[cfe-dev] Question about "CodeGenFunction::EmitLoadOfScalar" with vector type of 3 elements

[aleksey.bader@gmail.com via cfe-dev]

Hi JinGu,

I don't think it should be a problem for OpenCL. 3-component vector is
aligned as 4-component vector (see section 6.1.5 "Alignment of Type" of
OpenCL C kernel language specification v2.0).
AFAIK, almost all existing OpenCL compilers are based on clang and there
seems to be no problems with handling load/store operations this way.
Could you elaborate on the case where this approach doesn't work?

Thanks,
Alexey

On Mon, Mar 6, 2017 at 6:47 PM, jingu at codeplay.com via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi All,
>
>
> I have a question about "CodeGenFunction::EmitLoadOfScalar". I am
> compiling code with vector type of 3 elements like int3 or float3. Clang
> converts the vector load to different vector load with 4 element vector
> type because there is code on "CodeGenFunction::EmitLoadOfScalar" as
> follows:
>
> 1312   // For better performance, handle vector loads differently.
> 1313   if (Ty->isVectorType()) {
> 1314     const llvm::Type *EltTy = Addr.getElementType();
> 1315
> 1316     const auto *VTy = cast<llvm::VectorType>(EltTy);
> 1317
> 1318     // Handle vectors of size 3 like size 4 for better performance.
> 1319     if (VTy->getNumElements() == 3) {
> 1320
> 1321       // Bitcast to vec4 type.
> 1322       llvm::VectorType *vec4Ty = llvm::VectorType::get(VTy->get
> ElementType(),
> 1323                                                          4);
> 1324       Address Cast = Builder.CreateElementBitCast(Addr, vec4Ty,
> "castToVec4");
> 1325       // Now load value.
> 1326       llvm::Value *V = Builder.CreateLoad(Cast, Volatile, "loadVec4");
>
> 4 element vector load could generate aligned vector load in the end and it
> would be better in usual. But it is not good for other target or language
> like OpenCL which supports 3 element vector type natively. Can we consider
> this situation on "CodeGenFunction::EmitLoadOfScalar" like this "if
> (!getLangOpts().OpenCL)" or with target specific property on
> TargetCodeGenInfo?
>
> If I missed something, please let me know.
>
> Thanks,
> JinGu Kang
>
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