tinygo/compiler/llvm.go

package compiler

import (
	"fmt"
	"go/token"
	"go/types"
	"math/big"
	"strings"

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

// This file contains helper functions for LLVM that are not exposed in the Go
// bindings.

// createTemporaryAlloca creates a new alloca in the entry block and adds
// lifetime start information in the IR signalling that the alloca won't be used
// before this point.
//
// This is useful for creating temporary allocas for intrinsics. Don't forget to
// end the lifetime using emitLifetimeEnd after you're done with it.
func (b *builder) createTemporaryAlloca(t llvm.Type, name string) (alloca, bitcast, size llvm.Value) {
	return llvmutil.CreateTemporaryAlloca(b.Builder, b.mod, t, name)
}

// insertBasicBlock inserts a new basic block after the current basic block.
// This is useful when inserting new basic blocks while converting a
// *ssa.BasicBlock to a llvm.BasicBlock and the LLVM basic block needs some
// extra blocks.
// It does not update b.blockExits, this must be done by the caller.
func (b *builder) insertBasicBlock(name string) llvm.BasicBlock {
	currentBB := b.Builder.GetInsertBlock()
	nextBB := llvm.NextBasicBlock(currentBB)
	if nextBB.IsNil() {
		// Last basic block in the function, so add one to the end.
		return b.ctx.AddBasicBlock(b.llvmFn, name)
	}
	// Insert a basic block before the next basic block - that is, at the
	// current insert location.
	return b.ctx.InsertBasicBlock(nextBB, name)
}

// emitLifetimeEnd signals the end of an (alloca) lifetime by calling the
// llvm.lifetime.end intrinsic. It is commonly used together with
// createTemporaryAlloca.
func (b *builder) emitLifetimeEnd(ptr, size llvm.Value) {
	llvmutil.EmitLifetimeEnd(b.Builder, b.mod, ptr, size)
}

// emitPointerPack packs the list of values into a single pointer value using
// bitcasts, or else allocates a value on the heap if it cannot be packed in the
// pointer value directly. It returns the pointer with the packed data.
// If the values are all constants, they are be stored in a constant global and
// deduplicated.
func (b *builder) emitPointerPack(values []llvm.Value) llvm.Value {
	valueTypes := make([]llvm.Type, len(values))
	for i, value := range values {
		valueTypes[i] = value.Type()
	}
	packedType := b.ctx.StructType(valueTypes, false)

	// Allocate memory for the packed data.
	size := b.targetData.TypeAllocSize(packedType)
	if size == 0 {
		return llvm.ConstPointerNull(b.i8ptrType)
	} else if len(values) == 1 && values[0].Type().TypeKind() == llvm.PointerTypeKind {
		return b.CreateBitCast(values[0], b.i8ptrType, "pack.ptr")
	} else if size <= b.targetData.TypeAllocSize(b.i8ptrType) {
		// Packed data fits in a pointer, so store it directly inside the
		// pointer.
		if len(values) == 1 && values[0].Type().TypeKind() == llvm.IntegerTypeKind {
			// Try to keep this cast in SSA form.
			return b.CreateIntToPtr(values[0], b.i8ptrType, "pack.int")
		}

		// Because packedType is a struct and we have to cast it to a *i8, store
		// it in a *i8 alloca first and load the *i8 value from there. This is
		// effectively a bitcast.
		packedAlloc, _, _ := b.createTemporaryAlloca(b.i8ptrType, "")

		if size < b.targetData.TypeAllocSize(b.i8ptrType) {
			// The alloca is bigger than the value that will be stored in it.
			// To avoid having some bits undefined, zero the alloca first.
			// Hopefully this will get optimized away.
			b.CreateStore(llvm.ConstNull(b.i8ptrType), packedAlloc)
		}

		// Store all values in the alloca.
		packedAllocCast := b.CreateBitCast(packedAlloc, llvm.PointerType(packedType, 0), "")
		for i, value := range values {
			indices := []llvm.Value{
				llvm.ConstInt(b.ctx.Int32Type(), 0, false),
				llvm.ConstInt(b.ctx.Int32Type(), uint64(i), false),
			}
			gep := b.CreateInBoundsGEP(packedType, packedAllocCast, indices, "")
			b.CreateStore(value, gep)
		}

