tinygo/compiler/defer.go

package compiler

// This file implements the 'defer' keyword in Go.
// Defer statements are implemented by transforming the function in the
// following way:
//   * Creating an alloca in the entry block that contains a pointer (initially
//     null) to the linked list of defer frames.
//   * Every time a defer statement is executed, a new defer frame is created
//     using alloca with a pointer to the previous defer frame, and the head
//     pointer in the entry block is replaced with a pointer to this defer
//     frame.
//   * On return, runtime.rundefers is called which calls all deferred functions
//     from the head of the linked list until it has gone through all defer
//     frames.

import (
	"go/types"
	"strconv"
	"strings"

	"github.com/tinygo-org/tinygo/compiler/llvmutil"
	"golang.org/x/tools/go/ssa"
	"tinygo.org/x/go-llvm"
)

// supportsRecover returns whether the compiler supports the recover() builtin
// for the current architecture.
func (b *builder) supportsRecover() bool {
	switch b.archFamily() {
	case "wasm32":
		// Probably needs to be implemented using the exception handling
		// proposal of WebAssembly:
		// https://github.com/WebAssembly/exception-handling
		return false
	case "riscv64", "xtensa":
		// TODO: add support for these architectures
		return false
	default:
		return true
	}
}

// hasDeferFrame returns whether the current function needs to catch panics and
// run defers.
func (b *builder) hasDeferFrame() bool {
	if b.fn.Recover == nil {
		return false
	}
	return b.supportsRecover()
}

// deferInitFunc sets up this function for future deferred calls. It must be
// called from within the entry block when this function contains deferred
// calls.
func (b *builder) deferInitFunc() {
	// Some setup.
	b.deferFuncs = make(map[*ssa.Function]int)
	b.deferInvokeFuncs = make(map[string]int)
	b.deferClosureFuncs = make(map[*ssa.Function]int)
	b.deferExprFuncs = make(map[ssa.Value]int)
	b.deferBuiltinFuncs = make(map[ssa.Value]deferBuiltin)

	if b.hasDeferFrame() {
		// Set up the defer frame with the current stack pointer.
		// This assumes that the stack pointer doesn't move outside of the
		// function prologue/epilogue (an invariant maintained by TinyGo but
		// possibly broken by the C alloca function).
		// The frame pointer is _not_ saved, because it is marked as clobbered
		// in the setjmp-like inline assembly.
		deferFrameType := b.getLLVMRuntimeType("deferFrame")
		b.deferFrame = b.CreateAlloca(deferFrameType, "deferframe.buf")
		// The field index must match the DeferPtr field in runtime.deferFrame,
		// defined in src/runtime/panic.go.
		b.deferPtr = b.CreateInBoundsGEP(deferFrameType, b.deferFrame, []llvm.Value{
			llvm.ConstInt(b.ctx.Int32Type(), 0, false),
			llvm.ConstInt(b.ctx.Int32Type(), 6, false), // DeferPtr field
		}, "deferPtr")
		stackPointer := b.readStackPointer()
		b.createRuntimeCall("setupDeferFrame", []llvm.Value{b.deferFrame, stackPointer}, "")

		// Create the landing pad block, which is where control transfers after
		// a panic.
		b.landingpad = b.ctx.AddBasicBlock(b.llvmFn, "lpad")
	} else {
		// Create defer list pointer.
		b.deferPtr = b.CreateAlloca(b.dataPtrType, "deferPtr")
		b.CreateStore(llvm.ConstPointerNull(b.dataPtrType), b.deferPtr)
	}
}

// createLandingPad fills in the landing pad block. This block runs the deferred
// functions and returns (by jumping to the recover block). If the function is
// still panicking after the defers are run, the panic will be re-raised in
// destroyDeferFrame.
func (b *builder) createLandingPad() {
	b.SetInsertPointAtEnd(b.landingpad)

	// Add debug info, if needed.
	// The location used is the closing bracket of the function.
	if b.Debug {
		pos := b.program.Fset.Position(b.fn.Syntax().End())
		b.SetCurrentDebugLocation(uint(pos.Line), uint(pos.Column), b.difunc, llvm.Metadata{})
	}

	b.createRunDefers()

