[LLVMdev] loop vectorizer says Bad stride

[Frank Winter]

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