		// Load value (the *i8) from the alloca.
		result := b.CreateLoad(b.i8ptrType, packedAlloc, "")

		// End the lifetime of the alloca, to help the optimizer.
		packedPtr := b.CreateBitCast(packedAlloc, b.i8ptrType, "")
		packedSize := llvm.ConstInt(b.ctx.Int64Type(), b.targetData.TypeAllocSize(packedAlloc.Type()), false)
		b.emitLifetimeEnd(packedPtr, packedSize)

		return result
	} else {
		// Check if the values are all constants.
		constant := true
		for _, v := range values {
			if !v.IsConstant() {
				constant = false
				break
			}
		}

		if constant {
			// The data is known at compile time, so store it in a constant global.
			// The global address is marked as unnamed, which allows LLVM to merge duplicates.
			global := llvm.AddGlobal(b.mod, packedType, b.pkg.Path()+"$pack")
			global.SetInitializer(b.ctx.ConstStruct(values, false))
			global.SetGlobalConstant(true)
			global.SetUnnamedAddr(true)
			global.SetLinkage(llvm.InternalLinkage)
			return llvm.ConstBitCast(global, b.i8ptrType)
		}

		// Packed data is bigger than a pointer, so allocate it on the heap.
		sizeValue := llvm.ConstInt(b.uintptrType, size, false)
		alloc := b.mod.NamedFunction("runtime.alloc")
		packedHeapAlloc := b.CreateCall(alloc.GlobalValueType(), alloc, []llvm.Value{
			sizeValue,
			llvm.ConstNull(b.i8ptrType),
			llvm.Undef(b.i8ptrType), // unused context parameter
		}, "")
		if b.NeedsStackObjects {
			b.trackPointer(packedHeapAlloc)
		}
		packedAlloc := b.CreateBitCast(packedHeapAlloc, llvm.PointerType(packedType, 0), "")

		// Store all values in the heap pointer.
		for i, value := range values {
			indices := []llvm.Value{
				llvm.ConstInt(b.ctx.Int32Type(), 0, false),
				llvm.ConstInt(b.ctx.Int32Type(), uint64(i), false),
			}
			gep := b.CreateInBoundsGEP(packedType, packedAlloc, indices, "")
			b.CreateStore(value, gep)
		}

		// Return the original heap allocation pointer, which already is an *i8.
		return packedHeapAlloc
	}
}

// emitPointerUnpack extracts a list of values packed using emitPointerPack.
func (b *builder) emitPointerUnpack(ptr llvm.Value, valueTypes []llvm.Type) []llvm.Value {
	packedType := b.ctx.StructType(valueTypes, false)

	// Get a correctly-typed pointer to the packed data.
	var packedAlloc, packedRawAlloc llvm.Value
	size := b.targetData.TypeAllocSize(packedType)
	if size == 0 {
		// No data to unpack.
	} else if len(valueTypes) == 1 && valueTypes[0].TypeKind() == llvm.PointerTypeKind {
		// A single pointer is always stored directly.
		return []llvm.Value{b.CreateBitCast(ptr, valueTypes[0], "unpack.ptr")}
	} else if size <= b.targetData.TypeAllocSize(b.i8ptrType) {
		// Packed data stored directly in pointer.
		if len(valueTypes) == 1 && valueTypes[0].TypeKind() == llvm.IntegerTypeKind {
			// Keep this cast in SSA form.
			return []llvm.Value{b.CreatePtrToInt(ptr, valueTypes[0], "unpack.int")}
		}
		// Fallback: load it using an alloca.
		packedRawAlloc, _, _ = b.createTemporaryAlloca(llvm.PointerType(b.i8ptrType, 0), "unpack.raw.alloc")
		packedRawValue := b.CreateBitCast(ptr, llvm.PointerType(b.i8ptrType, 0), "unpack.raw.value")
		b.CreateStore(packedRawValue, packedRawAlloc)
		packedAlloc = b.CreateBitCast(packedRawAlloc, llvm.PointerType(packedType, 0), "unpack.alloc")
	} else {
		// Packed data stored on the heap. Bitcast the passed-in pointer to the
		// correct pointer type.
		packedAlloc = b.CreateBitCast(ptr, llvm.PointerType(packedType, 0), "unpack.raw.ptr")
	}
	// Load each value from the packed data.
	values := make([]llvm.Value, len(valueTypes))
	for i, valueType := range valueTypes {
		if b.targetData.TypeAllocSize(valueType) == 0 {
			// This value has length zero, so there's nothing to load.
			values[i] = llvm.ConstNull(valueType)
			continue
		}
		indices := []llvm.Value{
			llvm.ConstInt(b.ctx.Int32Type(), 0, false),
			llvm.ConstInt(b.ctx.Int32Type(), uint64(i), false),
		}
		gep := b.CreateInBoundsGEP(packedType, packedAlloc, indices, "")
		values[i] = b.CreateLoad(valueType, gep, "")
	}
	if !packedRawAlloc.IsNil() {
		allocPtr := b.CreateBitCast(packedRawAlloc, b.i8ptrType, "")
		allocSize := llvm.ConstInt(b.ctx.Int64Type(), b.targetData.TypeAllocSize(b.uintptrType), false)
		b.emitLifetimeEnd(allocPtr, allocSize)
	}
	return values
}

