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

[Félix Cloutier via llvm-dev]

Yes, there is no way for any pass to find out by itself whether that function has side effects, and I didn't tell it that it doesn't have any, so that seems overly aggressive.

As far as finding and fixing the problem goes: I used to build LLVM and Clang from source, but I stopped because a full build takes around 3 hours on this machine and I don't get a lot out of it. I use LLVM on my spare time and I'm about as casual as a compiler back-end user can be. Sanjoy, I'd be happy if you looked into it.

Félix

> Le 24 août 2016 à 05:47:46, Renato Golin <renato.golin at linaro.org> a écrit :
> 
> On 24 August 2016 at 11:45, Alexandre Isoard <alexandre.isoard at gmail.com> wrote:
>> However, he also stated that in the case the functions returns void it is
>> not removed, which suggest there is something fishier going on.
> 
> Looking back at the example (and LangRef, for function attributes),
> looks like it's not legal to remove because the function does *not*
> have "readnone" or "readonly", so LLVM can't prove it side-effect
> free.
> 
> Sanjoy's fix is probably on the right track.
> 
> --renato

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