Thanks for raising this issue. Looks like no one checked the assembly that's being generated on Windows.
I agree that we should do something about this. As both the preserve_most and preserve_all calling conventions are still experimental anyway, I think they can still both be changed in LLVM. Note that also the preserve_most attribute had issues originally, which Anton (CCed) fixed in https://reviews.llvm.org/D141020. My personal preference for a fix would be either (3) or (5), but both would need fixes on the LLVM side. If that doesn't work we can also do (2) in the meantime. On Mon, Nov 20, 2023 at 5:33 PM 'seth.b...@microsoft.com' via v8-dev < v8-dev@googlegroups.com> wrote: > Hi everyone, > > I noticed some unfortunate disassembly recently and am curious about > whether y'all are aware of this problem or have any ideas about how to fix > it. > > Problem description: > > The Clang preserve_most attribute defines a new calling convention where > all floating-point registers are caller-saved. This is a mismatch with the > normal Windows calling convention, and causes any caller of a > V8_PRESERVE_MOST function to save and restore xmm6 through xmm15, at a cost > of 126 bytes of extra instructions per calling function. When there are > many callers to a V8_PRESERVE_MOST function, this size cost adds up: for > example, there are many hundreds of instantiations of the template > AllocationDispatcher::Invoke in Chromium, and each one calls a > V8_PRESERVE_MOST function. I tried disabling V8_PRESERVE_MOST and found > that it reduced the size of a release build of chrome.dll by a whopping 2 > MB. > > The preserve_most calling convention is working correctly, as defined at > https://clang.llvm.org/docs/AttributeReference.html#preserve-most . > However, it's a bad fit for Windows. This problem has been raised in the > LLVM discussion forum: > https://discourse.llvm.org/t/conv-c-and-conv-preservemost-mix-badly-on-windows-x64/73054 > . > > The discussion question: what should we do about this? I could imagine a > few answers: > > 1. Do nothing; it's working fine and we're okay with the code-size cost > 2. Disable preserve_most for Windows builds, since the cost of saving and > restoring all of those floating-point registers outweighs the benefit of > avoiding similar stack access for general-purpose registers > 3. Redefine preserve_most in a platform-specific way so it treats xmm6 > through xmm15 as callee-save on Windows > 4. Define a new Clang attribute preserve_most_win which is like > preserve_most but with Windows semantics for floating-point registers > 5. Use preserve_all instead on Windows, to declare that all floating-point > registers are callee-save. This is appealing because it pushes the cost of > saving and restoring into the cold function, but when I tried it, I got a > bunch of internal compiler errors. > > Best, > Seth > > -- > -- > v8-dev mailing list > v8-dev@googlegroups.com > http://groups.google.com/group/v8-dev > --- > You received this message because you are subscribed to the Google Groups > "v8-dev" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to v8-dev+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/v8-dev/3e5b967d-553e-40dd-ad7b-4a606d750f85n%40googlegroups.com > <https://groups.google.com/d/msgid/v8-dev/3e5b967d-553e-40dd-ad7b-4a606d750f85n%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- Clemens Backes Software Engineer cleme...@google.com Google Germany GmbH Erika-Mann-Straße 33 80636 München Geschäftsführer: Paul Manicle, Liana Sebastian Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Diese E-Mail ist vertraulich. Falls sie diese fälschlicherweise erhalten haben sollten, leiten Sie diese bitte nicht an jemand anderes weiter, löschen Sie alle Kopien und Anhänge davon und lassen Sie mich bitte wissen, dass die E-Mail an die falsche Person gesendet wurde. This e-mail is confidential. If you received this communication by mistake, please don't forward it to anyone else, please erase all copies and attachments, and please let me know that it has gone to the wrong person. -- -- v8-dev mailing list v8-dev@googlegroups.com http://groups.google.com/group/v8-dev --- You received this message because you are subscribed to the Google Groups "v8-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to v8-dev+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/v8-dev/CAGO%3DqhCmOATMphOJtsRPV0XY2Sm%3Dwj%3D0v8_VOtc9YQr152NMFw%40mail.gmail.com.
