[PATCH] D40177: performance improvements for ThunderX2 T99

[Stefan Teleman via Phabricator via llvm-commits]

steleman added a comment.

In https://reviews.llvm.org/D40177#935813, @fhahn wrote:

> In https://reviews.llvm.org/D40177#932950, @steleman wrote:
>
> > > Also if I understand correctly, this should have an impact on scheduling instructions using WriteAtomic, like CASB.
> >
> > It doesn't. Maybe it was supposed to, in theory, but in reality it makes no difference whatsoever.
> >
> > The only noticeable difference is in instruction cost: when Unsupported == 1, the estimated instruction cost is higher than when Unsupported == 0. Which makes no difference whatsoever in practice, given that the real cost of LSE instructions is pretty high anyway.
> >
> > We've been emitting LSE instructions with no problems for many months. Whether or not Unsupported was 0 or 1 made no difference whatsoever.
>
>
> It is probably unlikely that atomic instructions are in hot loops of most benchmarks, so I am not surprised that you do not see any impact on runtimes (assuming that's what you meant with 'it makes no difference whatsoever). However there should be cases where it should make a difference when scheduling, but it probably requires some time to come up with a test case.


That's not what I meant. What I meant has nothing to do with hot loops in SPECcpu benchmarks.

You can convince yourself of what I am saying by compiling the following test program:

https://drive.google.com/file/d/1gZ_i738yJ1TlJcQQ_oo-vTghydPUU1ZR/view?usp=sharing

(lseadd.c)

You can change around the #define for ATOMIC_MEMORY_ORDERING in the test program.

clang { -O0|-O1|-O2|-O3 } -std=c99 -mcpu=thunderx2t99 -S lseadd.c -o lseadd-t99.S
clang { -O0|-O1|-O2|-O3 } -std=c99 -mcpu=cortex-a57 -S lseadd.c -o lseadd-a57.S

T99 supports LSE Atomics. Cortex-A57 does not. For Cortex-A57, LLVM will emit LL/SC for the atomics. For T99, LLVM will always emit LSE Atomics for this test program, regardless of whether Unsupported == 0 or Unsupported == 1 in the AArch64SchedThunderX2T99.td scheduler file.

When compiling for T99, no warning is being emitted about Unsupported == 1.

Also, when compiling LLVM, no warning is emitted by llvm-tblgen about Unsupported == 0 while having LSE Atomics available.


Repository:
  rL LLVM

https://reviews.llvm.org/D40177