src/cmd/compile/internal/ssagen/abi.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package ssagen

import (
	"fmt"
	"internal/buildcfg"
	"io/ioutil"
	"log"
	"os"
	"strings"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/staticdata"
	"cmd/compile/internal/typecheck"
	"cmd/compile/internal/types"
	"cmd/internal/obj"
	"cmd/internal/objabi"
)

// SymABIs records information provided by the assembler about symbol
// definition ABIs and reference ABIs.
type SymABIs struct {
	defs map[string]obj.ABI
	refs map[string]obj.ABISet

	localPrefix string
}

func NewSymABIs(myimportpath string) *SymABIs {
	var localPrefix string
	if myimportpath != "" {
		localPrefix = objabi.PathToPrefix(myimportpath) + "."
	}

	return &SymABIs{
		defs:        make(map[string]obj.ABI),
		refs:        make(map[string]obj.ABISet),
		localPrefix: localPrefix,
	}
}

// canonicalize returns the canonical name used for a linker symbol in
// s's maps. Symbols in this package may be written either as "".X or
// with the package's import path already in the symbol. This rewrites
// both to `"".`, which matches compiler-generated linker symbol names.
func (s *SymABIs) canonicalize(linksym string) string {
	// If the symbol is already prefixed with localPrefix,
	// rewrite it to start with "" so it matches the
	// compiler's internal symbol names.
	if s.localPrefix != "" && strings.HasPrefix(linksym, s.localPrefix) {
		return `"".` + linksym[len(s.localPrefix):]
	}
	return linksym
}

// ReadSymABIs reads a symabis file that specifies definitions and
// references of text symbols by ABI.
//
// The symabis format is a set of lines, where each line is a sequence
// of whitespace-separated fields. The first field is a verb and is
// either "def" for defining a symbol ABI or "ref" for referencing a
// symbol using an ABI. For both "def" and "ref", the second field is
// the symbol name and the third field is the ABI name, as one of the
// named cmd/internal/obj.ABI constants.
func (s *SymABIs) ReadSymABIs(file string) {
	data, err := ioutil.ReadFile(file)
	if err != nil {
		log.Fatalf("-symabis: %v", err)
	}

	for lineNum, line := range strings.Split(string(data), "\n") {
		lineNum++ // 1-based
		line = strings.TrimSpace(line)
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}

		parts := strings.Fields(line)
		switch parts[0] {
		case "def", "ref":
			// Parse line.
			if len(parts) != 3 {
				log.Fatalf(`%s:%d: invalid symabi: syntax is "%s sym abi"`, file, lineNum, parts[0])
			}
			sym, abistr := parts[1], parts[2]
			abi, valid := obj.ParseABI(abistr)
			if !valid {
				log.Fatalf(`%s:%d: invalid symabi: unknown abi "%s"`, file, lineNum, abistr)
			}

			sym = s.canonicalize(sym)

			// Record for later.
			if parts[0] == "def" {
				s.defs[sym] = abi
			} else {
				s.refs[sym] |= obj.ABISetOf(abi)
			}
		default:
			log.Fatalf(`%s:%d: invalid symabi type "%s"`, file, lineNum, parts[0])
		}
	}
}

// GenABIWrappers applies ABI information to Funcs and generates ABI
// wrapper functions where necessary.
func (s *SymABIs) GenABIWrappers() {
	// For cgo exported symbols, we tell the linker to export the
	// definition ABI to C. That also means that we don't want to
	// create ABI wrappers even if there's a linkname.
	//
	// TODO(austin): Maybe we want to create the ABI wrappers, but
	// ensure the linker exports the right ABI definition under
	// the unmangled name?
	cgoExports := make(map[string][]*[]string)
	for i, prag := range typecheck.Target.CgoPragmas {
		switch prag[0] {
		case "cgo_export_static", "cgo_export_dynamic":
			symName := s.canonicalize(prag[1])
			pprag := &typecheck.Target.CgoPragmas[i]
			cgoExports[symName] = append(cgoExports[symName], pprag)
		}
	}

