go/ir: consider fake exits when building (post-)dominator frontier

We correctly considered fake exits when building the (post-)dominator
tree, but forgot to do so when calculating the frontier. This would
lead to missing phi and sigma nodes, ultimately resulting in returning
wrong values.

Closes gh-949

(cherry picked from commit 390eb5b)

Author: dominikh

go/ir/builder_test.go

@@ -482,7 +482,7 @@ func h(error)
 			}
 		}
 	}
-	if expected := 1; phis != expected {
+	if expected := 3; phis != expected {
 		g.WriteTo(os.Stderr)
 		t.Errorf("expected %d Phi nodes (for the range index), got %d", expected, phis)
 	}

go/ir/dom.go

@@ -423,6 +423,10 @@ func printDomTreeDot(buf io.Writer, f *Function) {
 		for _, pred := range b.Preds {
 			fmt.Fprintf(buf, "\tn%d -> n%d [style=\"dotted\",weight=0];\n", pred.dom.pre, v.pre)
 		}
+
+		if f.fakeExits.Has(b) {
+			fmt.Fprintf(buf, "\tn%d -> n%d [style=\"dotted\",weight=0,color=red];\n", b.dom.pre, f.Exit.dom.pre)
+		}
 	}
 	fmt.Fprintln(buf, "}")
 }
@@ -456,6 +460,10 @@ func printPostDomTreeDot(buf io.Writer, f *Function) {
 		for _, pred := range b.Preds {
 			fmt.Fprintf(buf, "\tn%d -> n%d [style=\"dotted\",weight=0];\n", pred.pdom.pre, v.pre)
 		}
+
+		if f.fakeExits.Has(b) {
+			fmt.Fprintf(buf, "\tn%d -> n%d [style=\"dotted\",weight=0,color=red];\n", b.dom.pre, f.Exit.dom.pre)
+		}
 	}
 	fmt.Fprintln(buf, "}")
 }

go/ir/lift.go

@@ -79,8 +79,16 @@ func (df domFrontier) add(u, v *BasicBlock) {
 // the DF -> IDF step.
 func (df domFrontier) build(fn *Function) {
 	for _, b := range fn.Blocks {
-		if len(b.Preds) >= 2 {
-			for _, p := range b.Preds {
+		preds := b.Preds[0:len(b.Preds):len(b.Preds)]
+		if b == fn.Exit {
+			for i, v := range fn.fakeExits.values {
+				if v {
+					preds = append(preds, fn.Blocks[i])
+				}
+			}
+		}
+		if len(preds) >= 2 {
+			for _, p := range preds {
 				runner := p
 				for runner != b.dom.idom {
 					df.add(runner, b)
@@ -105,8 +113,12 @@ func (rdf postDomFrontier) add(u, v *BasicBlock) {
 
 func (rdf postDomFrontier) build(fn *Function) {
 	for _, b := range fn.Blocks {
-		if len(b.Succs) >= 2 {
-			for _, s := range b.Succs {
+		succs := b.Succs[0:len(b.Succs):len(b.Succs)]
+		if fn.fakeExits.Has(b) {
+			succs = append(succs, fn.Exit)
+		}
+		if len(succs) >= 2 {
+			for _, s := range succs {
 				runner := s
 				for runner != b.pdom.idom {
 					rdf.add(runner, b)

staticcheck/testdata/src/CheckPureFunctions/CheckPureFunctions.go

@@ -2,6 +2,7 @@ package pkg
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 	"strings"
 )
@@ -36,3 +37,12 @@ func fn3() {
 	stubPointer()
 	stubInt()
 }
+
+func fn4() error {
+	// Test for https://github.com/dominikh/go-tools/issues/949
+	if true {
+		return fmt.Errorf("")
+	}
+	for {
+	}
+}