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.
