cmd/compile/internal/noder: elide statically known "if" statements

In go.dev/cl/517775, I moved the frontend's deadcode elimination pass
into unified IR. But I also made a small enhancement: a branch like
"if x || true" is now detected as always taken, so the else branch can
be eliminated.

However, the inliner also has an optimization for delaying the
introduction of the result temporary variables when there's a single
return statement (added in go.dev/cl/266199). Consequently, the
inliner turns "if x || true { return true }; return true" into:

	if x || true {
		~R0 := true
		goto .i0
	}
	.i0:
	// code that uses ~R0

In turn, this confuses phi insertion, because it doesn't recognize
that the "if" statement is always taken, and so ~R0 will always be
initialized.

With this CL, after inlining we instead produce:

	_ = x || true
	~R0 := true
	goto .i0
	.i0:

Fixes #62211.

Change-Id: Ic8a12c9eb85833ee4e5d114f60e6c47817fce538
Reviewed-on: https://go-review.googlesource.com/c/go/+/522096
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>

1 files changed, 27 insertions, 6 deletions

diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index 2a526dbe69..0efe2ea2d5 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -1677,7 +1677,11 @@ func (r *reader) closeAnotherScope() {
 // @@@ Statements
 
 func (r *reader) stmt() ir.Node {
-	switch stmts := r.stmts(); len(stmts) {
+	return block(r.stmts())
+}
+
+func block(stmts []ir.Node) ir.Node {
+	switch len(stmts) {
 	case 0:
 		return nil
 	case 1:
@@ -1687,7 +1691,7 @@ func (r *reader) stmt() ir.Node {
 	}
 }
 
-func (r *reader) stmts() []ir.Node {
+func (r *reader) stmts() ir.Nodes {
 	assert(ir.CurFunc == r.curfn)
 	var res ir.Nodes
 
@@ -1912,12 +1916,29 @@ func (r *reader) ifStmt() ir.Node {
 	pos := r.pos()
 	init := r.stmts()
 	cond := r.expr()
-	then := r.blockStmt()
-	els := r.stmts()
+	staticCond := r.Int()
+	var then, els []ir.Node
+	if staticCond >= 0 {
+		then = r.blockStmt()
+	} else {
+		r.lastCloseScopePos = r.pos()
+	}
+	if staticCond <= 0 {
+		els = r.stmts()
+	}
 	r.closeAnotherScope()
 
-	if ir.IsConst(cond, constant.Bool) && len(init)+len(then)+len(els) == 0 {
-		return nil // drop empty if statement
+	if staticCond != 0 {
+		// We may have removed a dead return statement, which can trip up
+		// later passes (#62211). To avoid confusion, we instead flatten
+		// the if statement into a block.
+
+		if cond.Op() != ir.OLITERAL {
+			init.Append(typecheck.Stmt(ir.NewAssignStmt(pos, ir.BlankNode, cond))) // for side effects
+		}
+		init.Append(then...)
+		init.Append(els...)
+		return block(init)
 	}
 
 	n := ir.NewIfStmt(pos, cond, then, els)