq   %rsi, %rax
        andl    $64, %ecx
        movq    %rdx, %rsi
        cmovne  %rax, %rsi
        cmove   %rax, %rdx
        vmovq   %rsi, %xmm0
        vpinsrq $1, %rdx, %xmm0, %xmm0
        ret


This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check with no new failures.  Ok for mainline?


2021-11-28  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-expand.c (ix86_expand_v1ti_to_ti): Perform the
	conversion via V2DImode using vec_extractv2didi on TARGET_SSE2.
	* config/i386/sse.md (rotlv1ti3, rotrv1ti3): Change constraint
	on QImode shift amounts from const_int_operand to general_operand.

gcc/testsuite/ChangeLog
	* gcc.target/i386/sse2-v1ti-rotate.c: New test case.


Thanks in advance,
Roger
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