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runtime: use correct constant when computing nsec remainder

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A code comment on amd64 for windows and plan9 contained a snippet for
splitting apart the sec and nsec components of a unix timestamp, with
produced assembly below, which was then cleaned up by hand. When arm64
was ported, that code snippet in the comment went through the compiler
to produce some code that was then pasted and cleaned up. Unfortunately,
the comment had a typo in it, containing 8 zeros instead of 9.

This resulted in the constant used in the assembly being wrong, spotted
by @bufflig's eagle eyes. So, this commit fixes the comment on all three
platforms, and the assembly on windows/arm64.

Fixes #48072.

Change-Id: I786fe89147328b0d25544f52c927ddfdb9f6f1cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/361474
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Patrik Nyblom <pnyb@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
This commit is contained in:
Jason A. Donenfeld 2021-11-05 00:10:31 +01:00
parent 256a8fc6ef
commit 0e5f287fde
3 changed files with 5 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
src/runtime/sys_plan9_amd64.s
View File

@ -94,7 +94,7 @@ TEXT runtime·walltime(SB),NOSPLIT,$8-12
MOVQ 0(SP), AX
// generated code for
// func f(x uint64) (uint64, uint64) { return x/1000000000, x%100000000 }
// func f(x uint64) (uint64, uint64) { return x/1000000000, x%1000000000 }
// adapted to reduce duplication
MOVQ AX, CX
MOVQ $1360296554856532783, AX

2
src/runtime/time_windows_amd64.s
View File

@ -25,7 +25,7 @@ TEXT time·now(SB),NOSPLIT,$0-24
IMULQ $100, AX
// generated code for
// func f(x uint64) (uint64, uint64) { return x/1000000000, x%100000000 }
// func f(x uint64) (uint64, uint64) { return x/1000000000, x%1000000000 }
// adapted to reduce duplication
MOVQ AX, CX
MOVQ $1360296554856532783, AX

9
src/runtime/time_windows_arm64.s
View File

@ -32,17 +32,14 @@ TEXT time·now(SB),NOSPLIT|NOFRAME,$0-24
// Code stolen from compiler output for:
//
// var x uint64
// func f() (sec uint64, nsec uint32) { return x / 1000000000, uint32(x % 100000000) }
// func f() (sec uint64, nsec uint32) { return x / 1000000000, uint32(x % 1000000000) }
//
LSR $1, R0, R1
MOVD $-8543223759426509416, R2
UMULH R2, R1, R1
UMULH R1, R2, R1
LSR $28, R1, R1
MOVD R1, sec+0(FP)
MOVD $-6067343680855748867, R1
UMULH R0, R1, R1
LSR $26, R1, R1
MOVD $100000000, R2
MOVD $1000000000, R2
MSUB R1, R0, R2, R0
MOVW R0, nsec+8(FP)
RET