[LLVMdev] loop vectorizer says Bad stride

Nadav Rotem nrotem at apple.com
Mon Oct 28 13:58:37 PDT 2013


Frank, 

It looks like the loop vectorizer is unable to tell that the two stores in your code never overlap. This is probably because of the sign-extend in your code. Can you extend the indices to 64bit ? 

Thanks,
Nadav

On Oct 28, 2013, at 1:38 PM, Frank Winter <fwinter at jlab.org> wrote:

> Verifying function
> running passes ...
> LV: Checking a loop in "bar"
> LV: Found a loop: L0
> LV: Found an induction variable.
> LV: We need to do 0 pointer comparisons.
> LV: Checking memory dependencies
> LV: Bad stride - Not an AddRecExpr pointer   %13 = getelementptr float* %arg2, i32 %1 SCEV: ((4 * (sext i32 {(256 + %arg0),+,1}<nw><%L0> to i64)) + %arg2)
> LV: Src Scev: {((4 * (sext i32 %arg0 to i64)) + %arg2),+,4}<%L0>Sink Scev: ((4 * (sext i32 {(256 + %arg0),+,1}<nw><%L0> to i64)) + %arg2)(Induction step: 1)
> LV: Distance for   store float %11, float* %12 to   store float %10, float* %13: ((4 * (sext i32 {(256 + %arg0),+,1}<nw><%L0> to i64)) + {(-4 * (sext i32 %arg0 to i64)),+,-4}<%L0>)
> Non-consecutive pointer access
> LV: We don't need a runtime memory check.
> LV: Can't vectorize due to memory conflicts
> LV: Not vectorizing.
> 
> Here the input IR:
> 
> define void @bar(i32 %arg0, i32 %arg1, float* noalias %arg2, float* noalias %arg3, float* noalias %arg4) {
> entrypoint:
>  br label %L0
> 
> L0:                                               ; preds = %L0, %entrypoint
>  %0 = phi i32 [ %16, %L0 ], [ %arg0, %entrypoint ]
>  %1 = add nsw i32 %0, 256
>  %2 = sext i32 %0 to i64
>  %3 = getelementptr float* %arg3, i64 %2
>  %4 = load float* %3, align 4
>  %5 = sext i32 %1 to i64
>  %6 = getelementptr float* %arg3, i64 %5
>  %7 = load float* %6, align 4
>  %8 = getelementptr float* %arg4, i64 %2
>  %9 = load float* %8, align 4
>  %10 = getelementptr float* %arg4, i64 %5
>  %11 = load float* %10, align 4
>  %12 = fadd float %11, %7
>  %13 = fadd float %9, %4
>  %14 = getelementptr float* %arg2, i64 %2
>  store float %13, float* %14, align 4
>  %15 = getelementptr float* %arg2, i64 %5
>  store float %12, float* %15, align 4
>  %16 = add nsw i32 %0, 1
>  %17 = icmp slt i32 %16, %arg1
>  br i1 %17, label %L0, label %L1
> 
> L1:                                               ; preds = %L0
>  ret void
> }
> 
> This function is IMO equivalent to
> 
> void main(int start, int end, float * restrict c, float * restrict a, float * restrict b)
> {
>  const int width = 256;
>  for (int i = start ; i < end ; ++i ) {
>    c[ i ]         = a[ i ]         + b[ i ];
>    c[ width + i ] = a[ width + i ] + b[ width + i ];
>  }
> }
> 
> With this version, the vectorizer doesnt complain about a bad stride and can parallelize the loop.
> 
> Here the output from "clang -emit-llvm -S loop.c" which can be parallelized:
> 
> 
> 
> target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
> 
> ; Function Attrs: nounwind uwtable
> define void @bar(float* noalias %c, float* noalias %a, float* noalias %b, i32 %start, i32 %end) #0 {
> entry:
>  %c.addr = alloca float*, align 8
>  %a.addr = alloca float*, align 8
>  %b.addr = alloca float*, align 8
>  %start.addr = alloca i32, align 4
>  %end.addr = alloca i32, align 4
>  %width = alloca i32, align 4
>  %i = alloca i32, align 4
>  store float* %c, float** %c.addr, align 8
>  store float* %a, float** %a.addr, align 8
>  store float* %b, float** %b.addr, align 8
>  store i32 %start, i32* %start.addr, align 4
>  store i32 %end, i32* %end.addr, align 4
>  store i32 256, i32* %width, align 4
>  %0 = load i32* %start.addr, align 4
>  store i32 %0, i32* %i, align 4
>  br label %for.cond
> 
> for.cond:                                         ; preds = %for.inc, %entry
>  %1 = load i32* %i, align 4
>  %2 = load i32* %end.addr, align 4
>  %cmp = icmp slt i32 %1, %2
>  br i1 %cmp, label %for.body, label %for.end
> 
> for.body:                                         ; preds = %for.cond
>  %3 = load i32* %i, align 4
>  %idxprom = sext i32 %3 to i64
>  %4 = load float** %a.addr, align 8
>  %arrayidx = getelementptr inbounds float* %4, i64 %idxprom
>  %5 = load float* %arrayidx, align 4
>  %6 = load i32* %i, align 4
>  %idxprom1 = sext i32 %6 to i64
>  %7 = load float** %b.addr, align 8
>  %arrayidx2 = getelementptr inbounds float* %7, i64 %idxprom1
>  %8 = load float* %arrayidx2, align 4
>  %add = fadd float %5, %8
>  %9 = load i32* %i, align 4
>  %idxprom3 = sext i32 %9 to i64
>  %10 = load float** %c.addr, align 8
>  %arrayidx4 = getelementptr inbounds float* %10, i64 %idxprom3
>  store float %add, float* %arrayidx4, align 4
>  %11 = load i32* %i, align 4
>  %add5 = add nsw i32 256, %11
>  %idxprom6 = sext i32 %add5 to i64
>  %12 = load float** %a.addr, align 8
>  %arrayidx7 = getelementptr inbounds float* %12, i64 %idxprom6
>  %13 = load float* %arrayidx7, align 4
>  %14 = load i32* %i, align 4
>  %add8 = add nsw i32 256, %14
>  %idxprom9 = sext i32 %add8 to i64
>  %15 = load float** %b.addr, align 8
>  %arrayidx10 = getelementptr inbounds float* %15, i64 %idxprom9
>  %16 = load float* %arrayidx10, align 4
>  %add11 = fadd float %13, %16
>  %17 = load i32* %i, align 4
>  %add12 = add nsw i32 256, %17
>  %idxprom13 = sext i32 %add12 to i64
>  %18 = load float** %c.addr, align 8
>  %arrayidx14 = getelementptr inbounds float* %18, i64 %idxprom13
>  store float %add11, float* %arrayidx14, align 4
>  br label %for.inc
> 
> for.inc:                                          ; preds = %for.body
>  %19 = load i32* %i, align 4
>  %inc = add nsw i32 %19, 1
>  store i32 %inc, i32* %i, align 4
>  br label %for.cond
> 
> for.end:                                          ; preds = %for.cond
>  ret void
> }
> 
> attributes #0 = { nounwind uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
> 
> !llvm.ident = !{!0}
> 
> !0 = metadata !{metadata !"clang version 3.4 (trunk 193120)"}
> 
> 
> Any ideas why the vectotizer doesn't like my code?
> 
> Frank
> 
> 
> 
> 
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