[PATCH] D69044: [X86] Allow up to 4 loads per inline memcmp()

Wed Oct 30 13:06:48 PDT 2019

spatel added reviewers: courbet, gchatelet.
spatel added a comment.

In D69044#1726941 <https://reviews.llvm.org/D69044#1726941>, @davezarzycki wrote:

> Some testing results.
>
> I built llvm+clang twice, both with core2 as the target CPU. Once without this change and once with this change. I verified that the 4-load-pair clang assembly to see that at least some memcmps generated three or more XMM load-pairs. That being said, more than two load XMM pairs was uncommon. I then ran `perf stat` against clang while it compiled X86ISelLowering.cpp (which takes about 37 seconds on my Xeon 8168 with turbo disabled).
>
> In terms of "wall clock" performance, allowing up to four load pairs is lost in the noise. (At best, there might be a 0.082% difference.) The 2-load-pair clang required 0.027% more instructions to execute versus the 4-load-pair clang, and almost 0.03% more branches. Both of these seem given the dynamic overhead of Libc's memcmp().
>
> Separably, I started writing a microbenchmark that used `llvm::StringSwitch` but it didn't feel right. Two (potentially overlapping) XMM registers can cover all values up to 32 bytes. That's big enough for the majority of real world scenarios.
>
> Overall, I've changed my mind about this proposal. I think the time and place for 4 (or more) load pairs was in the pre-vector (and therefore pre-64-bit) era, where going from 2 scalar load pairs to 4 scalar load pairs was a bigger win because the load sizes were so tiny.
>
> I suppose we could enable four load pairs on pre-SSE machines if people care. Otherwise and unless there objections, I'll close this proposal in a few days.

Thanks for running the experiments. I don't have a motivating case to change the current setting, so no objection from me. Adding Clement and Guillaume as reviewers in case they have data/thoughts.

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https://reviews.llvm.org/D69044