[RFC] Exposing ghccc calling convention as preserve_none to clang

weiguozhi · January 8, 2024, 10:58pm

I have sent out the following patch for review.

github.com/llvm/llvm-project

New calling convention preserve_none

llvm:main ← weiguozhi:carrot-preserve-none

opened 09:46PM - 03 Jan 24 UTC

weiguozhi

+419 -3

The new experimental calling convention preserve_none is the opposite side of ex…isting preserve_all. It tries to preserve as few general registers as possible. So all general registers are caller saved registers. It can also uses more general registers to pass arguments. This attribute doesn't impact floating-point registers. Floating-point registers still follow the c calling convention. Currently preserve_none is supported on X86-64 only. It changes the c calling convention in following fields: * RSP and RBP are the only preserved general registers, all other general registers are caller saved registers. * We can use [RDI, RSI, RDX, RCX, R8, R9, R11, R12, R13, R14, R15, RAX] to pass arguments. It can improve the performance of hot tailcall chain, because many callee saved registers' save/restore instructions can be removed if the tail functions are using preserve_none. In my experiment in protocol buffer, the parsing functions are improved by 3% to 10%.

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[RFC] Exposing ghccc calling convention as preserve_none to clang

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