tinygo/compiler/calls.go

package compiler

import (
	"github.com/aykevl/go-llvm"
	"golang.org/x/tools/go/ssa"
)

// This file implements the calling convention used by Go.
// The calling convention is like the C calling convention (or, whatever LLVM
// makes of it) with the following modifications:
//   * Struct parameters are fully expanded to individual fields (recursively),
//     when the number of fields (combined) is 3 or less.
//     Examples:
//       {i8*, i32}         -> i8*, i32
//       {{i8*, i32}, i16}  -> i8*, i32, i16
//       {{i64}}            -> i64
//       {}                 ->
//       {i8*, i32, i8, i8} -> {i8*, i32, i8, i8}
//     Note that all native Go data types that don't exist in LLVM (string,
//     slice, interface, fat function pointer) can be expanded this way, making
//     the work of LLVM optimizers easier.
//   * Closures have an extra context paramter appended at the end of the
//     argument list.
//   * Blocking functions have a coroutine pointer prepended to the argument
//     list, see src/runtime/scheduler.go for details.
//
// Some further notes:
//   * Function pointers are lowered to either a raw function pointer or a
//     closure struct: { i8*, function pointer }
//     The function pointer type depends on whether the exact same signature is
//     used anywhere else in the program for a call that needs a context
//     (closures, bound methods). If it isn't, it is lowered to a raw function
//     pointer.
//   * 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.

// The maximum number of arguments that can be expanded from a single struct. If
// a struct contains more fields, it is passed as a struct without expanding.
const MaxFieldsPerParam = 3

// Shortcut: create a call to runtime.<fnName> with the given arguments.
func (c *Compiler) createRuntimeCall(fnName string, args []llvm.Value, name string) llvm.Value {
	runtimePkg := c.ir.Program.ImportedPackage("runtime")
	member := runtimePkg.Members[fnName]
	if member == nil {
		panic("trying to call runtime." + fnName)
	}
	fn := c.ir.GetFunction(member.(*ssa.Function))
	return c.createCall(fn.LLVMFn, args, name)
}

// Create a call to the given function with the arguments possibly expanded.
func (c *Compiler) createCall(fn llvm.Value, args []llvm.Value, name string) llvm.Value {
	expanded := make([]llvm.Value, 0, len(args))
	for _, arg := range args {
		fragments := c.expandFormalParam(arg)
		expanded = append(expanded, fragments...)
	}
	return c.builder.CreateCall(fn, expanded, name)
}

// Expand an argument type to a list that can be used in a function call
// paramter list.
func (c *Compiler) expandFormalParamType(t llvm.Type) []llvm.Type {
	switch t.TypeKind() {
	case llvm.StructTypeKind:
		fields := c.flattenAggregateType(t)
		if len(fields) <= MaxFieldsPerParam {
			return fields
		} else {
			// failed to lower
			return []llvm.Type{t}
		}
	default:
		// TODO: split small arrays
		return []llvm.Type{t}
	}
}

// Equivalent of expandFormalParamType for parameter values.
func (c *Compiler) expandFormalParam(v llvm.Value) []llvm.Value {
	switch v.Type().TypeKind() {
	case llvm.StructTypeKind:
		fieldTypes := c.flattenAggregateType(v.Type())
		if len(fieldTypes) <= MaxFieldsPerParam {
			fields := c.flattenAggregate(v)
			if len(fields) != len(fieldTypes) {
				panic("type and value param lowering don't match")
			}
			return fields
		} else {
			// failed to lower
			return []llvm.Value{v}
		}
	default:
		// TODO: split small arrays
		return []llvm.Value{v}
	}
}

// Try to flatten a struct type to a list of types. Returns a 1-element slice
// with the passed in type if this is not possible.
func (c *Compiler) flattenAggregateType(t llvm.Type) []llvm.Type {
	switch t.TypeKind() {
	case llvm.StructTypeKind:
		fields := make([]llvm.Type, 0, t.StructElementTypesCount())
		for _, subfield := range t.StructElementTypes() {
			subfields := c.flattenAggregateType(subfield)
			fields = append(fields, subfields...)
		}
		return fields
	default:
		return []llvm.Type{t}
	}
}

// Break down a struct into its elementary types for argument passing. The value
// equivalent of flattenAggregateType
func (c *Compiler) flattenAggregate(v llvm.Value) []llvm.Value {
	switch v.Type().TypeKind() {
	case llvm.StructTypeKind:
		fields := make([]llvm.Value, 0, v.Type().StructElementTypesCount())
		for i := range v.Type().StructElementTypes() {
			subfield := c.builder.CreateExtractValue(v, i, "")
			subfields := c.flattenAggregate(subfield)
			fields = append(fields, subfields...)
		}
		return fields
	default:
		return []llvm.Value{v}
	}
}

// Collapse a list of fields into its original value.
func (c *Compiler) collapseFormalParam(t llvm.Type, fields []llvm.Value) llvm.Value {
	param, remaining := c.collapseFormalParamInternal(t, fields)
	if len(remaining) != 0 {
		panic("failed to expand back all fields")
	}
	return param
}

// Returns (value, remainingFields). Used by collapseFormalParam.
func (c *Compiler) collapseFormalParamInternal(t llvm.Type, fields []llvm.Value) (llvm.Value, []llvm.Value) {
	switch t.TypeKind() {
	case llvm.StructTypeKind:
		if len(c.flattenAggregateType(t)) <= MaxFieldsPerParam {
			value, err := c.getZeroValue(t)
			if err != nil {
				panic("could not get zero value of struct: " + err.Error())
			}
			for i, subtyp := range t.StructElementTypes() {
				structField, remaining := c.collapseFormalParamInternal(subtyp, fields)
				fields = remaining
				value = c.builder.CreateInsertValue(value, structField, i, "")
			}
			return value, fields
		} else {
			// this struct was not flattened
			return fields[0], fields[1:]
		}
	default:
		return fields[0], fields[1:]
	}
}