[llvm-dev] Non-Temporal hints from Loop Vectorizer

[Simon Pilgrim via llvm-dev]

On 20/01/2018 18:16, hameeza ahmed wrote:
> Actually i am working on vector accelerator which will perform those 
> instructions which are non temporal.
>
> for instance if i have this loop
>
> for(i=0;i<2048;i++)
> a[i]=b[i]+c[i];
>
> currently it emits following IR;
>
>
>   %0 = getelementptr inbounds [2048 x i32], [2048 x i32]* @b, i64 0, 
> i64 %index
>   %1 = bitcast i32* %0 to <16 x i32>*
>   %wide.load = load <16 x i32>, <16 x i32>* %1, align 16, !tbaa !1
>   %8 = getelementptr inbounds [2048 x i32], [2048 x i32]* @c, i64 0, 
> i64 %index
>   %9 = bitcast i32* %8 to <16 x i32>*
>   %wide.load14 = load <16 x i32>, <16 x i32>* %9, align 16, !tbaa !1
>   %16 = add nsw <16 x i32> %wide.load14, %wide.load
>   %20 = getelementptr inbounds [2048 x i32], [2048 x i32]* @a, i64 0, 
> i64 %index
>   %21 = bitcast i32* %20 to <16 x i32>*
>   store <16 x i32> %16, <16 x i32>* %21, align 16, !tbaa !1
>
>
> However, i want it to emit following IR
>
>   %0 = getelementptr inbounds [2048 x i32], [2048 x i32]* @b, i64 0, 
> i64 %index
>   %1 = bitcast i32* %0 to <16 x i32>*
>   %wide.load = load <16 x i32>, <16 x i32>* %1, align 16, !tbaa !1, 
> !nontemporal !1
>   %8 = getelementptr inbounds [2048 x i32], [2048 x i32]* @c, i64 0, 
> i64 %index
>   %9 = bitcast i32* %8 to <16 x i32>*
>   %wide.load14 = load <16 x i32>, <16 x i32>* %9, align 16, !tbaa 
> !1, !nontemporal !1
>   %16 = add nsw <16 x i32> %wide.load14, %wide.load, !nontemporal !1
>   %20 = getelementptr inbounds [2048 x i32], [2048 x i32]* @a, i64 0, 
> i64 %index
>   %21 = bitcast i32* %20 to <16 x i32>*
>   store <16 x i32> %16, <16 x i32>* %21, align 16, !tbaa 
> !1, !nontemporal !1
>
> so that i can offload load, add, store to accelerator hardware. is it 
> possible here? do i need a separate pass to detect whether the loop 
> has non temporal data or polly will help here? what do you say?
 From C/C++ you just need to use the 
__builtin_nontemporal_store/__builtin_nontemporal_load builtins to tag 
the stores/loads with the nontemporal flag.

for(i=0;i<2048;i++) {
   __builtin_nontemporal_store( __builtin_nontemporal_load(b+i) + 
__builtin_nontemporal_load(c + i), a + i );
}

There may be an attribute you can tag pointers with instead but I don't 
know off hand.

> On Sat, Jan 20, 2018 at 11:02 PM, Simon Pilgrim 
> <llvm-dev at redking.me.uk <mailto:llvm-dev at redking.me.uk>> wrote:
>
>     On 20/01/2018 17:44, hameeza ahmed via llvm-dev wrote:
>
>         Hello,
>
>         My work deals with non-temporal loads and stores i found
>         non-temporal meta data in llvm documentation but its not shown
>         in IR.
>
>         How to get non-temporal meta data?
>
>     llvm\test\CodeGen\X86\nontemporal-loads.ll shows how to create nt
>     vector loads in IR - is that what you're after?
>
>     Simon.
>
>

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