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cmd/compile: large variable not initialized to 0 #42054

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zephyrtronium opened this issue Oct 18, 2020 · 8 comments
Closed

cmd/compile: large variable not initialized to 0 #42054

zephyrtronium opened this issue Oct 18, 2020 · 8 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
Milestone
Go1.16

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@zephyrtronium
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What version of Go are you using (go version)?

Initially spotted on playground (1.14.9), but also reproduces with 1.15.2 and:

$ go version
go version devel +3036b76df0 Sat Oct 10 16:06:07 2020 +0000 windows/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output
PS E:\go> go env
set GO111MODULE=on
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\zephyr\AppData\Local\go-build
set GOENV=C:\Users\zephyr\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=E:\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=E:\go
set GOPRIVATE=
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=E:\gotip
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=E:\gotip\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=NUL
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\zephyr\AppData\Local\Temp\go-build322924098=/tmp/go-build -gno-record-gcc-switches

What did you do?

Create a large array and print the first few elements of it:

https://play.golang.org/p/66eTJdT8Ctu

What did you expect to see?

Either a slice of 0s, or a compilation error. (I was originally trying to demonstrate an error that can be reported by dowidth. An error does occur when the size of the array is greater than 2e9 but less than 1<<32, but it originates elsewhere.)

What did you see instead?

[1 1 0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 1 0 1], with other apparently random combinations of 1s and 0s for other sizes. Testing locally, I also see different combinations on the same size between my (slightly dated) copy of tip and 1.15.2.
@ianlancetaylor
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CC @randall77 @mdempsky

@ianlancetaylor ianlancetaylor added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Oct 19, 2020
@ianlancetaylor ianlancetaylor added this to the Go1.16 milestone Oct 19, 2020
@mdempsky
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I think this is actually a linker issue. Looking at go tool compile -S output, it looks like we're correctly emitting code to generate a 4GiB variable. It seems like the linker is truncating the size to 32-bits somewhere. I thought it was here:

go/src/cmd/internal/obj/objfile.go

Line 334 in 9499a2e

o.SetSiz(uint32(s.Size))

but that code was newly merged from the dev.link branch, and the issue reproduces identically under Go 1.15 (including the compiler's -S output looking right).

/cc @cherrymui @thanm

@cherrymui
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but that code was newly merged from the dev.link branch, and the issue reproduces identically under Go 1.15 (including the compiler's -S output looking right).

Go 1.15 has the same code as well (in a different place).

Go 1.14 rejects it.

main.x: symbol too large (4294967296 bytes)
too much data in section SNOPTRBSS (over 2e+09 bytes)

We never supported symbol larger than 2GB (issue #9862), so the object file uses 32-bit for symbol size. I'll add a check before truncation.

@ALTree ALTree changed the title cmd/compile: Large variable not initialized to 0 cmd/compile: large variable not initialized to 0 Oct 19, 2020
@go101
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go101 commented Oct 19, 2020 •
edited

It looks that gc 1.15.3 rejects var x [1<<31]byte but accepts var x [1<<32]byte.

[edit]: it looks 31 is the only rejected number. And it is ok when x is declared locally.

@gopherbot
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Change https://golang.org/cl/263641 mentions this issue: cmd/internal/obj: reject too large symbols

@go101
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go101 commented Oct 19, 2020 •
edited

What is the difficulty to declare very large sized arrays?
If var x = make([]byte, 1<<32) can be allocated without problem, then why to limit the length of package-level arrays?

@go101
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go101 commented Oct 20, 2020

It looks the fix makes the old code "var x [1<<31]byte" fail to compile now. An incompatibility?

@go101
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go101 commented Oct 20, 2020

Sorry, I forgot "var x [1<<31]byte" doesn't compile before. ;)

@golang golang locked and limited conversation to collaborators Oct 20, 2021
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@mdempsky @zephyrtronium @ianlancetaylor @gopherbot @cherrymui @go101