	// Apply ABI defs and refs to Funcs and generate wrappers.
	//
	// This may generate new decls for the wrappers, but we
	// specifically *don't* want to visit those, lest we create
	// wrappers for wrappers.
	for _, fn := range typecheck.Target.Decls {
		if fn.Op() != ir.ODCLFUNC {
			continue
		}
		fn := fn.(*ir.Func)
		nam := fn.Nname
		if ir.IsBlank(nam) {
			continue
		}
		sym := nam.Sym()
		var symName string
		if sym.Linkname != "" {
			symName = s.canonicalize(sym.Linkname)
		} else {
			// These names will already be canonical.
			symName = sym.Pkg.Prefix + "." + sym.Name
		}

		// Apply definitions.
		defABI, hasDefABI := s.defs[symName]
		if hasDefABI {
			fn.ABI = defABI
		}

		if fn.Pragma&ir.CgoUnsafeArgs != 0 {
			// CgoUnsafeArgs indicates the function (or its callee) uses
			// offsets to dispatch arguments, which currently using ABI0
			// frame layout. Pin it to ABI0.
			fn.ABI = obj.ABI0
		}

		// If cgo-exported, add the definition ABI to the cgo
		// pragmas.
		cgoExport := cgoExports[symName]
		for _, pprag := range cgoExport {
			// The export pragmas have the form:
			//
			//   cgo_export_* <local> [<remote>]
			//
			// If <remote> is omitted, it's the same as
			// <local>.
			//
			// Expand to
			//
			//   cgo_export_* <local> <remote> <ABI>
			if len(*pprag) == 2 {
				*pprag = append(*pprag, (*pprag)[1])
			}
			// Add the ABI argument.
			*pprag = append(*pprag, fn.ABI.String())
		}

		// Apply references.
		if abis, ok := s.refs[symName]; ok {
			fn.ABIRefs |= abis
		}
		// Assume all functions are referenced at least as
		// ABIInternal, since they may be referenced from
		// other packages.
		fn.ABIRefs.Set(obj.ABIInternal, true)

		// If a symbol is defined in this package (either in
		// Go or assembly) and given a linkname, it may be
		// referenced from another package, so make it
		// callable via any ABI. It's important that we know
		// it's defined in this package since other packages
		// may "pull" symbols using linkname and we don't want
		// to create duplicate ABI wrappers.
		//
		// However, if it's given a linkname for exporting to
		// C, then we don't make ABI wrappers because the cgo
		// tool wants the original definition.
		hasBody := len(fn.Body) != 0
		if sym.Linkname != "" && (hasBody || hasDefABI) && len(cgoExport) == 0 {
			fn.ABIRefs |= obj.ABISetCallable
		}

		// Double check that cgo-exported symbols don't get
		// any wrappers.
		if len(cgoExport) > 0 && fn.ABIRefs&^obj.ABISetOf(fn.ABI) != 0 {
			base.Fatalf("cgo exported function %s cannot have ABI wrappers", fn)
		}

		if !buildcfg.Experiment.RegabiWrappers {
			// We'll generate ABI aliases instead of
			// wrappers once we have LSyms in InitLSym.
			continue
		}

		forEachWrapperABI(fn, makeABIWrapper)
	}
}

// InitLSym defines f's obj.LSym and initializes it based on the
// properties of f. This includes setting the symbol flags and ABI and
// creating and initializing related DWARF symbols.
//
// InitLSym must be called exactly once per function and must be
// called for both functions with bodies and functions without bodies.
// For body-less functions, we only create the LSym; for functions
// with bodies call a helper to setup up / populate the LSym.
func InitLSym(f *ir.Func, hasBody bool) {
	if f.LSym != nil {
		base.FatalfAt(f.Pos(), "InitLSym called twice on %v", f)
	}

