Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

runtime: mask bucket shift on s390x? #31165

Closed
josharian opened this issue Mar 30, 2019 · 3 comments

Comments

Projects
None yet
3 participants
@josharian
Copy link
Contributor

commented Mar 30, 2019

// bucketShift returns 1<<b, optimized for code generation.
func bucketShift(b uint8) uintptr {
	if sys.GoarchAmd64|sys.GoarchAmd64p32|sys.Goarch386 != 0 {
		b &= sys.PtrSize*8 - 1 // help x86 archs remove shift overflow checks
	}
	return uintptr(1) << b
}

IIRC, s390x also makes use of masks to remove shift overflow checks. @mundaym care to send a CL adding s390x if that's correct? (And close this issue if not?)

@josharian josharian added this to the Go1.13 milestone Mar 30, 2019

@mundaym

This comment has been minimized.

Copy link
Member

commented Mar 31, 2019

Yes, s390x does (and so does ppc64{,le} as well actually). My vote would be just to get rid of the if statement since in the worst case it just adds one extra AND instruction, and most architectures can generate better code with the mask even if they don't currently.

If we are particularly worried about this code path for some reason we could add an extra bunch of generic rules to detect this pattern and turn it into a rotate instruction, since rotate and shift are equivalent operations when the input is 1 and the shift amount is masked to the bit-width of the input.

@mundaym

This comment has been minimized.

Copy link
Member

commented Mar 31, 2019

Something like this would do the trick I think:

(Lsh32x(8|16|32|64) x:(Const32 [1]) (And(8|16|32|64) (Const(8|16|32|64) [31]) y)) -> (RotateLeft32 x y)
(Lsh64x(8|16|32|64) x:(Const64 [1]) (And(8|16|32|64) (Const(8|16|32|64) [63]) y)) -> (RotateLeft64 x y)
@gopherbot

This comment has been minimized.

Copy link

commented Mar 31, 2019

Change https://golang.org/cl/170122 mentions this issue: runtime: always mask shift amount regardless of architecture

@gopherbot gopherbot closed this in 7b62e98 Mar 31, 2019

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
You can’t perform that action at this time.