	// Continue at the 'recover' block, which returns to the parent in an
	// appropriate way.
	b.CreateBr(b.blockInfo[b.fn.Recover.Index].entry)
}

// Create a checkpoint (similar to setjmp). This emits inline assembly that
// stores the current program counter inside the ptr address (actually
// ptr+sizeof(ptr)) and then returns a boolean indicating whether this is the
// normal flow (false) or we jumped here from somewhere else (true).
func (b *builder) createCheckpoint(ptr llvm.Value) llvm.Value {
	// Construct inline assembly equivalents of setjmp.
	// The assembly works as follows:
	//   * Registers are either clobbered or, on 386, saved for longjmp to
	//     restore if the ABI requires them to survive calls.
	//   * The assembly stores the address just past the end of the assembly
	//     into the jump buffer.
	//   * The return value (eax, rax, r0, etc) is set to zero in the inline
	//     assembly but set to an unspecified non-zero value when jumping using
	//     a longjmp.
	var asmString, constraints string
	resultType := b.uintptrType
	switch b.archFamily() {
	case "i386":
		asmString = `
xorl %eax, %eax
movl $$1f, 4(%ebx)
movl %ebx, 8(%ebx)
movl %esi, 12(%ebx)
movl %edi, 16(%ebx)
movl %ebp, 20(%ebx)
1:`
		constraints = "={eax},{ebx},~{ecx},~{edx},~{xmm0},~{xmm1},~{xmm2},~{xmm3},~{xmm4},~{xmm5},~{xmm6},~{xmm7},~{fpsr},~{fpcr},~{flags},~{dirflag},~{memory}"
		// This doesn't include the floating point stack because TinyGo uses
		// newer floating point instructions.
	case "x86_64":
		asmString = `
leaq 1f(%rip), %rax
movq %rax, 8(%rbx)
xorq %rax, %rax
1:`
		constraints = "={rax},{rbx},~{rbx},~{rcx},~{rdx},~{rsi},~{rdi},~{rbp},~{r8},~{r9},~{r10},~{r11},~{r12},~{r13},~{r14},~{r15},~{xmm0},~{xmm1},~{xmm2},~{xmm3},~{xmm4},~{xmm5},~{xmm6},~{xmm7},~{xmm8},~{xmm9},~{xmm10},~{xmm11},~{xmm12},~{xmm13},~{xmm14},~{xmm15},~{xmm16},~{xmm17},~{xmm18},~{xmm19},~{xmm20},~{xmm21},~{xmm22},~{xmm23},~{xmm24},~{xmm25},~{xmm26},~{xmm27},~{xmm28},~{xmm29},~{xmm30},~{xmm31},~{fpsr},~{fpcr},~{flags},~{dirflag},~{memory}"
		// This list doesn't include AVX/AVX512 registers because TinyGo
		// doesn't currently enable support for AVX instructions.
	case "arm":
		// Note: the following assembly takes into account that the PC is
		// always 4 bytes ahead on ARM. The PC that is stored always points
		// to the instruction just after the assembly fragment so that
		// tinygo_longjmp lands at the correct instruction.
		if b.isThumb() {
			// Instructions are 2 bytes in size.
			asmString = `
movs r0, #0
mov r2, pc
str r2, [r1, #4]`
		} else {
			// Instructions are 4 bytes in size.
			asmString = `
str pc, [r1, #4]
movs r0, #0`
		}
		constraints = "={r0},{r1},~{r1},~{r2},~{r3},~{r4},~{r5},~{r6},~{r7},~{r8},~{r9},~{r10},~{r11},~{r12},~{lr},~{q0},~{q1},~{q2},~{q3},~{q4},~{q5},~{q6},~{q7},~{q8},~{q9},~{q10},~{q11},~{q12},~{q13},~{q14},~{q15},~{cpsr},~{memory}"
	case "aarch64":
		asmString = `
adr x2, 1f
str x2, [x1, #8]
mov x0, #0
1:
`
		constraints = "={x0},{x1},~{x1},~{x2},~{x3},~{x4},~{x5},~{x6},~{x7},~{x8},~{x9},~{x10},~{x11},~{x12},~{x13},~{x14},~{x15},~{x16},~{x17},~{x19},~{x20},~{x21},~{x22},~{x23},~{x24},~{x25},~{x26},~{x27},~{x28},~{lr},~{q0},~{q1},~{q2},~{q3},~{q4},~{q5},~{q6},~{q7},~{q8},~{q9},~{q10},~{q11},~{q12},~{q13},~{q14},~{q15},~{q16},~{q17},~{q18},~{q19},~{q20},~{q21},~{q22},~{q23},~{q24},~{q25},~{q26},~{q27},~{q28},~{q29},~{q30},~{nzcv},~{ffr},~{memory}"
