compiler: use Tarjan's SCC algorithm to detect loops for defer

The compiler needs to know whether a defer is in a loop to determine whether to allocate stack or heap memory.
Previously, this performed a DFS of the CFG every time a defer was found.
This resulted in time complexity jointly proportional to the number of defers and the number of blocks in the function.

Now, the compiler will instead use Tarjan's strongly connected components algorithm to find cycles in linear time.
The search is performed lazily, so this has minimal performance impact on functions without defers.

In order to implement Tarjan's SCC algorithm, additional state needed to be attached to the blocks.
I chose to merge all of the per-block state into a single slice to simplify memory management.

compiler/compiler.go

@@ -152,10 +152,12 @@ type builder struct {
 	llvmFnType        llvm.Type
 	llvmFn            llvm.Value
 	info              functionInfo
-	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
+	locals            map[ssa.Value]llvm.Value // local variables
+	blockInfo         []blockInfo
 	currentBlock      *ssa.BasicBlock
+	currentBlockInfo  *blockInfo
+	tarjanStack       []uint
+	tarjanIndex       uint
 	phis              []phiNode
 	deferPtr          llvm.Value
 	deferFrame        llvm.Value
@@ -187,11 +189,22 @@ func newBuilder(c *compilerContext, irbuilder llvm.Builder, f *ssa.Function) *bu
 		info:            c.getFunctionInfo(f),
 		locals:          make(map[ssa.Value]llvm.Value),
 		dilocals:        make(map[*types.Var]llvm.Metadata),
-		blockEntries:    make(map[*ssa.BasicBlock]llvm.BasicBlock),
-		blockExits:      make(map[*ssa.BasicBlock]llvm.BasicBlock),
 	}
 }
 
+type blockInfo struct {
+	// entry is the LLVM basic block corresponding to the start of this *ssa.Block.
+	entry llvm.BasicBlock
+
+	// exit is the LLVM basic block corresponding to the end of this *ssa.Block.
+	// It will be different than entry if any of the block's instructions contain internal branches.
+	exit llvm.BasicBlock
+
+	// tarjan holds state for applying Tarjan's strongly connected components algorithm to the CFG.
+	// This is used by defer.go to determine whether to stack- or heap-allocate defer data.
+	tarjan tarjanNode
+}
+
 type deferBuiltin struct {
 	callName string
 	pos      token.Pos
@@ -1220,14 +1233,29 @@ func (b *builder) createFunctionStart(intrinsic bool) {
 		// intrinsic (like an atomic operation). Create the entry block
 		// manually.
 		entryBlock = b.ctx.AddBasicBlock(b.llvmFn, "entry")
-	} else {
-		for _, block := range b.fn.DomPreorder() {
-			llvmBlock := b.ctx.AddBasicBlock(b.llvmFn, block.Comment)
-			b.blockEntries[block] = llvmBlock
-			b.blockExits[block] = llvmBlock
+		// Intrinsics may create internal branches (e.g. nil checks).
+		// They will attempt to access b.currentBlockInfo to update the exit block.
+		// Create some fake block info for them to access.
+		blockInfo := []blockInfo{
+			{
+				entry: entryBlock,
+				exit:  entryBlock,
+			},
 		}
+		b.blockInfo = blockInfo
+		b.currentBlockInfo = &blockInfo[0]
+	} else {
+		blocks := b.fn.Blocks
+		blockInfo := make([]blockInfo, len(blocks))
+		for _, block := range b.fn.DomPreorder() {
+			info := &blockInfo[block.Index]
+			llvmBlock := b.ctx.AddBasicBlock(b.llvmFn, block.Comment)
+			info.entry = llvmBlock
+			info.exit = llvmBlock
+		}
+		b.blockInfo = blockInfo
 		// Normal functions have an entry block.
-		entryBlock = b.blockEntries[b.fn.Blocks[0]]
+		entryBlock = blockInfo[0].entry
 	}
 	b.SetInsertPointAtEnd(entryBlock)
 
@@ -1323,8 +1351,9 @@ func (b *builder) createFunction() {
 		if b.DumpSSA {
 			fmt.Printf("%d: %s:\n", block.Index, block.Comment)
 		}
-		b.SetInsertPointAtEnd(b.blockEntries[block])
 		b.currentBlock = block
+		b.currentBlockInfo = &b.blockInfo[block.Index]
+		b.SetInsertPointAtEnd(b.currentBlockInfo.entry)
 		for _, instr := range block.Instrs {
 			if instr, ok := instr.(*ssa.DebugRef); ok {
 				if !b.Debug {
@@ -1384,7 +1413,7 @@ func (b *builder) createFunction() {
 		block := phi.ssa.Block()
 		for i, edge := range phi.ssa.Edges {
 			llvmVal := b.getValue(edge, getPos(phi.ssa))
-			llvmBlock := b.blockExits[block.Preds[i]]
+			llvmBlock := b.blockInfo[block.Preds[i].Index].exit
 			phi.llvm.AddIncoming([]llvm.Value{llvmVal}, []llvm.BasicBlock{llvmBlock})
 		}
 	}
@@ -1498,11 +1527,11 @@ func (b *builder) createInstruction(instr ssa.Instruction) {
 	case *ssa.If:
 		cond := b.getValue(instr.Cond, getPos(instr))
 		block := instr.Block()
-		blockThen := b.blockEntries[block.Succs[0]]
-		blockElse := b.blockEntries[block.Succs[1]]
+		blockThen := b.blockInfo[block.Succs[0].Index].entry
+		blockElse := b.blockInfo[block.Succs[1].Index].entry
 		b.CreateCondBr(cond, blockThen, blockElse)
 	case *ssa.Jump:
-		blockJump := b.blockEntries[instr.Block().Succs[0]]
+		blockJump := b.blockInfo[instr.Block().Succs[0].Index].entry
 		b.CreateBr(blockJump)
 	case *ssa.MapUpdate:
 		m := b.getValue(instr.Map, getPos(instr))