// makeGlobalArray creates a new LLVM global with the given name and integers as
// contents, and returns the global and initializer type.
// Note that it is left with the default linkage etc., you should set
// linkage/constant/etc properties yourself.
func (c *compilerContext) makeGlobalArray(buf []byte, name string, elementType llvm.Type) (llvm.Type, llvm.Value) {
	globalType := llvm.ArrayType(elementType, len(buf))
	global := llvm.AddGlobal(c.mod, globalType, name)
	value := llvm.Undef(globalType)
	for i := 0; i < len(buf); i++ {
		ch := uint64(buf[i])
		value = c.builder.CreateInsertValue(value, llvm.ConstInt(elementType, ch, false), i, "")
	}
	global.SetInitializer(value)
	return globalType, global
}

// createObjectLayout returns a LLVM value (of type i8*) that describes where
// there are pointers in the type t. If all the data fits in a word, it is
// returned as a word. Otherwise it will store the data in a global.
//
// The value contains two pieces of information: the length of the object and
// which words contain a pointer (indicated by setting the given bit to 1). For
// arrays, only the element is stored. This works because the GC knows the
// object size and can therefore know how this value is repeated in the object.
//
// For details on what's in this value, see src/runtime/gc_precise.go.
func (c *compilerContext) createObjectLayout(t llvm.Type, pos token.Pos) llvm.Value {
	// Use the element type for arrays. This works even for nested arrays.
	for {
		kind := t.TypeKind()
		if kind == llvm.ArrayTypeKind {
			t = t.ElementType()
			continue
		}
		if kind == llvm.StructTypeKind {
			fields := t.StructElementTypes()
			if len(fields) == 1 {
				t = fields[0]
				continue
			}
		}
		break
	}

	// Do a few checks to see whether we need to generate any object layout
	// information at all.
	objectSizeBytes := c.targetData.TypeAllocSize(t)
	pointerSize := c.targetData.TypeAllocSize(c.i8ptrType)
	pointerAlignment := c.targetData.PrefTypeAlignment(c.i8ptrType)
	if objectSizeBytes < pointerSize {
		// Too small to contain a pointer.
		layout := (uint64(1) << 1) | 1
		return llvm.ConstIntToPtr(llvm.ConstInt(c.uintptrType, layout, false), c.i8ptrType)
	}
	bitmap := c.getPointerBitmap(t, pos)
	if bitmap.BitLen() == 0 {
		// There are no pointers in this type, so we can simplify the layout.
		// TODO: this can be done in many other cases, e.g. when allocating an
		// array (like [4][]byte, which repeats a slice 4 times).
		layout := (uint64(1) << 1) | 1
		return llvm.ConstIntToPtr(llvm.ConstInt(c.uintptrType, layout, false), c.i8ptrType)
	}
	if objectSizeBytes%uint64(pointerAlignment) != 0 {
		// This shouldn't happen except for packed structs, which aren't
		// currently used.
		c.addError(pos, "internal error: unexpected object size for object with pointer field")
		return llvm.ConstNull(c.i8ptrType)
	}
	objectSizeWords := objectSizeBytes / uint64(pointerAlignment)

	pointerBits := pointerSize * 8
	var sizeFieldBits uint64
	switch pointerBits {
	case 16:
		sizeFieldBits = 4
	case 32:
		sizeFieldBits = 5
	case 64:
		sizeFieldBits = 6
	default:
		panic("unknown pointer size")
	}
	layoutFieldBits := pointerBits - 1 - sizeFieldBits

