[llvm] r185776 - Eliminate trivial redundant loads across nocapture+readonly calls to uncaptured

[Duncan Sands]

Hi Nick,

On 07/07/13 12:15, Nick Lewycky wrote:
> Author: nicholas
> Date: Sun Jul  7 05:15:16 2013
> New Revision: 185776
>
> URL: http://llvm.org/viewvc/llvm-project?rev=185776&view=rev
> Log:
> Eliminate trivial redundant loads across nocapture+readonly calls to uncaptured
> pointer arguments.

...

> --- llvm/trunk/test/Transforms/GVN/readattrs.ll (added)
> +++ llvm/trunk/test/Transforms/GVN/readattrs.ll Sun Jul  7 05:15:16 2013
> @@ -0,0 +1,17 @@
> +; RUN: opt -gvn -S -o - < %s | FileCheck %s
> +
> +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"
> +target triple = "x86_64-unknown-linux-gnu"
> +
> +declare void @use(i8* readonly nocapture)
> +
> +define i8 @test() {
> +  %a = alloca i8
> +  store i8 1, i8* %a
> +  call void @use(i8* %a)
> +  %b = load i8* %a
> +  ret i8 %b
> +; CHECK: define i8 @test
> +; CHECK: call void @use(i8* %a)
> +; CHECK-NEXT: ret i8 1
> +}

Why is nocapture relevant here?  It would be correct to eliminate the second
load in this example even if @use captured it.

Ciao, Duncan.