From: Austin Clements <austin@google.com>
Date: Wed, 14 Apr 2021 03:46:18 +0000 (-0400)
Subject: runtime: minor refactoring of _rt0_amd64_lib
X-Git-Tag: go1.17beta1~629
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=3e0b1cdb5d;p=gostls13.git

runtime: minor refactoring of _rt0_amd64_lib

This function bounces between the C and Go ABIs a few times. This CL
narrows the scope of the Go -> C transition to just around the branch
that calls C. This lets us take advantage of C callee-save registers
to simplify the code a little.

Change-Id: I1ffa0b9e50325425c5ec66596978aeb6450a6b57
Reviewed-on: https://go-review.googlesource.com/c/go/+/309929
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
---

diff --git a/src/runtime/asm_amd64.s b/src/runtime/asm_amd64.s
index a88cd17e6b..1c74d70d36 100644
--- a/src/runtime/asm_amd64.s
+++ b/src/runtime/asm_amd64.s
@@ -28,10 +28,7 @@ TEXT main(SB),NOSPLIT,$-8
 // c-archive) or when the shared library is loaded (for c-shared).
 // We expect argc and argv to be passed in the usual C ABI registers
 // DI and SI.
-TEXT _rt0_amd64_lib(SB),NOSPLIT,$0x50
-	// Align stack per ELF ABI requirements.
-	MOVQ	SP, AX
-	ANDQ	$~15, SP
+TEXT _rt0_amd64_lib(SB),NOSPLIT,$0x40
 	// Save C ABI callee-saved registers, as caller may need them.
 	MOVQ	BX, 0x10(SP)
 	MOVQ	BP, 0x18(SP)
@@ -39,7 +36,6 @@ TEXT _rt0_amd64_lib(SB),NOSPLIT,$0x50
 	MOVQ	R13, 0x28(SP)
 	MOVQ	R14, 0x30(SP)
 	MOVQ	R15, 0x38(SP)
-	MOVQ	AX, 0x40(SP)
 
 	MOVQ	DI, _rt0_amd64_lib_argc<>(SB)
 	MOVQ	SI, _rt0_amd64_lib_argv<>(SB)
@@ -51,9 +47,15 @@ TEXT _rt0_amd64_lib(SB),NOSPLIT,$0x50
 	MOVQ	_cgo_sys_thread_create(SB), AX
 	TESTQ	AX, AX
 	JZ	nocgo
+
+	// We're calling back to C.
+	// Align stack per ELF ABI requirements.
+	MOVQ	SP, BX  // Callee-save in C ABI
+	ANDQ	$~15, SP
 	MOVQ	$_rt0_amd64_lib_go(SB), DI
 	MOVQ	$0, SI
 	CALL	AX
+	MOVQ	BX, SP
 	JMP	restore
 
 nocgo:
@@ -69,7 +71,6 @@ restore:
 	MOVQ	0x28(SP), R13
 	MOVQ	0x30(SP), R14
 	MOVQ	0x38(SP), R15
-	MOVQ	0x40(SP), SP
 	RET
 
 // _rt0_amd64_lib_go initializes the Go runtime.