		if b.GOOS != "darwin" && b.GOOS != "windows" {
			// These registers cause the following warning when compiling for
			// MacOS and Windows:
			//     warning: inline asm clobber list contains reserved registers:
			//     X18, FP
			//     Reserved registers on the clobber list may not be preserved
			//     across the asm statement, and clobbering them may lead to
			//     undefined behaviour.
			constraints += ",~{x18},~{fp}"
		}
		// TODO: SVE registers, which we don't use in TinyGo at the moment.
	case "avr":
		// Note: the Y register (R28:R29) is a fixed register and therefore
		// needs to be saved manually. TODO: do this only once per function with
		// a defer frame, not for every call.
		resultType = b.ctx.Int8Type()
		asmString = `
ldi r24, pm_lo8(1f)
ldi r25, pm_hi8(1f)
std z+2, r24
std z+3, r25
std z+4, r28
std z+5, r29
ldi r24, 0
1:`
		constraints = "={r24},z,~{r0},~{r2},~{r3},~{r4},~{r5},~{r6},~{r7},~{r8},~{r9},~{r10},~{r11},~{r12},~{r13},~{r14},~{r15},~{r16},~{r17},~{r18},~{r19},~{r20},~{r21},~{r22},~{r23},~{r25},~{r26},~{r27}"
	case "mips":
		// $4 flag (zero or non-zero)
		// $5 defer frame
		asmString = `
.set noat
move $$4, $$zero
jal 1f
1:
addiu $$ra, 8
sw $$ra, 4($$5)
.set at`
		constraints = "={$4},{$5},~{$1},~{$2},~{$3},~{$5},~{$6},~{$7},~{$8},~{$9},~{$10},~{$11},~{$12},~{$13},~{$14},~{$15},~{$16},~{$17},~{$18},~{$19},~{$20},~{$21},~{$22},~{$23},~{$24},~{$25},~{$26},~{$27},~{$28},~{$29},~{$30},~{$31},~{memory}"
		if !strings.Contains(b.Features, "+soft-float") {
			// Using floating point registers together with GOMIPS=softfloat
			// results in a crash: "This value type is not natively supported!"
			// So only add them when using hardfloat.
			constraints += ",~{$f0},~{$f1},~{$f2},~{$f3},~{$f4},~{$f5},~{$f6},~{$f7},~{$f8},~{$f9},~{$f10},~{$f11},~{$f12},~{$f13},~{$f14},~{$f15},~{$f16},~{$f17},~{$f18},~{$f19},~{$f20},~{$f21},~{$f22},~{$f23},~{$f24},~{$f25},~{$f26},~{$f27},~{$f28},~{$f29},~{$f30},~{$f31}"
		}
	case "riscv32":
		asmString = `
la a2, 1f
sw a2, 4(a1)
li a0, 0
1:`
		constraints = "={a0},{a1},~{a1},~{a2},~{a3},~{a4},~{a5},~{a6},~{a7},~{s0},~{s1},~{s2},~{s3},~{s4},~{s5},~{s6},~{s7},~{s8},~{s9},~{s10},~{s11},~{t0},~{t1},~{t2},~{t3},~{t4},~{t5},~{t6},~{ra},~{f0},~{f1},~{f2},~{f3},~{f4},~{f5},~{f6},~{f7},~{f8},~{f9},~{f10},~{f11},~{f12},~{f13},~{f14},~{f15},~{f16},~{f17},~{f18},~{f19},~{f20},~{f21},~{f22},~{f23},~{f24},~{f25},~{f26},~{f27},~{f28},~{f29},~{f30},~{f31},~{memory}"
	default:
		// This case should have been handled by b.supportsRecover().
		b.addError(b.fn.Pos(), "unknown architecture for defer: "+b.archFamily())
	}
	asmType := llvm.FunctionType(resultType, []llvm.Type{b.dataPtrType}, false)
	asm := llvm.InlineAsm(asmType, asmString, constraints, false, false, 0, false)
	result := b.CreateCall(asmType, asm, []llvm.Value{ptr}, "setjmp")
	result.AddCallSiteAttribute(-1, b.ctx.CreateEnumAttribute(llvm.AttributeKindID("returns_twice"), 0))
	isZero := b.CreateICmp(llvm.IntEQ, result, llvm.ConstInt(resultType, 0, false), "setjmp.result")
	return isZero
}

