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cmd/cgo: use go:notinheap for anonymous structs

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They can't reasonably be allocated on the heap. Not a huge deal, but
it has an interesting and useful side effect.

After CL 249917, the compiler and runtime treat pointers to
go:notinheap types as uintptrs instead of real pointers (no write
barrier, not processed during stack scanning, ...). That feature is
exactly what we want for cgo to fix #40954. All the cases we have of
pointers declared in C, but which might actually be filled with
non-pointer data, are of this form (JNI's jobject heirarch, Darwin's
CFType heirarchy, ...).

Fixes #40954

Change-Id: I44a3b9bc2513d4287107e39d0cbbd0efd46a3aae
Reviewed-on: https://go-review.googlesource.com/c/go/+/250940
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Keith Randall 2020-08-26 14:17:35 -07:00
parent 4f915911e8
commit 42b023d7b9
4 changed files with 55 additions and 1 deletions
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15
src/cmd/cgo/gcc.go
View file

@ -2448,6 +2448,18 @@ func (c *typeConv) loadType(dtype dwarf.Type, pos token.Pos, parent string) *Typ
tt := *t
tt.C = &TypeRepr{"%s %s", []interface{}{dt.Kind, tag}}
tt.Go = c.Ident("struct{}")
if dt.Kind == "struct" {
// We don't know what the representation of this struct is, so don't let
// anyone allocate one on the Go side. As a side effect of this annotation,
// pointers to this type will not be considered pointers in Go. They won't
// get writebarrier-ed or adjusted during a stack copy. This should handle
// all the cases badPointerTypedef used to handle, but hopefully will
// continue to work going forward without any more need for cgo changes.
tt.NotInHeap = true
// TODO: we should probably do the same for unions. Unions can't live
// on the Go heap, right? It currently doesn't work for unions because
// they are defined as a type alias for struct{}, not a defined type.
}
typedef[name.Name] = &tt
break
}
@ -2518,6 +2530,7 @@ func (c *typeConv) loadType(dtype dwarf.Type, pos token.Pos, parent string) *Typ
}
t.Go = name
t.BadPointer = sub.BadPointer
t.NotInHeap = sub.NotInHeap
if unionWithPointer[sub.Go] {
unionWithPointer[t.Go] = true
}
@ -2528,6 +2541,7 @@ func (c *typeConv) loadType(dtype dwarf.Type, pos token.Pos, parent string) *Typ
tt := *t
tt.Go = sub.Go
tt.BadPointer = sub.BadPointer
tt.NotInHeap = sub.NotInHeap
typedef[name.Name] = &tt
}
@ -3026,6 +3040,7 @@ func (c *typeConv) anonymousStructTypedef(dt *dwarf.TypedefType) bool {
// non-pointers in this type.
// TODO: Currently our best solution is to find these manually and list them as
// they come up. A better solution is desired.
// Note: DEPRECATED. There is now a better solution. Search for NotInHeap in this file.
func (c *typeConv) badPointerTypedef(dt *dwarf.TypedefType) bool {
if c.badCFType(dt) {
return true