[LLVMdev] Load value and broadcast in LLVM

[Sanjay Patel]

Zhi -

If your IR is not ending up as the expected splat instructions (simple AVX
examples below), please file a bug.

$ cat broadcast.ll
define <2 x double> @v2f64(double* %d) {
  %ld = load double, double* %d
  %v = insertelement <2 x double> undef, double %ld, i32 0
  %sh = shufflevector <2 x double> %v, <2 x double> undef, <2 x i32><i32 0,
i32 0>
  ret <2 x double> %sh
}

define <4 x double> @v4f64(double* %d) {
  %ld = load double, double* %d
  %v = insertelement <4 x double> undef, double %ld, i32 0
  %sh = shufflevector <4 x double> %v, <4 x double> undef, <4 x i32><i32 0,
i32 0, i32 0, i32 0>
  ret <4 x double> %sh
}

$ ./llc broadcast.ll -o - -mattr=avx
_v2f64:                                 ## @v2f64
    vmovddup    (%rdi), %xmm0   ## xmm0 = mem[0,0]
    retq

_v4f64:                                 ## @v4f64
    vbroadcastsd    (%rdi), %ymm0
    retq




On Mon, May 4, 2015 at 12:12 PM, Shahid, Asghar-ahmad <
Asghar-ahmad.Shahid at amd.com> wrote:

>  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> 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] *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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