Jakub Jelinek wrote:
> On Thu, Apr 12, 2018 at 03:53:13PM +0000, Wilco Dijkstra wrote:

>> The tailcall issue is just a distraction. Historically the handling of 
>> mempcpy 
>> has been horribly inefficient in both GCC and GLIBC for practically all 
>> targets.
>> This is why it was decided to defer to memcpy.
> I guess we need to agree to disagree.  But we have a P1 PR that we need to
> resolve and it is one of the last 6 blockers we have.  I'm not suggesting to
> revert PR70140, just let use mempcpy libcall if it is what the user wrote and
> we aren't expanding it inline.

Frankly I don't see why it is a P1 regression. Do you have a benchmark that
regresses significantly (a few percent, not by a few bytes)? I already showed
the AArch64 results for GLIBC, do you have x86 results that prove things are
much worse?

>> So generally it's a good idea to change mempcpy into memcpy by default. It's
>> not slower than calling mempcpy even if you have a fast implementation, it's 
>> faster
>> if you use an up to date GLIBC which calls memcpy, and it's significantly 
>> better
>> when using an old GLIBC.
> mempcpy is quite good on many targets even in old GLIBCs.

Only true if with "many" you mean x86, x86_64 and IIRC sparc.


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