cmd/compile: remove large intermediate slice from gc.scopePCs

Three loops can be converted into one.
Minor reviewer-recommended refactoring.
Passes toolstash-check.

Updates golang#27739.

Change-Id: Ia87a11d88ae3ce56fcc4267fe6c5a9c13bf7f533
Reviewed-on: https://go-review.googlesource.com/c/go/+/176577
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>

src/cmd/compile/internal/gc/scope.go

@@ -55,51 +55,25 @@ func scopeVariables(dwarfVars []*dwarf.Var, varScopes []ScopeID, dwarfScopes []d
 	}
 }
 
-// A scopedPCs represents a non-empty half-open interval of PCs that
-// share a common source position.
-type scopedPCs struct {
-	start, end int64
-	pos        src.XPos
-	scope      ScopeID
-}
-
 // scopePCs assigns PC ranges to their scopes.
 func scopePCs(fnsym *obj.LSym, marks []Mark, dwarfScopes []dwarf.Scope) {
 	// If there aren't any child scopes (in particular, when scope
 	// tracking is disabled), we can skip a whole lot of work.
 	if len(marks) == 0 {
 		return
 	}
-
-	// Break function text into scopedPCs.
-	var pcs []scopedPCs
 	p0 := fnsym.Func.Text
+	scope := findScope(marks, p0.Pos)
 	for p := fnsym.Func.Text; p != nil; p = p.Link {
 		if p.Pos == p0.Pos {
 			continue
 		}
-		if p0.Pc < p.Pc {
-			pcs = append(pcs, scopedPCs{start: p0.Pc, end: p.Pc, pos: p0.Pos})
-		}
+		dwarfScopes[scope].AppendRange(dwarf.Range{Start: p0.Pc, End: p.Pc})
 		p0 = p
+		scope = findScope(marks, p0.Pos)
 	}
 	if p0.Pc < fnsym.Size {
-		pcs = append(pcs, scopedPCs{start: p0.Pc, end: fnsym.Size, pos: p0.Pos})
-	}
-
-	// Assign scopes to each chunk of instructions.
-	for i := range pcs {
-		pcs[i].scope = findScope(marks, pcs[i].pos)
-	}
-
-	// Create sorted PC ranges for each DWARF scope.
-	for _, pc := range pcs {
-		r := &dwarfScopes[pc.scope].Ranges
-		if i := len(*r); i > 0 && (*r)[i-1].End == pc.start {
-			(*r)[i-1].End = pc.end
-		} else {
-			*r = append(*r, dwarf.Range{Start: pc.start, End: pc.end})
-		}
+		dwarfScopes[scope].AppendRange(dwarf.Range{Start: p0.Pc, End: fnsym.Size})
 	}
 }
 

src/cmd/internal/dwarf/dwarf.go

@@ -145,6 +145,20 @@ func (s *Scope) UnifyRanges(c *Scope) {
 	s.Ranges = out
 }
 
+// AppendRange adds r to s, if r is non-empty.
+// If possible, it extends the last Range in s.Ranges; if not, it creates a new one.
+func (s *Scope) AppendRange(r Range) {
+	if r.End <= r.Start {
+		return
+	}
+	i := len(s.Ranges)
+	if i > 0 && s.Ranges[i-1].End == r.Start {
+		s.Ranges[i-1].End = r.End
+		return
+	}
+	s.Ranges = append(s.Ranges, r)
+}
+
 type InlCalls struct {
 	Calls []InlCall
 }