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[dev.regabi] cmd/compile: don't promote Byval CaptureVars if Addrtaken

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We decide during escape analysis whether to pass closure variables by
value or reference. One of the factors that's considered is whether a
variable has had its address taken.

However, this analysis is based only on the user-written source code,
whereas order+walk may introduce rewrites that take the address of a
variable (e.g., passing a uint16 key by reference to the size-generic
map runtime builtins).

Typically this would be harmless, albeit suboptimal. But in #43701 it
manifested as needing a stack object for a function where we didn't
realize we needed one up front when we generate symbols.

Probably we should just generate symbols on demand, now that those
routines are all concurrent-safe, but this is a first fix.

Thanks to Alberto Donizetti for reporting the issue, and Cuong Manh Le
for initial investigation.

Fixes #43701.

Change-Id: I16d87e9150723dcb16de7b43f2a8f3cd807a9437
Reviewed-on: https://go-review.googlesource.com/c/go/+/284075
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
This commit is contained in:
Matthew Dempsky 2021-01-15 00:39:24 -08:00
parent b7a698c73f
commit ab523fc510
2 changed files with 27 additions and 2 deletions
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11
src/cmd/compile/internal/ssagen/ssa.go
View File

@ -490,8 +490,15 @@ func buildssa(fn *ir.Func, worker int) *ssa.Func {
ptr := s.newValue1I(ssa.OpOffPtr, types.NewPtr(typ), offset, clo)
offset += typ.Size()
if n.Byval() && TypeOK(n.Type()) {
// If it is a small variable captured by value, downgrade it to PAUTO.
// If n is a small variable captured by value, promote
// it to PAUTO so it can be converted to SSA.
//
// Note: While we never capture a variable by value if
// the user took its address, we may have generated
// runtime calls that did (#43701). Since we don't
// convert Addrtaken variables to SSA anyway, no point
// in promoting them either.
if n.Byval() && !n.Addrtaken() && TypeOK(n.Type()) {
n.Class = ir.PAUTO
fn.Dcl = append(fn.Dcl, n)
s.assign(n, s.load(n.Type(), ptr), false, 0)

18
test/fixedbugs/issue43701.go Normal file
View File

@ -0,0 +1,18 @@
// compile
// Copyright 2021 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package p
func f() {
var st struct {
s string
i int16
}
_ = func() {
var m map[int16]int
m[st.i] = 0
}
}