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runtime: minor refactoring of _rt0_amd64_lib

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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>
This commit is contained in:
Austin Clements 2021-04-13 23:46:18 -04:00
parent b1c4cc5589
commit 3e0b1cdb5d
1 changed files with 7 additions and 6 deletions
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13
src/runtime/asm_amd64.s
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@ -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.