[llvm] r265762 - Don't IPO over functions that can be de-refined

[Benjamin Kramer via llvm-commits]

On Fri, Apr 8, 2016 at 8:12 PM, Sanjoy Das
<sanjoy at playingwithpointers.com> wrote:
>
> Benjamin Kramer wrote:
>> $ cat foo.cc
>> inline void foo(const char *, ...) {}
>>
>> int main() {
>>    foo("bar");
>> }
>>
>> $ clang-old -S -o - log.cc -O2|grep -c bar
>> 0
>> $ clang-new -S -o - log.cc -O2|grep -c bar
>> 1
>>
>> Is this something we want to fix or is that how it's supposed to work
>> now. GCC happily discards foo() here.
>
> This is a correct optimization that the old LLVM did for the wrong
> reasons.  What most likely happened in the old LLVM was we'd infer
> readnone on foo() and then elide the readnone call to foo in main.
> Now the first step isn't happening anymore, because it is unsound.
>
> If this specific case is important to you, there probably are things
> we can do get the above optimization (e.g. inlining through simple
> varargs functions).  If this can wait, then whatever we do to win back
> non-inlining IPO for normal C++ inline function should kick in here
> as well.

It's not a high priority and can wait, I was just curious if this was
intentional
and if so if we have plans to recover the missed optimization. Improving
non-inlining IPO sounds good :)