	// Try to emit the value as an inline integer. This is possible in most
	// cases.
	if objectSizeWords < layoutFieldBits {
		// If it can be stored directly in the pointer value, do so.
		// The runtime knows that if the least significant bit of the pointer is
		// set, the pointer contains the value itself.
		layout := bitmap.Uint64()<<(sizeFieldBits+1) | (objectSizeWords << 1) | 1
		return llvm.ConstIntToPtr(llvm.ConstInt(c.uintptrType, layout, false), c.i8ptrType)
	}

	// Unfortunately, the object layout is too big to fit in a pointer-sized
	// integer. Store it in a global instead.

	// Try first whether the global already exists. All objects with a
	// particular name have the same type, so this is possible.
	globalName := "runtime/gc.layout:" + fmt.Sprintf("%d-%0*x", objectSizeWords, (objectSizeWords+15)/16, bitmap)
	global := c.mod.NamedGlobal(globalName)
	if !global.IsNil() {
		return llvm.ConstBitCast(global, c.i8ptrType)
	}

	// Create the global initializer.
	bitmapBytes := make([]byte, int(objectSizeWords+7)/8)
	bitmap.FillBytes(bitmapBytes)
	reverseBytes(bitmapBytes) // big-endian to little-endian
	var bitmapByteValues []llvm.Value
	for _, b := range bitmapBytes {
		bitmapByteValues = append(bitmapByteValues, llvm.ConstInt(c.ctx.Int8Type(), uint64(b), false))
	}
	initializer := c.ctx.ConstStruct([]llvm.Value{
		llvm.ConstInt(c.uintptrType, objectSizeWords, false),
		llvm.ConstArray(c.ctx.Int8Type(), bitmapByteValues),
	}, false)

	global = llvm.AddGlobal(c.mod, initializer.Type(), globalName)
	global.SetInitializer(initializer)
	global.SetUnnamedAddr(true)
	global.SetGlobalConstant(true)
	global.SetLinkage(llvm.LinkOnceODRLinkage)
	if c.targetData.PrefTypeAlignment(c.uintptrType) < 2 {
		// AVR doesn't have alignment by default.
		global.SetAlignment(2)
	}
	if c.Debug && pos != token.NoPos {
		// Creating a fake global so that the value can be inspected in GDB.
		// For example, the layout for strings.stringFinder (as of Go version
		// 1.15) has the following type according to GDB:
		//   type = struct {
		//       uintptr numBits;
		//       uint8 data[33];
		//   }
		// ...that's sort of a mixed C/Go type, but it is readable. More
		// importantly, these object layout globals can be read and printed by
		// GDB which may be useful for debugging.
		position := c.program.Fset.Position(pos)
		diglobal := c.dibuilder.CreateGlobalVariableExpression(c.difiles[position.Filename], llvm.DIGlobalVariableExpression{
			Name: globalName,
			File: c.getDIFile(position.Filename),
			Line: position.Line,
			Type: c.getDIType(types.NewStruct([]*types.Var{
				types.NewVar(pos, nil, "numBits", types.Typ[types.Uintptr]),
				types.NewVar(pos, nil, "data", types.NewArray(types.Typ[types.Byte], int64(len(bitmapByteValues)))),
			}, nil)),
			LocalToUnit: false,
			Expr:        c.dibuilder.CreateExpression(nil),
		})
		global.AddMetadata(0, diglobal)
	}

