perf(gengo): eliminate dead IF/ELSEIF branches with literal conds

IF .T. collapses to its body; IF .F. forwards to the first live
ELSEIF or ELSE. For dynamic main conditions the chain is still
filtered: ELSEIF .F. drops out, ELSEIF .T. truncates and becomes
the ELSE. Verified with /tmp/test_deadif.prg — five dead labels
all removed from gen output, runtime emits only live branches.

FiveSql2 43/43, Harbour compat 56/56.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

compiler/gengo/gengo.go

@@ -1265,7 +1265,85 @@ func (g *Generator) emitCallAsStmt(call *ast.CallExpr, locals localMap) {
 	g.writeln(fmt.Sprintf("t.Do(%d)", len(call.Args)))
 }
 
+// boolLiteralValue returns (value, true) if e is a bare .T./.F. literal.
+// Used by emitIf to skip dead branches.
+func boolLiteralValue(e ast.Expr) (bool, bool) {
+	lit, ok := e.(*ast.LiteralExpr)
+	if !ok {
+		return false, false
+	}
+	switch lit.Kind {
+	case token.TRUE:
+		return true, true
+	case token.FALSE:
+		return false, true
+	}
+	return false, false
+}
+
 func (g *Generator) emitIf(s *ast.IfStmt, locals localMap) {
+	// Dead-branch elimination for literal conditions. An IF .T. collapses
+	// to its body; an IF .F. collapses to its first live ELSEIF/ELSE.
+	// We resolve the main Cond here and recurse on the remainder if it
+	// turns into a new IF chain.
+	if v, ok := boolLiteralValue(s.Cond); ok {
+		if v {
+			for _, stmt := range s.Body {
+				g.emitStmt(stmt, locals)
+			}
+			return
+		}
+		// IF .F. — scan ElseIfs for first non-.F. branch.
+		for i, ei := range s.ElseIfs {
+			if v2, ok2 := boolLiteralValue(ei.Cond); ok2 {
+				if v2 {
+					for _, stmt := range ei.Body {
+						g.emitStmt(stmt, locals)
+					}
+					return
+				}
+				continue // ELSEIF .F. — dead, skip
+			}
+			// Non-literal ELSEIF becomes the new IF head.
+			newIf := &ast.IfStmt{
+				IfPos:    ei.ElseIfPos,
+				Cond:     ei.Cond,
+				Body:     ei.Body,
+				ElseIfs:  s.ElseIfs[i+1:],
+				ElseBody: s.ElseBody,
+			}
+			g.emitIf(newIf, locals)
+			return
+		}
+		// All ElseIfs were .F. — only ELSE body remains.
+		for _, stmt := range s.ElseBody {
+			g.emitStmt(stmt, locals)
+		}
+		return
+	}
+
+	// Main cond is dynamic. Still filter dead ELSEIFs (.F. removed;
+	// an ELSEIF .T. truncates the chain and becomes the ELSE).
+	elseIfs := s.ElseIfs
+	elseBody := s.ElseBody
+	if len(elseIfs) > 0 {
+		filtered := make([]*ast.ElseIfClause, 0, len(elseIfs))
+		for _, ei := range elseIfs {
+			if v, ok := boolLiteralValue(ei.Cond); ok {
+				if v {
+					// ELSEIF .T. — chain stops here; body becomes ELSE.
+					elseBody = ei.Body
+					elseIfs = filtered
+					goto emit
+				}
+				continue // ELSEIF .F. — dead
+			}
+			filtered = append(filtered, ei)
+		}
+		elseIfs = filtered
+	}
+
+emit:
 	g.emitExpr(s.Cond)
 	g.writeln("if t.PopLogical() {")
 	g.indent++
@@ -1274,7 +1352,7 @@ func (g *Generator) emitIf(s *ast.IfStmt, locals localMap) {
 	}
 	g.indent--
 
-	for _, ei := range s.ElseIfs {
+	for _, ei := range elseIfs {
 		g.writeIndent()
 		g.write("} else {\n")
 		g.indent++
@@ -1287,10 +1365,10 @@ func (g *Generator) emitIf(s *ast.IfStmt, locals localMap) {
 		g.indent--
 	}
 
-	if len(s.ElseBody) > 0 {
+	if len(elseBody) > 0 {
 		g.writeln("} else {")
 		g.indent++
-		for _, stmt := range s.ElseBody {
+		for _, stmt := range elseBody {
 			g.emitStmt(stmt, locals)
 		}
 		g.indent--
@@ -1299,7 +1377,7 @@ func (g *Generator) emitIf(s *ast.IfStmt, locals localMap) {
 	g.writeln("}")
 
 	// Close nested elseif braces
-	for range s.ElseIfs {
+	for range elseIfs {
 		g.writeln("}")
 	}
 }