llvm dev - Aug 2016 - LLVM 3.9 RC2's SCCP pass removing calls to external functions?!

Hi,

I've been porting a project to LLVM 3.9 and found that the SCCP pass may
remove calls to external functions if their return type is declared to be an
empty struct. For instance:
> %empty = type {}
> 
> declare %empty @foo()
> 
> define i32 @main() {
>   %1 = call %empty @foo()
>   ret i32 0
> }

opt -sccp -S file.ll returns:
> %empty = type {}
> 
> declare %empty @foo()
> 
> define i32 @main() {
>   ret i32 0
> }

It doesn't look right to remove a call to an external function (especially
since the call isn't removed if `@foo` returns a `void` instead). It
didn't happen with LLVM 3.8.

I'm using the Mac OS X prebuilt binaries for RC2.

Assuming that this is a bug, what are the next steps?

Félix

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Hi Félix,

Félix Cloutier via llvm-dev wrote:
 > Assuming that this is a bug, what are the next steps?

Looks like you already have a very small test case -- have you tried
sticking it in a debugger to see why SCCP thinks removing the call is
okay?

Alternatively, file a bug at llvm.org/bugs and someone will get to it.

-- Sanjoy

Hi Félix,

Sanjoy Das wrote:
 > Félix Cloutier via llvm-dev wrote:
 >  > Assuming that this is a bug, what are the next steps?
 >
 > Looks like you already have a very small test case -- have you tried
 > sticking it in a debugger to see why SCCP thinks removing the call is
 > okay?
 >
 > Alternatively, file a bug at llvm.org/bugs and someone will get to it.

The third thing you can do is run a bisection to find the problematic
commit, since you said this was a regression.

-- Sanjoy