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cmd/compile: incorrect loop variable detection #53653

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randall77 opened this issue Jul 1, 2022 · 2 comments
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cmd/compile: incorrect loop variable detection #53653

randall77 opened this issue Jul 1, 2022 · 2 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done.
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Go1.19

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@randall77
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func main() {
	for i := math.MinInt64 + 2; i >= math.MinInt64; i-- {
		if i == math.MaxInt64 {
			break
		}
		println(i)
	}
}

This program will loop forever when it shouldn't. You can "fix" it by passing -gcflags=-N.

Similar to #53600, this is a bug in induction variable detection. Just reversing the direction when step is negative in cmd/compile/internal/ssa/loopbce.go (added in CL 104041 ) isn't quite right. The "distance until overflow" isn't preserved under mirroring.

@dr2chase @rasky
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Change https://go.dev/cl/415854 mentions this issue: cmd/compile: fix prove pass when lower condition is >= minint

@randall77 randall77 mentioned this issue Jul 2, 2022
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Change https://go.dev/cl/415874 mentions this issue: cmd/compile: rework induction variable detector

@dr2chase dr2chase added the NeedsFix The path to resolution is known, but the work has not been done. label Jul 3, 2022
@dmitshur dmitshur added this to the Go1.19 milestone Jul 6, 2022
jproberts pushed a commit to jproberts/go that referenced this issue Aug 10, 2022
@randall77 @jproberts
cmd/compile: rework induction variable detector
e2abf12 
Induction variable detection is still not quite right. I've added
another failing test.

Redo the overflow/underflow detector so it is more obviously correct.

Update golang#53600
Fixes golang#53653
Fixes golang#53663

Change-Id: Id95228e282fdbf6bd80b26e1c41d62e935ba08ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/415874
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: David Chase <drchase@google.com>
@golang golang locked and limited conversation to collaborators Jul 6, 2023
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