compiler: fix invalid incoming block in complex typeassert flow

A single *ssa.BasicBlock may be split in multiple LLVM basic blocks due
to typeassert instructions. This means the incoming block and outgoing
block are different. PHI nodes need to get the result from the outgoing
block, which was fixed before, but incoming branches need to branch to
the incoming block, not the outgoing block.

Branching to the outgoing block led to a LLVM verification error when
compiling the fmt package.

Originally found in (*fmt.pp).handleMethods.

compiler/compiler.go

@@ -68,8 +68,9 @@ type Compiler struct {
 
 type Frame struct {
 	fn           *ir.Function
-	locals       map[ssa.Value]llvm.Value // local variables
-	blocks       map[*ssa.BasicBlock]llvm.BasicBlock
+	locals       map[ssa.Value]llvm.Value            // local variables
+	blockEntries map[*ssa.BasicBlock]llvm.BasicBlock // a *ssa.BasicBlock may be split up
+	blockExits   map[*ssa.BasicBlock]llvm.BasicBlock // these are the exit blocks
 	currentBlock *ssa.BasicBlock
 	phis         []Phi
 	blocking     bool
@@ -917,10 +918,11 @@ func (c *Compiler) wrapInterfaceInvoke(f *ir.Function) (llvm.Value, error) {
 
 func (c *Compiler) parseFuncDecl(f *ir.Function) (*Frame, error) {
 	frame := &Frame{
-		fn:       f,
-		locals:   make(map[ssa.Value]llvm.Value),
-		blocks:   make(map[*ssa.BasicBlock]llvm.BasicBlock),
-		blocking: c.ir.IsBlocking(f),
+		fn:           f,
+		locals:       make(map[ssa.Value]llvm.Value),
+		blockEntries: make(map[*ssa.BasicBlock]llvm.BasicBlock),
+		blockExits:   make(map[*ssa.BasicBlock]llvm.BasicBlock),
+		blocking:     c.ir.IsBlocking(f),
 	}
 
 	var retType llvm.Type
@@ -1346,13 +1348,14 @@ func (c *Compiler) parseFunc(frame *Frame) error {
 	// Pre-create all basic blocks in the function.
 	for _, block := range frame.fn.DomPreorder() {
 		llvmBlock := c.ctx.AddBasicBlock(frame.fn.LLVMFn, block.Comment)
-		frame.blocks[block] = llvmBlock
+		frame.blockEntries[block] = llvmBlock
+		frame.blockExits[block] = llvmBlock
 	}
 	if frame.blocking {
 		frame.cleanupBlock = c.ctx.AddBasicBlock(frame.fn.LLVMFn, "task.cleanup")
 		frame.suspendBlock = c.ctx.AddBasicBlock(frame.fn.LLVMFn, "task.suspend")
 	}
-	entryBlock := frame.blocks[frame.fn.Blocks[0]]
+	entryBlock := frame.blockEntries[frame.fn.Blocks[0]]
 	c.builder.SetInsertPointAtEnd(entryBlock)
 
 	// Load function parameters
@@ -1479,9 +1482,9 @@ func (c *Compiler) parseFunc(frame *Frame) error {
 	// Fill blocks with instructions.
 	for _, block := range frame.fn.DomPreorder() {
 		if c.DumpSSA {
-			fmt.Printf("%s:\n", block.Comment)
+			fmt.Printf("%d: %s:\n", block.Index, block.Comment)
 		}
-		c.builder.SetInsertPointAtEnd(frame.blocks[block])
+		c.builder.SetInsertPointAtEnd(frame.blockEntries[block])
 		frame.currentBlock = block
 		for _, instr := range block.Instrs {
 			if _, ok := instr.(*ssa.DebugRef); ok {
@@ -1512,7 +1515,7 @@ func (c *Compiler) parseFunc(frame *Frame) error {
 			if err != nil {
 				return err
 			}
-			llvmBlock := frame.blocks[block.Preds[i]]
+			llvmBlock := frame.blockExits[block.Preds[i]]
 			phi.llvm.AddIncoming([]llvm.Value{llvmVal}, []llvm.BasicBlock{llvmBlock})
 		}
 	}
@@ -1651,12 +1654,12 @@ func (c *Compiler) parseInstr(frame *Frame, instr ssa.Instruction) error {
 			return err
 		}
 		block := instr.Block()
-		blockThen := frame.blocks[block.Succs[0]]
-		blockElse := frame.blocks[block.Succs[1]]
+		blockThen := frame.blockEntries[block.Succs[0]]
+		blockElse := frame.blockEntries[block.Succs[1]]
 		c.builder.CreateCondBr(cond, blockThen, blockElse)
 		return nil
 	case *ssa.Jump:
-		blockJump := frame.blocks[instr.Block().Succs[0]]
+		blockJump := frame.blockEntries[instr.Block().Succs[0]]
 		c.builder.CreateBr(blockJump)
 		return nil
 	case *ssa.MapUpdate:
@@ -2793,7 +2796,7 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 		prevBlock := c.builder.GetInsertBlock()
 		okBlock := c.ctx.AddBasicBlock(frame.fn.LLVMFn, "typeassert.ok")
 		nextBlock := c.ctx.AddBasicBlock(frame.fn.LLVMFn, "typeassert.next")
-		frame.blocks[frame.currentBlock] = nextBlock // adjust outgoing block for phi nodes
+		frame.blockExits[frame.currentBlock] = nextBlock // adjust outgoing block for phi nodes
 		c.builder.CreateCondBr(commaOk, okBlock, nextBlock)
 
 		// Retrieve the value from the interface if the type assert was

testdata/interface.go

@@ -20,6 +20,8 @@ func main() {
 	println("Stringer.String():", s.String())
 	var itf interface{} = s
 	println("Stringer.(*Thing).String():", itf.(Stringer).String())
+
+	println("nested switch:", nestedSwitch('v', 3))
 }
 
 func printItf(val interface{}) {
@@ -46,6 +48,17 @@ func printItf(val interface{}) {
 	}
 }
 
+func nestedSwitch(verb rune, arg interface{}) bool {
+	switch verb {
+	case 'v', 's':
+		switch arg.(type) {
+		case int:
+			return true
+		}
+	}
+	return false
+}
+
 type Thing struct {
 	name string
 }

testdata/interface.txt

@@ -16,3 +16,4 @@ is Tuple: 0 8 16 24
 SmallPair.Print: 3 5
 Stringer.String(): foo
 Stringer.(*Thing).String(): foo
+nested switch: true