[LLVMdev] <8 x i1> legalization overhead on X86 with AVX2

[Lizunov, Andrey E]

Hello everyone,

I'm facing a problem with an inefficient vector code generation for X86 with AVX2.
The problem with the example below is unnecessary conversions of <8 x i32> to <8 x i16> and back when a boolean vector crosses BB boundary. I suppose that is because <8 x i1> is legalized to <8 x 16>. As far as I understand there is no such issue if all i1 vectors are computed inside a BB because they are optimized out by Machine Code Optimizations.

Here is a short reproducer I've written by hand (based on a real LLVM IR sequence produced by a vectorizer I'm working with).
bash$ ./llc -mcpu=core-avx2 -filetype=asm -o legalization.overhead.s legalization.overhead.ll -asm-verbose

define <8 x i32> @foo(<8 x i32> %arg0, <8 x i32> %arg1,         |foo:                                    # @foo
                      <8 x i32> %arg2, i1 %switch) {            |        .cfi_startproc
BB0:                                                            |# BB#0:                                 # %BB0
  %boolv0 = icmp sgt <8 x i32> %arg0, zeroinitializer           |        pushq   %rbp
  br i1 %switch, label %BB1, label %BB2                         |.Ltmp2:
BB1:                                                            |        .cfi_def_cfa_offset 16
  %boolv1 = icmp sgt <8 x i32> %arg1, zeroinitializer           |.Ltmp3:
  br label %BB2                                                 |        .cfi_offset %rbp, -16
BB2:                                                            |        movq    %rsp, %rbp
  %boolvx = phi <8 x i1> [ %boolv0, %BB0 ], [ %boolv1, %BB1 ]   |.Ltmp4:
  %boolv2 = icmp sgt <8 x i32> %arg2, zeroinitializer           |        .cfi_def_cfa_register %rbp
  %merge = and <8 x i1> %boolvx, %boolv2                        |        vpxor   %ymm3, %ymm3, %ymm3
  %res = sext <8 x i1> %merge to <8 x i32>                      |        vpcmpgtd        %ymm3, %ymm0, %ymm0
  ret <8 x i32> %res                                            |        vpshufb .LCPI0_0(%rip), %ymm0, %ymm0
}                                                               |        # ymm0 = ymm0[0,1,4,5,8,9,12,13,NULL,NULL,NULL,NULL,16,17,20,21,24,25,28,29,NULL...]
~                                                               |        vpermq  $8, %ymm0, %ymm0        # ymm0 = ymm0[0,2,0,0]
~                                                               |        testb   $1, %dil
~                                                               |        je      .LBB0_2
~                                                               |# BB#1:                                 # %BB1
~                                                               |        vpcmpgtd        %ymm3, %ymm1, %ymm0
~                                                               |        vpshufb .LCPI0_0(%rip), %ymm0, %ymm0
~                                                               |        vpermq  $8, %ymm0, %ymm0        # ymm0 = ymm0[0,2,0,0]
~                                                               |.LBB0_2:                                # %BB2
~                                                               |        vpcmpgtd        %ymm3, %ymm2, %ymm1
~                                                               |        vpshufb .LCPI0_0(%rip), %ymm1, %ymm1
~                                                               |        vpermq  $8, %ymm1, %ymm1        # ymm1 = ymm1[0,2,0,0]
~                                                               |        vpand   %xmm1, %xmm0, %xmm0
~                                                               |        vpmovzxwd       %xmm0, %ymm0
~                                                               |        vpslld  $31, %ymm0, %ymm0
~                                                               |        vpsrad  $31, %ymm0, %ymm0
~                                                               |        popq    %rbp
~                                                               |        ret
Instances of the problematic ASM sequences:

1.      <8 x i32> to <8 x i16>
vpshufb .LCPI0_0(%rip), %ymm0, %ymm0
vpermq  $8, %ymm0, %ymm0        # ymm0 = ymm0[0,2,0,0]


2.      <8 x i16> to <8 x i32>
vpmovzxwd       %xmm0, %ymm0
vpslld  $31, %ymm0, %ymm0
vpsrad  $31, %ymm0, %ymm0

Currently I'm trying to figure out the best way to completely remove those unnecessary conversions and would appreciate any help from the community.

Thank you!
Andrey Lizunov

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