	return llvm.ConstBitCast(global, c.i8ptrType)
}

// getPointerBitmap scans the given LLVM type for pointers and sets bits in a
// bigint at the word offset that contains a pointer. This scan is recursive.
func (c *compilerContext) getPointerBitmap(typ llvm.Type, pos token.Pos) *big.Int {
	alignment := c.targetData.PrefTypeAlignment(c.i8ptrType)
	switch typ.TypeKind() {
	case llvm.IntegerTypeKind, llvm.FloatTypeKind, llvm.DoubleTypeKind:
		return big.NewInt(0)
	case llvm.PointerTypeKind:
		return big.NewInt(1)
	case llvm.StructTypeKind:
		ptrs := big.NewInt(0)
		if typ.StructName() == "runtime.funcValue" {
			// Hack: the type runtime.funcValue contains an 'id' field which is
			// of type uintptr, but before the LowerFuncValues pass it actually
			// contains a pointer (ptrtoint) to a global. This trips up the
			// interp package. Therefore, make the id field a pointer for now.
			typ = c.ctx.StructType([]llvm.Type{c.i8ptrType, c.i8ptrType}, false)
		}
		for i, subtyp := range typ.StructElementTypes() {
			subptrs := c.getPointerBitmap(subtyp, pos)
			if subptrs.BitLen() == 0 {
				continue
			}
			offset := c.targetData.ElementOffset(typ, i)
			if offset%uint64(alignment) != 0 {
				// This error will let the compilation fail, but by continuing
				// the error can still easily be shown.
				c.addError(pos, "internal error: allocated struct contains unaligned pointer")
				continue
			}
			subptrs.Lsh(subptrs, uint(offset)/uint(alignment))
			ptrs.Or(ptrs, subptrs)
		}
		return ptrs
	case llvm.ArrayTypeKind:
		subtyp := typ.ElementType()
		subptrs := c.getPointerBitmap(subtyp, pos)
		ptrs := big.NewInt(0)
		if subptrs.BitLen() == 0 {
			return ptrs
		}
		elementSize := c.targetData.TypeAllocSize(subtyp)
		if elementSize%uint64(alignment) != 0 {
			// This error will let the compilation fail (but continues so that
			// other errors can be shown).
			c.addError(pos, "internal error: allocated array contains unaligned pointer")
			return ptrs
		}
		for i := 0; i < typ.ArrayLength(); i++ {
			ptrs.Lsh(ptrs, uint(elementSize)/uint(alignment))
			ptrs.Or(ptrs, subptrs)
		}
		return ptrs
	default:
		// Should not happen.
		panic("unknown LLVM type")
	}
}

// archFamily returns the archtecture from the LLVM triple but with some
// architecture names ("armv6", "thumbv7m", etc) merged into a single
// architecture name ("arm").
func (c *compilerContext) archFamily() string {
	arch := strings.Split(c.Triple, "-")[0]
	if strings.HasPrefix(arch, "arm64") {
		return "aarch64"
	}
	if strings.HasPrefix(arch, "arm") || strings.HasPrefix(arch, "thumb") {
		return "arm"
	}
	return arch
}

// isThumb returns whether we're in ARM or in Thumb mode. It panics if the
// features string is not one for an ARM architecture.
func (c *compilerContext) isThumb() bool {
	var isThumb, isNotThumb bool
	for _, feature := range strings.Split(c.Features, ",") {
		if feature == "+thumb-mode" {
			isThumb = true
		}
		if feature == "-thumb-mode" {
			isNotThumb = true
		}
	}
	if isThumb == isNotThumb {
		panic("unexpected feature flags")
	}
	return isThumb
}

// readStackPointer emits a LLVM intrinsic call that returns the current stack
// pointer as an *i8.
func (b *builder) readStackPointer() llvm.Value {
	stacksave := b.mod.NamedFunction("llvm.stacksave")
	if stacksave.IsNil() {
		fnType := llvm.FunctionType(b.i8ptrType, nil, false)
		stacksave = llvm.AddFunction(b.mod, "llvm.stacksave", fnType)
	}
	return b.CreateCall(stacksave.GlobalValueType(), stacksave, nil, "")
}

// createZExtOrTrunc lets the input value fit in the output type bits, by zero
// extending or truncating the integer.
func (b *builder) createZExtOrTrunc(value llvm.Value, t llvm.Type) llvm.Value {
	valueBits := value.Type().IntTypeWidth()
	resultBits := t.IntTypeWidth()
	if valueBits > resultBits {
		value = b.CreateTrunc(value, t, "")
	} else if valueBits < resultBits {
		value = b.CreateZExt(value, t, "")
	}
	return value
}

// Reverse a slice of bytes. From the wiki:
// https://github.com/golang/go/wiki/SliceTricks#reversing
func reverseBytes(buf []byte) {
	for i := len(buf)/2 - 1; i >= 0; i-- {
		opp := len(buf) - 1 - i
		buf[i], buf[opp] = buf[opp], buf[i]
	}
}