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cmd/compile: unsafe.Sizeof with generic type parameter causes panic #47716

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tdakkota opened this issue Aug 16, 2021 · 6 comments
Closed

cmd/compile: unsafe.Sizeof with generic type parameter causes panic #47716

tdakkota opened this issue Aug 16, 2021 · 6 comments
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@tdakkota
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@tdakkota tdakkota commented Aug 16, 2021

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

$ go version
go version devel go1.18-4d3cc84774 Thu Aug 12 20:22:27 2021 +0000 windows/amd64

Does this issue reproduce with the latest release?

No.

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

go env Output
$ go env
set GO111MODULE=auto
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\tdakkota\AppData\Local\go-build
set GOENV=C:\Users\tdakkota\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\tdakkota\go\pkg\mod
set GOOS=windows
set GOPATH=C:\Users\tdakkota\go
set GOPROXY=https://proxy.golang.org,direct
set GOROOT=C:\Users\tdakkota\GolandProjects\gh\go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Users\tdakkota\GolandProjects\gh\go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=devel go1.18-4d3cc84774 Thu Aug 12 20:22:27 2021 +0000
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
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\tdakkota\AppData\Local\Temp\go-build471959828=/tmp/go-build -gno-record-gcc-switches

What did you do?

https://go2goplay.golang.org/p/f14OcREOTZ9

What did you expect to see?

Successful compilation or some compiler error.

What did you see instead?

panic: <nil> not an Int

goroutine 1 [running]:
go/constant.Int64Val({0x0, 0x0})
	/root/master/go/src/go/constant/value.go:504 +0x119
cmd/compile/internal/noder.(*irgen).validateBuiltin(0xc0000a6300, {0xc000024f96, 0x6}, 0xc0000897c0)
	/root/master/go/src/cmd/compile/internal/noder/validate.go:84 +0xd7
cmd/compile/internal/noder.(*irgen).validate(0xc0000a6300, {0xe49e80, 0xc0000897c0})
	/root/master/go/src/cmd/compile/internal/noder/validate.go:70 +0x157
cmd/compile/internal/noder.(*irgen).generate.func1({0xe49e80, 0xc0000897c0})
	/root/master/go/src/cmd/compile/internal/noder/irgen.go:259 +0x30
cmd/compile/internal/syntax.Crawl.func1({0xe49e80, 0xc0000897c0})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:41 +0x30
cmd/compile/internal/syntax.inspector.Visit(0xc00002ee80, {0xe49e80, 0xc0000897c0})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:24 +0x31
cmd/compile/internal/syntax.walker.node({{0xe471c0, 0xc00002ee80}}, {0xe49e80, 0xc0000897c0})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:76 +0x6b
cmd/compile/internal/syntax.walker.node({{0xe471c0, 0xc00002ee80}}, {0xe4a308, 0xc000353f00})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:267 +0x165c
cmd/compile/internal/syntax.walker.stmtList({{0xe471c0, 0xc00002ee80}}, {0xc000353f80, 0x2, 0xc000353e80})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:348 +0x91
cmd/compile/internal/syntax.walker.node({{0xe471c0, 0xc00002ee80}}, {0xe49e30, 0xc000089780})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:238 +0x13f2
cmd/compile/internal/syntax.walker.node({{0xe471c0, 0xc00002ee80}}, {0xe4a0b0, 0xc000065c80})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:125 +0x949
cmd/compile/internal/syntax.walker.declList({{0xe471c0, 0xc00002ee80}}, {0xc0000ba400, 0x6, 0xc0000976d8})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:336 +0x91
cmd/compile/internal/syntax.walker.node({{0xe471c0, 0xc00002ee80}}, {0xe4a060, 0xc000377a90})
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:85 +0xfee
cmd/compile/internal/syntax.Walk(...)
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:57
cmd/compile/internal/syntax.Inspect(...)
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:18
cmd/compile/internal/syntax.Crawl({0xe4a060, 0xc000377a90}, 0xc00002ee70)
	/root/master/go/src/cmd/compile/internal/syntax/walk.go:40 +0x75
cmd/compile/internal/noder.(*irgen).generate(0xc0000a6300, {0xc00000e448, 0x1, 0x203000})
	/root/master/go/src/cmd/compile/internal/noder/irgen.go:258 +0x1dd
cmd/compile/internal/noder.check2({0xc00000e448, 0x1, 0x1})
	/root/master/go/src/cmd/compile/internal/noder/irgen.go:92 +0x16d
cmd/compile/internal/noder.LoadPackage({0xc00001e260, 0x1, 0x0})
	/root/master/go/src/cmd/compile/internal/noder/noder.go:90 +0x365
cmd/compile/internal/gc.Main(0xd19b48)
	/root/master/go/src/cmd/compile/internal/gc/main.go:190 +0xaf3
main.main()
	/root/master/go/src/cmd/compile/main.go:55 +0xdd

It seems g.info.Types[call].Value is nil here, but g.info.Types[call] is not zero value.

got, ok := constant.Int64Val(g.info.Types[call].Value)

@findleyr
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@findleyr findleyr commented Aug 16, 2021

Thanks for the report, and the investigation.

I'm guessing this is a consequence of https://golang.org/cl/335413; Sizeof is no longer necessarily constant.

CC @griesemer @mdempsky

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@mknyszek mknyszek added this to the Go1.18 milestone Aug 16, 2021
@griesemer
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@griesemer griesemer commented Aug 16, 2021

Simpler reproducer:

package p

import "unsafe"

func _[T any](x T) {
	_ = unsafe.Sizeof(x)
}

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@griesemer
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@griesemer griesemer commented Aug 16, 2021

Here's a partial fix that avoids the crash. But we should probably check that the compiler's size computation agrees:

diff --git a/src/cmd/compile/internal/noder/validate.go b/src/cmd/compile/internal/noder/validate.go
index 68a059b96f..bc9092b8f3 100644
--- a/src/cmd/compile/internal/noder/validate.go
+++ b/src/cmd/compile/internal/noder/validate.go
@@ -81,7 +81,16 @@ func (g *irgen) validateBuiltin(name string, call *syntax.CallExpr) {
                // Check that types2+gcSizes calculates sizes the same
                // as cmd/compile does.
 
-               got, ok := constant.Int64Val(g.info.Types[call].Value)
+               tv := g.info.Types[call]
+               if !tv.IsValue() {
+                       base.FatalfAt(g.pos(call), "expected a value")
+               }
+
+               if tv.Value == nil {
+                       break // unsafe op is not a constant
+               }
+
+               got, ok := constant.Int64Val(tv.Value)
                if !ok {
                        base.FatalfAt(g.pos(call), "expected int64 constant value")
                }

cc: @danscales

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@mdempsky
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@mdempsky mdempsky commented Aug 16, 2021

To check, the current status is that unsafe.{Align,Size,Offset}of will now have type uintptr but not necessarily be constant, if the argument is a type parameter or derived from one? E.g., unsafe.Sizeof([]T(nil)) or unsafe.Offsetof(struct{a T; b byte}{}.b)?

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@griesemer
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@griesemer griesemer commented Aug 16, 2021

Per the current (in-progress) spec:

A (variable of) type T has variable size if T
is a type parameter, or if it is an array or struct type containing elements
or fields of variable size. Otherwise the size is constant.

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@gopherbot
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@gopherbot gopherbot commented Aug 22, 2021

Change https://golang.org/cl/344250 mentions this issue: cmd/compile: fixes for non-constant Sizeof/Alignof

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