[LLVMdev] SLP vectorizer on AVX feature

[Nadav Rotem]

Frank,

It sounds like the SLP vectorizer thinks that it is more profitable to use 128bit wide operations (because 256bit operations are double pumped on Sandybridge). Did you see a different result on Haswell?  

Thanks,
Nadav


> On Jul 1, 2015, at 11:06 AM, Frank Winter <fwinter at jlab.org> wrote:
> 
> I realized that the function parameters had no alignment attributes on them. However, even adding an alignment suitable for aligned loads on YMM, i.e. 32 bytes, didn't convince the vectorizer to use [8 x float].
> 
> define void @main(i64 %lo, i64 %hi, float* noalias align 32 %arg0, float* noalias align 32 %arg1, float* noalias align 32 %arg2) {
> ...
> 
> results still in code using only [4 x float].
> 
> Thanks,
> Frank
> 
> 
> On 07/01/2015 10:51 AM, Frank Winter wrote:
>> I seem to have problem to get the SLP vectorizer to make use of the full 8 floats available in a SIMD vector on a Sandy Bridge CPU with AVX. The function is attached, the CPU flags are:
>> 
>> flags        : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good xtopology nonstop_tsc aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 cx16 xtpr pdcm dca sse4_1 sse4_2 x2apic popcnt aes xsave avx lahf_lm ida arat epb xsaveopt pln pts dts tpr_shadow vnmi flexpriority ept vpid
>> 
>> I use LLVM 3.6 checked out yesterday
>> 
>> ~/toolchain/install/llvm-3.6/bin/opt -datalayout -basicaa -slp-vectorizer -instcombine < func_4x4x4_scalar_p_scalar.ll -S
>> 
>> the output goes like:
>> 
>> ; ModuleID = '<stdin>'
>> 
>> define void @main(i64 %lo, i64 %hi, float* noalias %arg0, float* noalias %arg1, float* noalias %arg2) {
>> entrypoint:
>>  %0 = bitcast float* %arg1 to <4 x float>*
>>  %1 = load <4 x float>* %0, align 4
>>  %2 = bitcast float* %arg2 to <4 x float>*
>>  %3 = load <4 x float>* %2, align 4
>>  %4 = fadd <4 x float> %3, %1
>>  %5 = bitcast float* %arg0 to <4 x float>*
>>  store <4 x float> %4, <4 x float>* %5, align 4
>> ....
>> 
>> So, it could make use of <8 x float> available in that machine. But it doesn't. Then I thought, that maybe the YMM registers get used when lowering the IR to machine code. However, the generated assembly doesn't seem to support this assumption :-(
>> 
>> 
>> main:
>>    .cfi_startproc
>>    xorl    %eax, %eax
>>    xorl    %esi, %esi
>>    .align    16, 0x90
>> .LBB0_1:
>>    vmovups    (%r8,%rax), %xmm0
>>    vaddps    (%rcx,%rax), %xmm0, %xmm0
>>    vmovups    %xmm0, (%rdx,%rax)
>>    addq    $4, %rsi
>>    addq    $16, %rax
>>    cmpq    $61, %rsi
>>    jb    .LBB0_1
>>    retq
>> 
>> I played with -mcpu and -march switches without success. In any case, the target architecture should be detected with the -datalayout pass, right?
>> 
>> Any idea what I am missing?
>> 
>> Frank
>> 
>> 
>> 
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> 
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