llvm dev - Feb 2021 - How to eliminate a redundant alloca, load and store?

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 }
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[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 }
>
>
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