[LLVMdev] MCJIT generates MOVAPS on unaligned address

[Arnold Schwaighofer]

Your .ll file does not have a data layout. Opt will not initialize the DataLayoutPass. The SLP vectorizer will not vectorize because there is no DataLayoutPass.

  debug-cmake/bin/opt -default-data-layout="e-m:e-i64:64-f80:128-n8:16:32:64-S128" -basicaa -slp-vectorizer -S </Users/arnold/Downloads/module_H7ktW0.ll | grep "<4 x" |  grep store 
  store <4 x float> %198, <4 x float>* %204, align 8

There is a bug in the SLPVectorizer however - it should be “align 4” - we get the alignment of the pointer type which is not what we want we want the alignment of the stored/loaded value. It should be

if (!Alignment)
  Alignment = DL->getABITypeAlignment(SI->getValueOperand()->getType());

I am not sure that would fix your issue though, because that would mean we return the wrong alignment not none.

If the call below returns 0 then something has gone wrong in setting up the data layout in your compilation pipeline.


> On Aug 7, 2014, at 2:18 PM, Frank Winter <fwinter at jlab.org> wrote:
> 
> It's not reproducible with 'opt'. I call the SLP pass from my application and only then the wrong IR gets generated.
> 
> On the attached module I call via the function pass manager:
> 
> 1) TargetLibraryInfo with the target triple
> 2) Set the data layout
> 3) Basic Alias Analysis
> 4) SLP vectorizer
> 
> This produces the wrong IR. On the other hand running the attached module through 'opt -slp-vectorizer' results in no code changes.
> 
> What could I be missing here?
> 
> Frank
> 
> 
> On 08/07/2014 04:29 PM, Arnold Schwaighofer wrote:
>>> On Aug 7, 2014, at 12:42 PM, Frank Winter <fwinter at jlab.org> wrote:
>>> 
>>> MCJIT when lowering to x86-64 generates a MOVAPS (Move Aligned Packed Single-Precision Floating-Point Values) on a non-aligned memory address:
>>> 
>>>    movaps    88(%rdx), %xmm0
>>> 
>>> where %rdx comes in as a function argument with only natural alignment (float*). This x86 instruction requires the memory address to be 16 byte aligned which 88 plus something aligned to 4 byte isn't.
>>> 
>>> Here the according IR code which was produced from the SLP vectorizer:
>>> 
>>> define void @func(float* noalias %arg0, float* noalias %arg1, float* noalias %arg2) {
>>> entrypoint:
>>> ...
>>>  %104 = getelementptr float* %arg0, i32 22
>>> ...
>>>  %204 = bitcast float* %104 to <4 x float>*
>>>  store <4 x float> %198, <4 x float>* %204
>>> 
>>> This in itself not wrong. However, shouldn't the lowering pass recognize the wrong alignment?
>> The LLVM IR is wrong. Omitting the align directive on the store means abi alignment of the target. The backend is “right” wrt to LLVM IR semantics to produce the movaps.
>> 
>> The error is in the  producer (looks like the SLP vectorizer) of said vector store. Could you provide a full test case where running the SLP vectorizer (opt -slp-vectorize < t.ll) produces such an output?
>> 
>> The following code in the SLP vectorizer should have made sure that we created an alignment of “4 bytes” given a data layout (http://llvm.org/docs/LangRef.html#data-layout) that specifies f32:32:32.
>> 
>>     case Instruction::Store: {
>>       StoreInst *SI = cast<StoreInst>(VL0);
>>       unsigned Alignment = SI->getAlignment();
>>       ...
>>       StoreInst *S = Builder.CreateStore(VecValue, VecPtr);
>>       if (!Alignment)
>>         Alignment = DL->getABITypeAlignment(SI->getPointerOperand()->getType()); // << Get the 4byte alignment for the scalar float store from the data layout string.
>>       S->setAlignment(Alignment);
> 
> 
> <module_H7ktW0.ll.gz>