	if nam := f.Nname; !ir.IsBlank(nam) {
		f.LSym = nam.LinksymABI(f.ABI)
		if f.Pragma&ir.Systemstack != 0 {
			f.LSym.Set(obj.AttrCFunc, true)
		}
		if f.ABI == obj.ABIInternal || !buildcfg.Experiment.RegabiWrappers {
			// Function values can only point to
			// ABIInternal entry points. This will create
			// the funcsym for either the defining
			// function or its wrapper as appropriate.
			//
			// If we're using ABI aliases instead of
			// wrappers, we only InitLSym for the defining
			// ABI of a function, so we make the funcsym
			// when we see that.
			staticdata.NeedFuncSym(f)
		}
		if !buildcfg.Experiment.RegabiWrappers {
			// Create ABI aliases instead of wrappers.
			forEachWrapperABI(f, makeABIAlias)
		}
	}
	if hasBody {
		setupTextLSym(f, 0)
	}
}

func forEachWrapperABI(fn *ir.Func, cb func(fn *ir.Func, wrapperABI obj.ABI)) {
	need := fn.ABIRefs &^ obj.ABISetOf(fn.ABI)
	if need == 0 {
		return
	}

	for wrapperABI := obj.ABI(0); wrapperABI < obj.ABICount; wrapperABI++ {
		if !need.Get(wrapperABI) {
			continue
		}
		cb(fn, wrapperABI)
	}
}

// makeABIAlias creates a new ABI alias so calls to f via wrapperABI
// will be resolved directly to f's ABI by the linker.
func makeABIAlias(f *ir.Func, wrapperABI obj.ABI) {
	// These LSyms have the same name as the native function, so
	// we create them directly rather than looking them up.
	// The uniqueness of f.lsym ensures uniqueness of asym.
	asym := &obj.LSym{
		Name: f.LSym.Name,
		Type: objabi.SABIALIAS,
		R:    []obj.Reloc{{Sym: f.LSym}}, // 0 size, so "informational"
	}
	asym.SetABI(wrapperABI)
	asym.Set(obj.AttrDuplicateOK, true)
	base.Ctxt.ABIAliases = append(base.Ctxt.ABIAliases, asym)
}

// makeABIWrapper creates a new function that will be called with
// wrapperABI and calls "f" using f.ABI.
func makeABIWrapper(f *ir.Func, wrapperABI obj.ABI) {
	if base.Debug.ABIWrap != 0 {
		fmt.Fprintf(os.Stderr, "=-= %v to %v wrapper for %v\n", wrapperABI, f.ABI, f)
	}

	// Q: is this needed?
	savepos := base.Pos
	savedclcontext := typecheck.DeclContext
	savedcurfn := ir.CurFunc

	base.Pos = base.AutogeneratedPos
	typecheck.DeclContext = ir.PEXTERN

	// At the moment we don't support wrapping a method, we'd need machinery
	// below to handle the receiver. Panic if we see this scenario.
	ft := f.Nname.Type()
	if ft.NumRecvs() != 0 {
		panic("makeABIWrapper support for wrapping methods not implemented")
	}

	// Manufacture a new func type to use for the wrapper.
	var noReceiver *ir.Field
	tfn := ir.NewFuncType(base.Pos,
		noReceiver,
		typecheck.NewFuncParams(ft.Params(), true),
		typecheck.NewFuncParams(ft.Results(), false))

	// Reuse f's types.Sym to create a new ODCLFUNC/function.
	fn := typecheck.DeclFunc(f.Nname.Sym(), tfn)
	fn.ABI = wrapperABI

	fn.SetABIWrapper(true)
	fn.SetDupok(true)

	// ABI0-to-ABIInternal wrappers will be mainly loading params from
	// stack into registers (and/or storing stack locations back to
	// registers after the wrapped call); in most cases they won't
	// need to allocate stack space, so it should be OK to mark them
	// as NOSPLIT in these cases. In addition, my assumption is that
	// functions written in assembly are NOSPLIT in most (but not all)
	// cases. In the case of an ABIInternal target that has too many
	// parameters to fit into registers, the wrapper would need to
	// allocate stack space, but this seems like an unlikely scenario.
	// Hence: mark these wrappers NOSPLIT.
	//
	// ABIInternal-to-ABI0 wrappers on the other hand will be taking
	// things in registers and pushing them onto the stack prior to
	// the ABI0 call, meaning that they will always need to allocate
	// stack space. If the compiler marks them as NOSPLIT this seems
	// as though it could lead to situations where the linker's
	// nosplit-overflow analysis would trigger a link failure. On the
	// other hand if they not tagged NOSPLIT then this could cause
	// problems when building the runtime (since there may be calls to
	// asm routine in cases where it's not safe to grow the stack). In
	// most cases the wrapper would be (in effect) inlined, but are
	// there (perhaps) indirect calls from the runtime that could run
	// into trouble here.
	// FIXME: at the moment all.bash does not pass when I leave out
	// NOSPLIT for these wrappers, so all are currently tagged with NOSPLIT.
	fn.Pragma |= ir.Nosplit

