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cmd/compile: add flag to FOR/RANGE to preserve loop semantics across inlines

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This modifies the loopvar change to be tied to the
package if it is specified that way, and preserves
the change across inlining.

Down the road, this will be triggered (and flow correctly)
if the changed semantics are tied to Go version specified
in go.mod (or rather, for the compiler, by the specified
version for compilation).

Includes tests.

Change-Id: If54e8b6dd23273b86be5ba47838c90d38af9bd1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/463595
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
This commit is contained in:
David Chase 2023-01-25 17:08:16 -05:00
parent c20d959163
commit 5a7793b7b8
11 changed files with 112 additions and 34 deletions
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27
src/cmd/compile/internal/loopvar/loopvar.go
View file

@ -21,10 +21,14 @@ import (
// subject to this change, that may (once transformed) be heap allocated in the
// process. (This allows checking after escape analysis to call out any such
// variables, in case it causes allocation/performance problems).
// For this code, the meaningful debug and hash flag settings
//
// base.Debug.LoopVar <= 0 => do not transform
// The decision to transform loops is normally encoded in the For/Range loop node
// field DistinctVars but is also dependent on base.LoopVarHash, and some values
// of base.Debug.LoopVar (which is set per-package). Decisions encoded in DistinctVars
// are preserved across inlining, so if package a calls b.F and loops in b.F are
// transformed, then they are always transformed, whether b.F is inlined or not.
//
// Per-package, the debug flag settings that affect this transformer:
//
// base.LoopVarHash != nil => use hash setting to govern transformation.
// note that LoopVarHash != nil sets base.Debug.LoopVar to 1 (unless it is >= 11, for testing/debugging).
@ -32,13 +36,7 @@ import (
// base.Debug.LoopVar == 11 => transform ALL loops ignoring syntactic/potential escape. Do not log, can be in addition to GOEXPERIMENT.
//
// The effect of GOEXPERIMENT=loopvar is to change the default value (0) of base.Debug.LoopVar to 1 for all packages.
func ForCapture(fn *ir.Func) []*ir.Name {
if base.Debug.LoopVar <= 0 { // code in base:flags.go ensures >= 1 if loopvarhash != ""
// TODO remove this when the transformation is made sensitive to inlining; this is least-risk for 1.21
return nil
}
// if a loop variable is transformed it is appended to this slice for later logging
var transformed []*ir.Name
@ -86,7 +84,7 @@ func ForCapture(fn *ir.Func) []*ir.Name {
}
// scanChildrenThenTransform processes node x to:
// 1. if x is a for/range, note declared iteration variables possiblyLeaked (PL)
// 1. if x is a for/range w/ DistinctVars, note declared iteration variables possiblyLeaked (PL)
// 2. search all of x's children for syntactically escaping references to v in PL,
// meaning either address-of-v or v-captured-by-a-closure
// 3. for all v in PL that had a syntactically escaping reference, transform the declaration
@ -122,7 +120,9 @@ func ForCapture(fn *ir.Func) []*ir.Name {
}
case *ir.RangeStmt:
if !x.Def {
if !(x.Def && x.DistinctVars) {
// range loop must define its iteration variables AND have distinctVars.
x.DistinctVars = false
break
}
noteMayLeak(x.Key)
@ -130,9 +130,13 @@ func ForCapture(fn *ir.Func) []*ir.Name {
ir.DoChildren(n, scanChildrenThenTransform)
x.Key = maybeReplaceVar(x.Key, x)
x.Value = maybeReplaceVar(x.Value, x)
x.DistinctVars = false
return false
case *ir.ForStmt:
if !x.DistinctVars {
break
}
forAllDefInInit(x, noteMayLeak)
ir.DoChildren(n, scanChildrenThenTransform)
var leaked []*ir.Name
@ -335,6 +339,7 @@ func ForCapture(fn *ir.Func) []*ir.Name {
// (11) post' = {}
x.Post = nil
}
x.DistinctVars = false
return false
}