// createInvokeCheckpoint saves the function state at the given point, to
// continue at the landing pad if a panic happened. This is implemented using a
// setjmp-like construct.
func (b *builder) createInvokeCheckpoint() {
	isZero := b.createCheckpoint(b.deferFrame)
	continueBB := b.insertBasicBlock("")
	b.CreateCondBr(isZero, continueBB, b.landingpad)
	b.SetInsertPointAtEnd(continueBB)
	b.currentBlockInfo.exit = continueBB
}

// createFaultCheckpoint is like createInvokeCheckpoint but for use in fault
// blocks (e.g., bounds check failures). Unlike createInvokeCheckpoint, it does
// not update currentBlockInfo.exit because the fault block is a dead-end that
// does not participate in phi node resolution.
func (b *builder) createFaultCheckpoint() {
	isZero := b.createCheckpoint(b.deferFrame)
	continueBB := b.insertBasicBlock("")
	b.CreateCondBr(isZero, continueBB, b.landingpad)
	b.SetInsertPointAtEnd(continueBB)
}

// isInLoop checks if there is a path from the current block to itself.
// Use Tarjan's strongly connected components algorithm to search for cycles.
// A one-node SCC is a cycle iff there is an edge from the node to itself.
// A multi-node SCC is always a cycle.
// https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm
func (b *builder) isInLoop() bool {
	if b.currentBlockInfo.tarjan.lowLink == 0 {
		b.strongConnect(b.currentBlock)
	}
	return b.currentBlockInfo.tarjan.cyclic
}

func (b *builder) strongConnect(block *ssa.BasicBlock) {
	// Assign a new index.
	// Indices start from 1 so that 0 can be used as a sentinel.
	assignedIndex := b.tarjanIndex + 1
	b.tarjanIndex = assignedIndex

	// Apply the new index.
	blockIndex := block.Index
	node := &b.blockInfo[blockIndex].tarjan
	node.lowLink = assignedIndex

	// Push the node onto the stack.
	node.onStack = true
	b.tarjanStack = append(b.tarjanStack, uint(blockIndex))

	// Process the successors.
	for _, successor := range block.Succs {
		// Look up the successor's state.
		successorIndex := successor.Index
		if successorIndex == blockIndex {
			// Handle a self-cycle specially.
			node.cyclic = true
			continue
		}
		successorNode := &b.blockInfo[successorIndex].tarjan

		switch {
		case successorNode.lowLink == 0:
			// This node has not yet been visisted.
			b.strongConnect(successor)

		case !successorNode.onStack:
			// This node has been visited, but is in a different SCC.
			// Ignore it, and do not update lowLink.
			continue
		}

