Hola LLVMers,
I’m looking to make use of the vectorization primitives in the Intel chip with the code we generate from LLVM and so I’ve started experimenting with it. What is the state of the machine code generated for vectors? In my tinkering, I seem to be getting some wonky machine instructions, but I’m most likely just doing something wrong and I’m hoping you can set me in the correct course.
My minimal function creates a float4 vector with a specified scalar in all the elements. It then extracts the third element and returns it.
We are currently using the JIT and I’m currently synced to about a week after the 2.0 branch, so I’m admittedly stale by about a month.
In LLVM IR:
; ModuleID = ‘test vectors’
define float @vSelect3(float %x) {
body:
%pv = alloca <4 x float> ; <<4 x float>*> [#uses=1]
%v = load <4 x float>* %pv ; <<4 x float>> [#uses=1]
%v1 = insertelement <4 x float> %v, float %x, i32 0 ; <<4 x
[#uses=1]
%v2 = insertelement <4 x float> %v1, float %x, i32 1 ; <<4 x
[#uses=1]
%v3 = insertelement <4 x float> %v2, float %x, i32 2 ; <<4 x
[#uses=1]
%v4 = insertelement <4 x float> %v3, float %x, i32 3 ; <<4 x
[#uses=1]
%s = extractelement <4 x float> %v4, i32 3 ; [#uses
=1]
ret float %s
}
In Intel assembly, I get the following:
00000000`01b80010 83ec20 sub esp,20h
00000000`01b80013 f30f10442424 movss xmm0,dword ptr [esp+24h] ß this loads x into the low float of xmm0
00000000`01b80019 0f284c2404 movaps xmm1,xmmword ptr [esp+4] ß this seg faults because esp+4 isn’t 16-byte aligned
What is that line trying to achieve? X is at [esp+24]. There weren’t any other parameters.
00000000`01b8001e f30f10c8 movss xmm1,xmm0
00000000`01b80022 8b442424 mov eax,dword ptr [esp+24h]
00000000`01b80026 660fc4c802 pinsrw xmm1,eax,2
00000000`01b8002b 89c1 mov ecx,eax
00000000`01b8002d c1e910 shr ecx,10h
00000000`01b80030 660fc4c903 pinsrw xmm1,ecx,3
00000000`01b80035 660fc4c804 pinsrw xmm1,eax,4
00000000`01b8003a 660fc4c905 pinsrw xmm1,ecx,5
00000000`01b8003f 660fc4c806 pinsrw xmm1,eax,6
00000000`01b80044 660fc4c907 pinsrw xmm1,ecx,7
00000000`01b80049 0fc6c903 shufps xmm1,xmm1,3
00000000`01b8004d f30f110c24 movss dword ptr [esp],xmm1
00000000`01b80052 d90424 fld dword ptr [esp]
00000000`01b80055 83c420 add esp,20h
00000000`01b80058 c3 ret
The code used to generate and run the program was:
#include “llvm/Module.h”
#include “llvm/DerivedTypes.h”
#include “llvm/Constants.h”
#include “llvm/Instructions.h”
#include “llvm/ModuleProvider.h”
#include “llvm/Analysis/Verifier.h”
#include “llvm/System/DynamicLibrary.h”
#include “llvm/ExecutionEngine/JIT.h”
#include “llvm/ExecutionEngine/Interpreter.h”
#include “llvm/ExecutionEngine/GenericValue.h”
#include “llvm/Support/ManagedStatic.h”
#include
using namespace llvm;
Value* makeVector(Value* s, unsigned int dim, BasicBlock* basicBlock)
{
AllocaInst* pV = new AllocaInst(VectorType::get(Type::FloatTy,dim),“pv”,basicBlock);
Value* v = new LoadInst(pV,“v”,basicBlock);
for (unsigned int i = 0 ; i < dim ; ++i)
v = new InsertElementInst(v,s,i,“v”,basicBlock);
return v;
}
Function* generateVectorAndSelect(Module* pModule)
{
std::vector<Type const*> params;
params.push_back(Type::FloatTy);
FunctionType* funcType = FunctionType::get(Type::FloatTy,params,NULL);
Function* func = cast(pModule->getOrInsertFunction(“vSelect3”,funcType));
BasicBlock* basicBlock = new BasicBlock(“body”,func);
Function::arg_iterator args = func->arg_begin();
Argument* x = args;
x->setName(“x”);
Value* v1 = makeVector(x,4,basicBlock);
Value* s = new ExtractElementInst(v1,3,“s”,basicBlock);
new ReturnInst(s,basicBlock);
return func;
}
// modified from the fibonacci example
int main(int argc, char **argv)
{
Module* pVectorModule = new Module(“test vectors”);
Function* pMain = generateVectorAndSelect(pVectorModule);
pVectorModule->print(std::cout);
GenericValue gv1, gv2, gvR;
gv1.FloatVal = 2.0f;
ExistingModuleProvider *pMP = new ExistingModuleProvider(pVectorModule);
pMP->getModule()->setDataLayout(“e-p:32:32:32-i1:8:8:8-i8:8:8:8-i32:32:32:32-f32:32:32:32”);
ExecutionEngine *pEE = ExecutionEngine::create(pMP, false);
std::vector args;
args.push_back(gv1);
GenericValue result = pEE->runFunction(pMain, args);
return 0;
}
Any help would be appreciated.
.
Thanks,
Chuck.