[LLVMdev] Load value and broadcast in LLVM

[Shahid, Asghar-ahmad]

Hi Zhi,

At IR level, yes there is an overhead of two more instruction, however, as Michel has pointed
backend may fold it to single instruction wherever there is such an instruction is available.

Regards,
Shahid

From: zhi chen [mailto:zchenhn at gmail.com]
Sent: Monday, May 04, 2015 10:32 PM
To: Shahid, Asghar-ahmad
Cc: LLVM Dev
Subject: Re: [LLVMdev] Load value and broadcast in LLVM

Hi Shahid,

Thank you so much for your response. You suggested approach is what I am right now using. However, it seems that the overhead is a little bit high because we are introducing two more instructions. I was wondering if there was a cheaper way to do it.

Best,
Zhi

On Mon, May 4, 2015 at 2:12 AM, Shahid, Asghar-ahmad <Asghar-ahmad.Shahid at amd.com<mailto:Asghar-ahmad.Shahid at amd.com>> wrote:
Hi Zhi,

If I get your question correctly, Yes, you can do it by using the IRBuilder’s CreateVectorSplat() API.

  /// \brief Return a vector value that contains \arg V broadcasted to \p
  /// NumElts elements.
  Value *CreateVectorSplat(unsigned NumElts, Value *V, const Twine &Name = "")

For your case, here the Value V will be your loaded value %0 and NumElts will be 2.

So after %0 = load double* %x, align 4, !tbaa !0
you will get a sequence of LLVM-IR

%1= insertelement <2 x double > %0, …
%2= shufflevector <2 x double > %1, …

%2 will be your desired  value.

Regards,
Shahid

From: llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu>] On Behalf Of zhi chen
Sent: Monday, May 04, 2015 1:29 PM
To: LLVM Dev
Subject: [LLVMdev] Load value and broadcast in LLVM

Is it possible to load a value into a vector register and broadcast it in LLVM?

For example, for the following address %x

 %x = getelementptr inbounds %struct._Ray* %ray, i32 0, i32 0, i32 0

instead of loading the value at %x into a scalar register %0:
%0 = load double* %x, align 4, !tbaa !0

I want to load it into a <2 x double> vector register %1 and make both of the two elements in %1 be the value at %x.

I guess one way to do this is to make getelementptr return a <2 x i32>* address, where the two addresses in <2 X 32> are the same. But I don't know if it is possible to do this in LLVM.

Any help would be appreciated.

Best,
Zhi


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