		// Update the lowLink index.
		// This always uses the min-of-lowlink instead of using index in the on-stack case.
		// This is done for two reasons:
		// 1. The lowLink update can be shared between the new-node and on-stack cases.
		// 2. The assigned index does not need to be saved - it is only needed for root node detection.
		if successorNode.lowLink < node.lowLink {
			node.lowLink = successorNode.lowLink
		}
	}

	if node.lowLink == assignedIndex {
		// This is a root node.
		// Pop the SCC off the stack.
		stack := b.tarjanStack
		top := stack[len(stack)-1]
		stack = stack[:len(stack)-1]
		blocks := b.blockInfo
		topNode := &blocks[top].tarjan
		topNode.onStack = false

		if top != uint(blockIndex) {
			// The root node is not the only node in the SCC.
			// Mark all nodes in this SCC as cyclic.
			topNode.cyclic = true
			for top != uint(blockIndex) {
				top = stack[len(stack)-1]
				stack = stack[:len(stack)-1]
				topNode = &blocks[top].tarjan
				topNode.onStack = false
				topNode.cyclic = true
			}
		}

		b.tarjanStack = stack
	}
}

// tarjanNode holds per-block state for isInLoop and strongConnect.
type tarjanNode struct {
	// lowLink is the index of the first visited node that is reachable from this block.
	// The lowLink indices are assigned by the SCC search, and do not correspond to b.Index.
	// A lowLink of 0 is used as a sentinel to mark a node which has not yet been visited.
	lowLink uint

	// onStack tracks whether this node is currently on the SCC search stack.
	onStack bool

	// cyclic indicates whether this block is in a loop.
	// If lowLink is 0, strongConnect must be called before reading this field.
	cyclic bool
}

type llvmValueList struct {
	values []llvm.Value
	types  []llvm.Type
}

func newLLVMValueList(values ...llvm.Value) llvmValueList {
	var list llvmValueList
	list.append(values...)
	return list
}

func (l *llvmValueList) append(values ...llvm.Value) {
	for _, value := range values {
		l.values = append(l.values, value)
		l.types = append(l.types, value.Type())
	}
}

func (l *llvmValueList) appendSSAValues(values []ssa.Value, lower func(ssa.Value) llvm.Value) {
	for _, value := range values {
		l.append(lower(value))
	}
}

func (b *builder) loadDeferredCallParams(structType llvm.Type, ptr llvm.Value) []llvm.Value {
	fieldTypes := structType.StructElementTypes()
	values := make([]llvm.Value, 0, len(fieldTypes)-2)
	zero := llvm.ConstInt(b.ctx.Int32Type(), 0, false)
	for i := 2; i < len(fieldTypes); i++ {
		fieldPtr := b.CreateInBoundsGEP(structType, ptr, []llvm.Value{
			zero,
			llvm.ConstInt(b.ctx.Int32Type(), uint64(i), false),
		}, "gep")
		values = append(values, b.CreateLoad(fieldTypes[i], fieldPtr, "param"))
	}
	return values
}

// createDefer emits a single defer instruction, to be run when this function
// returns.
func (b *builder) createDefer(instr *ssa.Defer) {
	// The pointer to the previous defer struct, which we will replace to
	// make a linked list.
	next := b.CreateLoad(b.dataPtrType, b.deferPtr, "defer.next")

	var values llvmValueList
	lowerArgument := func(value ssa.Value) llvm.Value {
		return b.getCallArgument(value, false)
	}
	if instr.Call.IsInvoke() {
		// Method call on an interface.

		// Get callback type number.
		key := b.getInvokeFunctionName(&instr.Call)
		if _, ok := b.deferInvokeFuncs[key]; !ok {
			b.deferInvokeFuncs[key] = len(b.allDeferFuncs)
			b.allDeferFuncs = append(b.allDeferFuncs, &instr.Call)
		}
		callback := llvm.ConstInt(b.uintptrType, uint64(b.deferInvokeFuncs[key]), false)

		// Collect all values to be put in the struct (starting with
		// runtime._defer fields, followed by the call parameters).
		itf := b.getValue(instr.Call.Value, getPos(instr)) // interface
		typecode := b.CreateExtractValue(itf, 0, "invoke.func.typecode")
		receiverValue := b.CreateExtractValue(itf, 1, "invoke.func.receiver")
		values = newLLVMValueList(callback, next, typecode, receiverValue)
		values.appendSSAValues(instr.Call.Args, lowerArgument)

	} else if callee, ok := instr.Call.Value.(*ssa.Function); ok {
		// Regular function call.
		if _, ok := b.deferFuncs[callee]; !ok {
			b.deferFuncs[callee] = len(b.allDeferFuncs)
			b.allDeferFuncs = append(b.allDeferFuncs, callee)
		}
		callback := llvm.ConstInt(b.uintptrType, uint64(b.deferFuncs[callee]), false)

		// Collect all values to be put in the struct (starting with
		// runtime._defer fields).
		values = newLLVMValueList(callback, next)
		exported := b.getFunctionInfo(callee).exported
		values.appendSSAValues(instr.Call.Args, func(value ssa.Value) llvm.Value {
			return b.getCallArgument(value, exported)
		})

	} else if makeClosure, ok := instr.Call.Value.(*ssa.MakeClosure); ok {
		// Immediately applied function literal with free variables.

