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math/big: write t*10 to multiply t by 10

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The compiler has caught up.
In fact the compiler is ahead; it knows about a magic multiply-by-5 instruction:

	// compute '0' + byte(r - t*10) in AX
	MOVQ	t, AX
	LEAQ	(AX)(AX*4), AX
	SHLQ	$1, AX
	MOVQ	r, CX
	SUBQ	AX, CX
	LEAL	48(CX), AX

For comparison, the shifty version compiles to:

	// compute '0' + byte(r - t*10) in AX
	MOVQ	t, AX
	MOVQ	AX, CX
	SHLQ	$3, AX
	MOVQ	r, DX
	SUBQ	AX, DX
	SUBQ	CX, DX
	SUBQ	CX, DX
	LEAL	48(DX), AX

Fixes #2671.

Change-Id: Ifbf23dbfeb19c0bb020fa44eb2f025943969fb6b
Reviewed-on: https://go-review.googlesource.com/23372
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Russ Cox 2016-05-24 02:45:11 -04:00
parent 7a9f6c2b56
commit ab4414773e
1 changed files with 1 additions and 1 deletions
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2
src/math/big/natconv.go
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@ -391,7 +391,7 @@ func (q nat) convertWords(s []byte, b Word, ndigits int, bb Word, table []diviso
// this appears to be faster for BenchmarkString10000Base10
// and smaller strings (but a bit slower for larger ones)
t := r / 10
s[i] = '0' + byte(r-t<<3-t-t) // TODO(gri) replace w/ t*10 once compiler produces better code
s[i] = '0' + byte(r-t*10)
r = t
}
}