[dev.link] cmd/link: move some work after deadcode

This is in preparation of delaying loading symbol content to
after dead code elimination.

Change-Id: Ia4ce82d9a42cc1a76c82f6d0eb9a54b0325ee3ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/197061
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

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

@@ -415,55 +415,40 @@ func (ctxt *Link) loadlib() {
 		}
 	}
 
-	switch ctxt.BuildMode {
-	case BuildModeCShared, BuildModePlugin:
-		s := ctxt.Syms.Lookup("runtime.islibrary", 0)
-		s.Type = sym.SNOPTRDATA
-		s.Attr |= sym.AttrDuplicateOK
-		s.AddUint8(1)
-	case BuildModeCArchive:
-		s := ctxt.Syms.Lookup("runtime.isarchive", 0)
-		s.Type = sym.SNOPTRDATA
-		s.Attr |= sym.AttrDuplicateOK
-		s.AddUint8(1)
-	}
-
 	iscgo = ctxt.Syms.ROLookup("x_cgo_init", 0) != nil
 
 	// We now have enough information to determine the link mode.
 	determineLinkMode(ctxt)
 
-	// Recalculate pe parameters now that we have ctxt.LinkMode set.
-	if ctxt.HeadType == objabi.Hwindows {
-		Peinit(ctxt)
-	}
-
-	if ctxt.HeadType == objabi.Hdarwin && ctxt.LinkMode == LinkExternal {
-		*FlagTextAddr = 0
-	}
+	// Now that we know the link mode, trim the dynexp list.
+	x := sym.AttrCgoExportDynamic
 
-	if ctxt.LinkMode == LinkExternal && ctxt.Arch.Family == sys.PPC64 && objabi.GOOS != "aix" {
-		toc := ctxt.Syms.Lookup(".TOC.", 0)
-		toc.Type = sym.SDYNIMPORT
+	if ctxt.LinkMode == LinkExternal {
+		x = sym.AttrCgoExportStatic
 	}
-
-	if ctxt.LinkMode == LinkExternal && !iscgo && ctxt.LibraryByPkg["runtime/cgo"] == nil && !(objabi.GOOS == "darwin" && (ctxt.Arch.Family == sys.AMD64 || ctxt.Arch.Family == sys.I386)) {
-		// This indicates a user requested -linkmode=external.
-		// The startup code uses an import of runtime/cgo to decide
-		// whether to initialize the TLS.  So give it one. This could
-		// be handled differently but it's an unusual case.
-		if lib := loadinternal(ctxt, "runtime/cgo"); lib != nil {
-			if lib.Shlib != "" {
-				ldshlibsyms(ctxt, lib.Shlib)
-			} else {
-				if ctxt.BuildMode == BuildModeShared || ctxt.linkShared {
-					Exitf("cannot implicitly include runtime/cgo in a shared library")
+	w := 0
+	for i := range dynexp {
+		if dynexp[i].Attr&x != 0 {
+			dynexp[w] = dynexp[i]
+			w++
 		}
-				loadobjfile(ctxt, lib)
 	}
+	dynexp = dynexp[:w]
+
+	// Resolve ABI aliases in the list of cgo-exported functions.
+	// This is necessary because we load the ABI0 symbol for all
+	// cgo exports.
+	for i, s := range dynexp {
+		if s.Type != sym.SABIALIAS {
+			continue
 		}
+		t := resolveABIAlias(s)
+		t.Attr |= s.Attr
+		t.SetExtname(s.Extname())
+		dynexp[i] = t
 	}
 
+	// In internal link mode, read the host object files.
 	if ctxt.LinkMode == LinkInternal {
 		// Drop all the cgo_import_static declarations.
 		// Turns out we won't be needing them.
@@ -480,85 +465,7 @@ func (ctxt *Link) loadlib() {
 				}
 			}
 		}
-	}
-
-	// The Android Q linker started to complain about underalignment of the our TLS
-	// section. We don't actually use the section on android, so dont't
-	// generate it.
-	if objabi.GOOS != "android" {
-		tlsg := ctxt.Syms.Lookup("runtime.tlsg", 0)
-
-		// runtime.tlsg is used for external linking on platforms that do not define
-		// a variable to hold g in assembly (currently only intel).
-		if tlsg.Type == 0 {
-			tlsg.Type = sym.STLSBSS
-			tlsg.Size = int64(ctxt.Arch.PtrSize)
-		} else if tlsg.Type != sym.SDYNIMPORT {
-			Errorf(nil, "runtime declared tlsg variable %v", tlsg.Type)
-		}
-		tlsg.Attr |= sym.AttrReachable
-		ctxt.Tlsg = tlsg
-	}
-
-	var moduledata *sym.Symbol
-	if ctxt.BuildMode == BuildModePlugin {
-		moduledata = ctxt.Syms.Lookup("local.pluginmoduledata", 0)
-		moduledata.Attr |= sym.AttrLocal
-	} else {
-		moduledata = ctxt.Syms.Lookup("runtime.firstmoduledata", 0)
-	}
-	if moduledata.Type != 0 && moduledata.Type != sym.SDYNIMPORT {
-		// If the module (toolchain-speak for "executable or shared
-		// library") we are linking contains the runtime package, it
-		// will define the runtime.firstmoduledata symbol and we
-		// truncate it back to 0 bytes so we can define its entire
-		// contents in symtab.go:symtab().
-		moduledata.Size = 0
 