		// Extract the context from the closure. We won't need the function
		// pointer.
		// TODO: ignore this closure entirely and put pointers to the free
		// variables directly in the defer struct, avoiding a memory allocation.
		closure := b.getValue(instr.Call.Value, getPos(instr))
		context := b.CreateExtractValue(closure, 0, "")

		// Get the callback number.
		fn := makeClosure.Fn.(*ssa.Function)
		if _, ok := b.deferClosureFuncs[fn]; !ok {
			b.deferClosureFuncs[fn] = len(b.allDeferFuncs)
			b.allDeferFuncs = append(b.allDeferFuncs, makeClosure)
		}
		callback := llvm.ConstInt(b.uintptrType, uint64(b.deferClosureFuncs[fn]), false)

		// Collect all values to be put in the struct (starting with
		// runtime._defer fields, followed by all parameters including the
		// context pointer).
		values = newLLVMValueList(callback, next)
		values.appendSSAValues(instr.Call.Args, lowerArgument)
		values.append(context)

	} else if builtin, ok := instr.Call.Value.(*ssa.Builtin); ok {
		var argTypes []types.Type
		var argValues []llvm.Value
		for _, arg := range instr.Call.Args {
			argTypes = append(argTypes, arg.Type())
			argValues = append(argValues, b.getValue(arg, getPos(instr)))
		}

		if _, ok := b.deferBuiltinFuncs[instr.Call.Value]; !ok {
			b.deferBuiltinFuncs[instr.Call.Value] = deferBuiltin{
				callName: builtin.Name(),
				pos:      builtin.Pos(),
				argTypes: argTypes,
				callback: len(b.allDeferFuncs),
			}
			b.allDeferFuncs = append(b.allDeferFuncs, instr.Call.Value)
		}
		callback := llvm.ConstInt(b.uintptrType, uint64(b.deferBuiltinFuncs[instr.Call.Value].callback), false)

		// Collect all values to be put in the struct (starting with
		// runtime._defer fields).
		values = newLLVMValueList(callback, next)
		values.append(argValues...)

	} else {
		funcValue := b.getValue(instr.Call.Value, getPos(instr))

		if _, ok := b.deferExprFuncs[instr.Call.Value]; !ok {
			b.deferExprFuncs[instr.Call.Value] = len(b.allDeferFuncs)
			b.allDeferFuncs = append(b.allDeferFuncs, &instr.Call)
		}

		callback := llvm.ConstInt(b.uintptrType, uint64(b.deferExprFuncs[instr.Call.Value]), false)

		// Collect all values to be put in the struct (starting with
		// runtime._defer fields, followed by all parameters including the
		// context pointer).
		values = newLLVMValueList(callback, next, funcValue)
		values.appendSSAValues(instr.Call.Args, lowerArgument)
	}

	// Make a struct out of the collected values to put in the deferred call
	// struct.
	deferredCallType := b.ctx.StructType(values.types, false)
	deferredCall := llvm.ConstNull(deferredCallType)
	for i, value := range values.values {
		deferredCall = b.CreateInsertValue(deferredCall, value, i, "")
	}

