src/cmd/link/internal/ld/deadcode.go

// Copyright 2016 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 ld

import (
	"cmd/internal/objabi"
	"cmd/link/internal/sym"
)

// deadcode marks all reachable symbols.
//
// The basis of the dead code elimination is a flood fill of symbols,
// following their relocations, beginning at *flagEntrySymbol.
//
// This flood fill is wrapped in logic for pruning unused methods.
// All methods are mentioned by relocations on their receiver's *rtype.
// These relocations are specially defined as R_METHODOFF by the compiler
// so we can detect and manipulated them here.
//
// There are three ways a method of a reachable type can be invoked:
//
//	1. direct call
//	2. through a reachable interface type
//	3. reflect.Value.Call, .Method, or reflect.Method.Func
//
// The first case is handled by the flood fill, a directly called method
// is marked as reachable.
//
// The second case is handled by decomposing all reachable interface
// types into method signatures. Each encountered method is compared
// against the interface method signatures, if it matches it is marked
// as reachable. This is extremely conservative, but easy and correct.
//
// The third case is handled by looking to see if any of:
//	- reflect.Value.Call is reachable
//	- reflect.Value.Method is reachable
// 	- reflect.Type.Method or MethodByName is called.
// If any of these happen, all bets are off and all exported methods
// of reachable types are marked reachable.
//
// Any unreached text symbols are removed from ctxt.Textp.
func deadcode(ctxt *Link) {
	deadcode2(ctxt)
}

func addToTextp(ctxt *Link) {
	// Remove dead text but keep file information (z symbols).
	textp := []*sym.Symbol{}
	for _, s := range ctxt.Textp {
		if s.Attr.Reachable() {
			textp = append(textp, s)
		}
	}

	// Put reachable text symbols into Textp.
	// do it in postorder so that packages are laid down in dependency order
	// internal first, then everything else
	ctxt.Library = postorder(ctxt.Library)
	for _, doInternal := range [2]bool{true, false} {
		for _, lib := range ctxt.Library {
			if isRuntimeDepPkg(lib.Pkg) != doInternal {
				continue
			}
			libtextp := lib.Textp[:0]
			for _, s := range lib.Textp {
				if s.Attr.Reachable() {
					textp = append(textp, s)
					libtextp = append(libtextp, s)
					if s.Unit != nil {
						s.Unit.Textp = append(s.Unit.Textp, s)
					}
				}
			}
			for _, s := range lib.DupTextSyms {
				if s.Attr.Reachable() && !s.Attr.OnList() {
					textp = append(textp, s)
					libtextp = append(libtextp, s)
					if s.Unit != nil {
						s.Unit.Textp = append(s.Unit.Textp, s)
					}
					s.Attr |= sym.AttrOnList
					// dupok symbols may be defined in multiple packages. its
					// associated package is chosen sort of arbitrarily (the
					// first containing package that the linker loads). canonicalize
					// it here to the package with which it will be laid down
					// in text.
					s.File = objabi.PathToPrefix(lib.Pkg)
				}
			}
			lib.Textp = libtextp
		}
	}
	ctxt.Textp = textp

	if len(ctxt.Shlibs) > 0 {
		// We might have overwritten some functions above (this tends to happen for the
		// autogenerated type equality/hashing functions) and we don't want to generated
		// pcln table entries for these any more so remove them from Textp.
		textp := make([]*sym.Symbol, 0, len(ctxt.Textp))
		for _, s := range ctxt.Textp {
			if s.Type != sym.SDYNIMPORT {
				textp = append(textp, s)
			}
		}
		ctxt.Textp = textp
	}
}