	// Generate call. Use tail call if no params and no returns,
	// but a regular call otherwise.
	//
	// Note: ideally we would be using a tail call in cases where
	// there are params but no returns for ABI0->ABIInternal wrappers,
	// provided that all params fit into registers (e.g. we don't have
	// to allocate any stack space). Doing this will require some
	// extra work in typecheck/walk/ssa, might want to add a new node
	// OTAILCALL or something to this effect.
	tailcall := tfn.Type().NumResults() == 0 && tfn.Type().NumParams() == 0 && tfn.Type().NumRecvs() == 0
	if base.Ctxt.Arch.Name == "ppc64le" && base.Ctxt.Flag_dynlink {
		// cannot tailcall on PPC64 with dynamic linking, as we need
		// to restore R2 after call.
		tailcall = false
	}
	if base.Ctxt.Arch.Name == "amd64" && wrapperABI == obj.ABIInternal {
		// cannot tailcall from ABIInternal to ABI0 on AMD64, as we need
		// to special registers (X15) when returning to ABIInternal.
		tailcall = false
	}

	var tail ir.Node
	if tailcall {
		tail = ir.NewTailCallStmt(base.Pos, f.Nname)
	} else {
		call := ir.NewCallExpr(base.Pos, ir.OCALL, f.Nname, nil)
		call.Args = ir.ParamNames(tfn.Type())
		call.IsDDD = tfn.Type().IsVariadic()
		tail = call
		if tfn.Type().NumResults() > 0 {
			n := ir.NewReturnStmt(base.Pos, nil)
			n.Results = []ir.Node{call}
			tail = n
		}
	}
	fn.Body.Append(tail)

	typecheck.FinishFuncBody()
	if base.Debug.DclStack != 0 {
		types.CheckDclstack()
	}

	typecheck.Func(fn)
	ir.CurFunc = fn
	typecheck.Stmts(fn.Body)

	typecheck.Target.Decls = append(typecheck.Target.Decls, fn)

	// Restore previous context.
	base.Pos = savepos
	typecheck.DeclContext = savedclcontext
	ir.CurFunc = savedcurfn
}

// setupTextLsym initializes the LSym for a with-body text symbol.
func setupTextLSym(f *ir.Func, flag int) {
	if f.Dupok() {
		flag |= obj.DUPOK
	}
	if f.Wrapper() {
		flag |= obj.WRAPPER
	}
	if f.ABIWrapper() {
		flag |= obj.ABIWRAPPER
	}
	if f.Needctxt() {
		flag |= obj.NEEDCTXT
	}
	if f.Pragma&ir.Nosplit != 0 {
		flag |= obj.NOSPLIT
	}
	if f.ReflectMethod() {
		flag |= obj.REFLECTMETHOD
	}

	// Clumsy but important.
	// For functions that could be on the path of invoking a deferred
	// function that can recover (runtime.reflectcall, reflect.callReflect,
	// and reflect.callMethod), we want the panic+recover special handling.
	// See test/recover.go for test cases and src/reflect/value.go
	// for the actual functions being considered.
	//
	// runtime.reflectcall is an assembly function which tailcalls
	// WRAPPER functions (runtime.callNN). Its ABI wrapper needs WRAPPER
	// flag as well.
	fnname := f.Sym().Name
	if base.Ctxt.Pkgpath == "runtime" && fnname == "reflectcall" {
		flag |= obj.WRAPPER
	} else if base.Ctxt.Pkgpath == "reflect" {
		switch fnname {
		case "callReflect", "callMethod":
			flag |= obj.WRAPPER
		}
	}

	base.Ctxt.InitTextSym(f.LSym, flag)
}