	// Put this struct in an allocation.
	var alloca llvm.Value
	if instr.Block() != b.currentBlock {
		panic("block mismatch")
	}
	if !b.isInLoop() {
		// This can safely use a stack allocation.
		alloca = llvmutil.CreateEntryBlockAlloca(b.Builder, deferredCallType, "defer.alloca")
	} else {
		// This may be hit a variable number of times, so use a heap allocation.
		size := b.targetData.TypeAllocSize(deferredCallType)
		sizeValue := llvm.ConstInt(b.uintptrType, size, false)
		layoutValue := b.createObjectLayout(deferredCallType, instr.Pos())
		align := b.targetData.ABITypeAlignment(deferredCallType)
		alloca = b.createAlloc(sizeValue, layoutValue, align, "defer.alloc.call")
	}
	if b.NeedsStackObjects {
		b.trackPointer(alloca)
	}
	b.CreateStore(deferredCall, alloca)

	// Push it on top of the linked list by replacing deferPtr.
	b.CreateStore(alloca, b.deferPtr)
}

// createRunDefers emits code to run all deferred functions.
func (b *builder) createRunDefers() {
	deferType := b.getLLVMRuntimeType("_defer")

	// Add a loop like the following:
	//     for stack != nil {
	//         _stack := stack
	//         stack = stack.next
	//         switch _stack.callback {
	//         case 0:
	//             // run first deferred call
	//         case 1:
	//             // run second deferred call
	//             // etc.
	//         default:
	//             unreachable
	//         }
	//     }

	// Create loop, in the order: loophead, loop, callback0, callback1, ..., unreachable, end.
	end := b.insertBasicBlock("rundefers.end")
	unreachable := b.ctx.InsertBasicBlock(end, "rundefers.default")
	loop := b.ctx.InsertBasicBlock(unreachable, "rundefers.loop")
	loophead := b.ctx.InsertBasicBlock(loop, "rundefers.loophead")
	b.CreateBr(loophead)

	// Create loop head:
	//     for stack != nil {
	b.SetInsertPointAtEnd(loophead)
	deferData := b.CreateLoad(b.dataPtrType, b.deferPtr, "")
	stackIsNil := b.CreateICmp(llvm.IntEQ, deferData, llvm.ConstPointerNull(deferData.Type()), "stackIsNil")
	b.CreateCondBr(stackIsNil, end, loop)

	// Create loop body:
	//     _stack := stack
	//     stack = stack.next
	//     switch stack.callback {
	b.SetInsertPointAtEnd(loop)
	nextStackGEP := b.CreateInBoundsGEP(deferType, deferData, []llvm.Value{
		llvm.ConstInt(b.ctx.Int32Type(), 0, false),
		llvm.ConstInt(b.ctx.Int32Type(), 1, false), // .next field
	}, "stack.next.gep")
	nextStack := b.CreateLoad(b.dataPtrType, nextStackGEP, "stack.next")
	b.CreateStore(nextStack, b.deferPtr)
	gep := b.CreateInBoundsGEP(deferType, deferData, []llvm.Value{
		llvm.ConstInt(b.ctx.Int32Type(), 0, false),
		llvm.ConstInt(b.ctx.Int32Type(), 0, false), // .callback field
	}, "callback.gep")
	callback := b.CreateLoad(b.uintptrType, gep, "callback")
	sw := b.CreateSwitch(callback, unreachable, len(b.allDeferFuncs))

	for i, callback := range b.allDeferFuncs {
		// Create switch case, for example:
		//     case 0:
		//         // run first deferred call
		block := b.insertBasicBlock("rundefers.callback" + strconv.Itoa(i))
		sw.AddCase(llvm.ConstInt(b.uintptrType, uint64(i), false), block)
		b.SetInsertPointAtEnd(block)
		switch callback := callback.(type) {
		case *ssa.CallCommon:
			// Call on an value or interface value.

			// Get the real defer struct type and cast to it.
			valueTypes := []llvm.Type{b.uintptrType, b.dataPtrType}

			if !callback.IsInvoke() {
				//Expect funcValue to be passed through the deferred call.
				valueTypes = append(valueTypes, b.getFuncType(callback.Signature()))
			} else {
				//Expect typecode
				valueTypes = append(valueTypes, b.dataPtrType, b.dataPtrType)
			}

			valueTypes = b.appendStoredValueTypes(valueTypes, callback.Args, false)

