AVX 512 Assembly Code Generation

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#1

Hello,

I am using llvm on my core i7 laptop which has no avx support.

my goal is to generate avx512 code (loop vectorization) for Knight landing/skylake .

my .c code is;

int a[256], b[256], c[256];
foo () {
int i;
for (i=0; i<256; i++) {
a[i] = b[i] + c[i];
}
}

i first generated its .ll file via clang

clang -S -emit-llvm test.c -o test.ll

then i optimized it;

opt -S -O3 test.ll -o test_o3.ll

then i used llc for code generation

llc -mcpu=skylake-avx512 -mattr=+avx512f test_o3.ll -o test_o3.s

llc -mcpu=knl -mattr=+avx512f test_o3.ll -o test_o3.s

here is my generated code;

.text
.file “filer_o3.ll”
.globl foo
.p2align 4, 0x90
.type foo,@function
foo: # @foo
.cfi_startproc

BB#0: # %min.iters.checked

pushq %rbp
.Ltmp0:
.cfi_def_cfa_offset 16
.Ltmp1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
.Ltmp2:
.cfi_def_cfa_register %rbp
movq $-1024, %rax # imm = 0xFC00
.p2align 4, 0x90
.LBB0_1: # %vector.body
# =>This Inner Loop Header: Depth=1
vmovdqa32 c+1024(%rax), %xmm0
vmovdqa32 c+1040(%rax), %xmm1
vpaddd b+1024(%rax), %xmm0, %xmm0
vpaddd b+1040(%rax), %xmm1, %xmm1
vmovdqa32 %xmm0, a+1024(%rax)
vmovdqa32 %xmm1, a+1040(%rax)
vmovdqa32 c+1056(%rax), %xmm0
vmovdqa32 c+1072(%rax), %xmm1
vpaddd b+1056(%rax), %xmm0, %xmm0
vpaddd b+1072(%rax), %xmm1, %xmm1
vmovdqa32 %xmm0, a+1056(%rax)
vmovdqa32 %xmm1, a+1072(%rax)
addq $64, %rax
jne .LBB0_1

BB#2: # %middle.block

popq %rbp
retq
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
.cfi_endproc

.type b,@object # @b
.comm b,1024,16
.type c,@object # @c
.comm c,1024,16
.type a,@object # @a
.comm a,1024,16

.ident “clang version 3.9.0 (tags/RELEASE_390/final)”
.section “.note.GNU-stack”,"",@progbits

in the generated code although there is use of vmov… instructions but no zmm register? only xmm registers.

Can you please specify where i am wrong. i have tried it several times by different parameters but always get xmm registers.

Thank You

#2

void foo() { Your problem is that vectorization happens in opt, not in llc. Telling llc that you wish to enable AVX-512 is not sufficient. In fact, if you run: clang -S -o - test.c -march=knl -O3 you’ll see AVX-512 vectorized code. If you want to run opt separately to generate the vectorized code, you need to tell it that it is targeting the KNL. Clang can add the necessary function attributes to do this. You’ll also want to run clang with optimizations enabled so that it will generate IR that is intended to be optimized, even if you then disable the actual optimizaitons to get the pre-opt IR. clang -S -emit-llvm test.c -march=knl -O3 -mllvm -disable-llvm-optzns then running opt as you have it below should produce the desired result. Finally, I recommend upgrading to Clang/LLVM 4.0. It produces better AVX-512 code than 3.9 did. -Hal