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cmd/compile: optimize comparisons to min/max (u)ints by checking for overflow #41664

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josharian opened this issue Sep 27, 2020 · 0 comments
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@josharian
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@josharian josharian commented Sep 27, 2020

package p

import "math"

func f(x int64) bool {
	return x == math.MinInt64
}

On amd64, the core of this compiles to:

	0x0005 00005 (z.go:6)	MOVQ	$-9223372036854775808, CX
	0x000f 00015 (z.go:6)	CMPQ	AX, CX
	0x0012 00018 (z.go:6)	SETEQ	"".~r1+16(SP)

It would be cheaper and smaller instead to decrement CX and check the flags for underflow. A similar trick can be used for checking min and max ints and uints of all sizes. It might also be useful in the division fix-up code, where we must check for min int divisor.

cc @randall77 @dr2chase @martisch @mundaym

@josharian josharian added this to the Unplanned milestone Sep 27, 2020
@andybons andybons added the NeedsFix label Sep 29, 2020
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