			// Extract the params from the struct (including receiver).
			deferredCallType := b.ctx.StructType(valueTypes, false)
			forwardParams := b.loadDeferredCallParams(deferredCallType, deferData)

			var fnPtr llvm.Value
			var fnType llvm.Type

			if !callback.IsInvoke() {
				// Isolate the func value.
				funcValue := forwardParams[0]
				forwardParams = forwardParams[1:]

				//Get function pointer and context
				var context llvm.Value
				fnPtr, context = b.decodeFuncValue(funcValue)
				fnType = b.getLLVMFunctionType(callback.Signature())

				//Pass context
				forwardParams = append(forwardParams, context)
			} else {
				// Move typecode from the start to the end of the list of
				// parameters.
				forwardParams = append(forwardParams[1:], forwardParams[0])
				fnPtr = b.getInvokeFunction(callback)
				fnType = fnPtr.GlobalValueType()

				// Add the context parameter. An interface call cannot also be a
				// closure but we have to supply the parameter anyway for platforms
				// with a strict calling convention.
				forwardParams = append(forwardParams, llvm.Undef(b.dataPtrType))
			}
			forwardParams = b.prependIndirectResult(callback.Signature(), false, forwardParams, "defer.result")

			b.createCall(fnType, fnPtr, forwardParams, "")

		case *ssa.Function:
			// Direct call.

			// Get the real defer struct type and cast to it.
			valueTypes := []llvm.Type{b.uintptrType, b.dataPtrType}
			exported := b.getFunctionInfo(callback).exported
			valueTypes = b.appendStoredParamTypes(valueTypes, getParams(callback.Signature), exported)
			deferredCallType := b.ctx.StructType(valueTypes, false)

			// Extract the params from the struct.
			forwardParams := b.loadDeferredCallParams(deferredCallType, deferData)

			// Plain TinyGo functions add some extra parameters to implement async functionality and function receivers.
			// These parameters should not be supplied when calling into an external C/ASM function.
			if !exported {
				// Add the context parameter. We know it is ignored by the receiving
				// function, but we have to pass one anyway.
				forwardParams = append(forwardParams, llvm.Undef(b.dataPtrType))
			}
			forwardParams = b.prependIndirectResult(callback.Signature, exported, forwardParams, "defer.result")

			// Call real function.
			fnType, fn := b.getFunction(callback)
			b.createInvoke(fnType, fn, forwardParams, "")

		case *ssa.MakeClosure:
			// Get the real defer struct type and cast to it.
			fn := callback.Fn.(*ssa.Function)
			valueTypes := []llvm.Type{b.uintptrType, b.dataPtrType}
			valueTypes = b.appendStoredParamTypes(valueTypes, getParams(fn.Signature), false)
			valueTypes = append(valueTypes, b.dataPtrType) // closure
			deferredCallType := b.ctx.StructType(valueTypes, false)

			// Extract the params from the struct.
			forwardParams := b.loadDeferredCallParams(deferredCallType, deferData)

			// Call deferred function.
			fnType, llvmFn := b.getFunction(fn)
			forwardParams = b.prependIndirectResult(fn.Signature, false, forwardParams, "defer.result")
			b.createCall(fnType, llvmFn, forwardParams, "")
		case *ssa.Builtin:
			db := b.deferBuiltinFuncs[callback]

			//Get parameter types
			valueTypes := []llvm.Type{b.uintptrType, b.dataPtrType}

			//Get signature from call results
			params := callback.Type().Underlying().(*types.Signature).Params()
			for v := range params.Variables() {
				valueTypes = append(valueTypes, b.getLLVMType(v.Type()))
			}

			deferredCallType := b.ctx.StructType(valueTypes, false)

			// Extract the params from the struct.
			argValues := b.loadDeferredCallParams(deferredCallType, deferData)

			_, err := b.createBuiltin(db.argTypes, argValues, db.callName, db.pos)
			if err != nil {
				b.diagnostics = append(b.diagnostics, err)
			}
		default:
			panic("unknown deferred function type")
		}

		// Branch back to the start of the loop.
		b.CreateBr(loophead)
	}

	// Create default unreachable block:
	//     default:
	//         unreachable
	//     }
	b.SetInsertPointAtEnd(unreachable)
	b.CreateUnreachable()

	// End of loop.
	b.SetInsertPointAtEnd(end)
}