[llvm-dev] How to eliminate a redundant alloca, load and store?

[Johannes Doerfert via llvm-dev]

[This time with list]

Short of making `@foo` private I cannot see a way right now.

Imagine:

```c
int was_called_with_foo;
void boo(int *A) { was_called_with_foo |= (A == &foo); }
```

which captures the fact you used @foo or not.

~ Johannes


On 2/22/21 7:51 PM, Riyaz Puthiyapurayil via llvm-dev wrote:
> In the following code, I know that the alloca, load and store are actually unnecessary (it is safe to pass @foo directly to the function boo). Are there any attributes one can set on function `boo` AND/OR its argument pointer AND/OR on the call so that the three instructions can be eliminated and the argument of call @boo be replaced with @foo (is there a transform that can actually do this today?)
>
> ; ModuleID = '<stdin>'
> source_filename = "test.c"
> target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
> target triple = "x86_64-unknown-linux-gnu"
>
> @foo = common dso_local local_unnamed_addr global i32 0, align 4
>
> ; Function Attrs: noinline nounwind uwtable
> define dso_local void @f() local_unnamed_addr #0 {
>    %A = alloca i32, align 4
>    %L = load i32, i32* @foo, align 4
>    store i32 %L, i32* %A, align 4
>   call void @boo(i32* nonnull %A) #2
>    ret void
> }
>
> declare dso_local void @boo(i32* readonly noalias nocapture) local_unnamed_addr argmemonly #1
>
> attributes #0 = { noinline nounwind uwtable "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
> attributes #1 = { "correctly-rounded-divide-sqrt-fp-math"="false" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-float"="false" }
> attributes #2 = { nounwind }
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev