From c166a43c6e6157b1309ea757324cc0a71c078e66 Mon Sep 17 00:00:00 2001
From: weiguozhi <57237827+weiguozhi@users.noreply.github.com>
Date: Mon, 5 Feb 2024 13:28:43 -0800
Subject: New calling convention preserve_none (#76868)

The new experimental calling convention preserve_none is the opposite
side of existing 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%.
---
 llvm/docs/LangRef.rst | 7 +++++++
 1 file changed, 7 insertions(+)

(limited to 'llvm/docs/LangRef.rst')

diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 96bdba5..fd2e3aa 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -421,6 +421,13 @@ added in the future:
     This calling convention, like the `PreserveMost` calling convention, will be
     used by a future version of the ObjectiveC runtime and should be considered
     experimental at this time.
+"``preserve_nonecc``" - The `PreserveNone` calling convention
+    This calling convention doesn't preserve any general registers. So all
+    general registers are caller saved registers. It also uses all general
+    registers to pass arguments. This attribute doesn't impact non-general
+    purpose registers (e.g. floating point registers, on X86 XMMs/YMMs).
+    Non-general purpose registers still follow the standard c calling
+    convention. Currently it is for x86_64 only.
 "``cxx_fast_tlscc``" - The `CXX_FAST_TLS` calling convention for access functions
     Clang generates an access function to access C++-style TLS. The access
     function generally has an entry block, an exit block and an initialization
-- 
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