Commit 6139019

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cmd/compile: pick position of implicit break statements more carefully

The previous version used the position of the switch statement, which makes for potentially jumpy stepping and introduces a large number of statements repeating the line (tricky for inserting breaks). It also shared a single OBREAK node and this was not really a syntax "tree". This improves both the nostmt test (by 6 lines) and reduces the total badness score from dwarf-goodness (by about 200). Change-Id: I1f71b231a26f152bdb6ce9bc8f95828bb222f665 Reviewed-on: https://go-review.googlesource.com/c/go/+/188218 Run-TryBot: David Chase <drchase@google.com> Reviewed-by: Jeremy Faller <jeremy@golang.org>

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+10

-4

lines changed

src/cmd/compile/internal/gc
- swt.go

1 file changed

+10

-4

lines changed

`‎src/cmd/compile/internal/gc/swt.go`

Lines changed: 10 additions & 4 deletions

Original file line number	Diff line number	Diff line change
`@@ -268,7 +268,6 @@ func walkExprSwitch(sw *Node) {`
`268`	`268`	`exprname: cond,`
`269`	`269`	`}`
`270`	`270`
`271`		`- br := nod(OBREAK, nil, nil)`
`272`	`271`	`var defaultGoto *Node`
`273`	`272`	`var body Nodes`
`274`	`273`	`for _, ncase := range sw.List.Slice() {`
`@@ -290,13 +289,17 @@ func walkExprSwitch(sw *Node) {`
`290`	`289`	`// Process body.`
`291`	`290`	`body.Append(npos(ncase.Pos, nodSym(OLABEL, nil, label)))`
`292`	`291`	`body.Append(ncase.Nbody.Slice()...)`
`293`		`- if !hasFall(ncase.Nbody.Slice()) {`
	`292`	`+ if fall, pos := hasFall(ncase.Nbody.Slice()); !fall {`
	`293`	`+ br := nod(OBREAK, nil, nil)`
	`294`	`+ br.Pos = pos`
`294`	`295`	`body.Append(br)`
`295`	`296`	`}`
`296`	`297`	`}`
`297`	`298`	`sw.List.Set(nil)`
`298`	`299`
`299`	`300`	`if defaultGoto == nil {`
	`301`	`+ br := nod(OBREAK, nil, nil)`
	`302`	`+ br.Pos = br.Pos.WithNotStmt()`
`300`	`303`	`defaultGoto = br`
`301`	`304`	`}`
`302`	`305`
`@@ -469,7 +472,7 @@ func allCaseExprsAreSideEffectFree(sw *Node) bool {`
`469`	`472`	`}`
`470`	`473`
`471`	`474`	`// hasFall reports whether stmts ends with a "fallthrough" statement.`
`472`		`-func hasFall(stmts []*Node) bool {`
	`475`	`+func hasFall(stmts []*Node) (bool, src.XPos) {`
`473`	`476`	`// Search backwards for the index of the fallthrough`
`474`	`477`	`// statement. Do not assume it'll be in the last`
`475`	`478`	`// position, since in some cases (e.g. when the statement`
`@@ -480,7 +483,10 @@ func hasFall(stmts []*Node) bool {`
`480`	`483`	`for i >= 0 && stmts[i].Op == OVARKILL {`
`481`	`484`	`i--`
`482`	`485`	`}`
`483`		`- return i >= 0 && stmts[i].Op == OFALL`
	`486`	`+ if i < 0 {`
	`487`	`+ return false, src.NoXPos`
	`488`	`+ }`
	`489`	`+ return stmts[i].Op == OFALL, stmts[i].Pos`
`484`	`490`	`}`
`485`	`491`
`486`	`492`	`// walkTypeSwitch generates an AST that implements sw, where sw is a`

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Commit 6139019

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`‎src/cmd/compile/internal/gc/swt.go`

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