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cmd/compile: Initializing an empty map causes divide by zero panic at init! #11354

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mpvl opened this issue Jun 23, 2015 · 4 comments
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cmd/compile: Initializing an empty map causes divide by zero panic at init! #11354

mpvl opened this issue Jun 23, 2015 · 4 comments
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@mpvl
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mpvl commented Jun 23, 2015

Seems to be fixed in 1.5, but for the record.

http://play.golang.org/p/5R2CZTLcB5 causes a divide by zero panic.

package main

type X int // any type of size > 0 seems to cause the bug.

// Use X in a map type.
// - Using X in a slice type does not trigger the bug.
// - Variable needs to be initialized to trigger the bug.
var foo = map[int]X{}

// Use X as follows:
// - Only an array size of 8 seems to cause the bug.
var bar = map[int][8]X{
        // adding fields here doesn't fix it.
} // DIVIDE BY ZERO PANIC AT THIS LINE.

func main() {}
@randall77
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Yes, this is definitely a bug in 1.4. Some types come out of the compiler with a nonzero size but a zero alignment. At least one of the culprits was the types used for map buckets, they were assigned a width but no alignment as map buckets don't need alignment info. But somehow those types are getting shared with other types that do need alignment info. This is why [8]X triggers the bug but not [7]X or [9]X.

This is fixed in 1.5 because we use dowidth() on the map types to compute their size (Dmitry made this change for other reasons, if I remember correctly).

I'm going to add a check to 1.5 to make sure it really is fixed and stays fixed.

@randall77 randall77 added this to the Go1.4.3 milestone Jun 24, 2015
@randall77 randall77 self-assigned this Jun 24, 2015
@gopherbot
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CL https://golang.org/cl/11406 mentions this issue.

gopherbot pushed a commit that referenced this issue Jun 25, 2015
@randall77
cmd/compile/internal/gc: make sure nonzero-sized objects have nonzero…
dbe9ab6 
… alignment

See issue #11354.

Change-Id: I6817d499e6c02e4a9c83f80bc66a2bee368a69ec
Reviewed-on: https://go-review.googlesource.com/11406
Reviewed-by: Russ Cox <rsc@golang.org>
@ianlancetaylor
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Closing because fixed in 1.5.

@mikioh mikioh changed the title Initializing an empty map causes divide by zero panic at init! cmd/compile: Initializing an empty map causes divide by zero panic at init! Jul 14, 2015
@adg adg removed this from the Go1.4.3 milestone Sep 3, 2015
@golang golang locked and limited conversation to collaborators Sep 4, 2016
@gopherbot
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CL https://golang.org/cl/38468 mentions this issue.

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@ianlancetaylor @mpvl @randall77 @adg @gopherbot