-		// In addition, on ARM, the runtime depends on the linker
-		// recording the value of GOARM.
-		if ctxt.Arch.Family == sys.ARM {
-			s := ctxt.Syms.Lookup("runtime.goarm", 0)
-			s.Type = sym.SDATA
-			s.Size = 0
-			s.AddUint8(uint8(objabi.GOARM))
-		}
-
-		if objabi.Framepointer_enabled(objabi.GOOS, objabi.GOARCH) {
-			s := ctxt.Syms.Lookup("runtime.framepointer_enabled", 0)
-			s.Type = sym.SDATA
-			s.Size = 0
-			s.AddUint8(1)
-		}
-	} else {
-		// If OTOH the module does not contain the runtime package,
-		// create a local symbol for the moduledata.
-		moduledata = ctxt.Syms.Lookup("local.moduledata", 0)
-		moduledata.Attr |= sym.AttrLocal
-	}
-	// In all cases way we mark the moduledata as noptrdata to hide it from
-	// the GC.
-	moduledata.Type = sym.SNOPTRDATA
-	moduledata.Attr |= sym.AttrReachable
-	ctxt.Moduledata = moduledata
-
-	// Now that we know the link mode, trim the dynexp list.
-	x := sym.AttrCgoExportDynamic
-
-	if ctxt.LinkMode == LinkExternal {
-		x = sym.AttrCgoExportStatic
-	}
-	w := 0
-	for i := range dynexp {
-		if dynexp[i].Attr&x != 0 {
-			dynexp[w] = dynexp[i]
-			w++
-		}
-	}
-	dynexp = dynexp[:w]
-
-	// In internal link mode, read the host object files.
-	if ctxt.LinkMode == LinkInternal {
 		hostobjs(ctxt)
 
 		// If we have any undefined symbols in external
@@ -613,38 +520,21 @@ func (ctxt *Link) loadlib() {
 	// Record whether we can use plugins.
 	ctxt.canUsePlugins = (ctxt.Syms.ROLookup("plugin.Open", sym.SymVerABIInternal) != nil)
 
-	// If there are no dynamic libraries needed, gcc disables dynamic linking.
-	// Because of this, glibc's dynamic ELF loader occasionally (like in version 2.13)
-	// assumes that a dynamic binary always refers to at least one dynamic library.
-	// Rather than be a source of test cases for glibc, disable dynamic linking
-	// the same way that gcc would.
-	//
-	// Exception: on OS X, programs such as Shark only work with dynamic
-	// binaries, so leave it enabled on OS X (Mach-O) binaries.
-	// Also leave it enabled on Solaris which doesn't support
-	// statically linked binaries.
-	if ctxt.BuildMode == BuildModeExe {
-		if havedynamic == 0 && ctxt.HeadType != objabi.Hdarwin && ctxt.HeadType != objabi.Hsolaris {
-			*FlagD = true
-		}
-	}
-
-	// If package versioning is required, generate a hash of the
-	// packages used in the link.
-	if ctxt.BuildMode == BuildModeShared || ctxt.BuildMode == BuildModePlugin || ctxt.CanUsePlugins() {
-		for _, lib := range ctxt.Library {
-			if lib.Shlib == "" {
-				genhash(ctxt, lib)
+	if ctxt.LinkMode == LinkExternal && !iscgo && ctxt.LibraryByPkg["runtime/cgo"] == nil && !(objabi.GOOS == "darwin" && (ctxt.Arch.Family == sys.AMD64 || ctxt.Arch.Family == sys.I386)) {
+		// This indicates a user requested -linkmode=external.
+		// The startup code uses an import of runtime/cgo to decide
+		// whether to initialize the TLS.  So give it one. This could
+		// be handled differently but it's an unusual case.
+		if lib := loadinternal(ctxt, "runtime/cgo"); lib != nil {
+			if lib.Shlib != "" {
+				ldshlibsyms(ctxt, lib.Shlib)
+			} else {
+				if ctxt.BuildMode == BuildModeShared || ctxt.linkShared {
+					Exitf("cannot implicitly include runtime/cgo in a shared library")
 				}
+				loadobjfile(ctxt, lib)
 			}
 		}
-
-	if ctxt.Arch == sys.Arch386 && ctxt.HeadType != objabi.Hwindows {
-		if (ctxt.BuildMode == BuildModeCArchive && ctxt.IsELF) || ctxt.BuildMode == BuildModeCShared || ctxt.BuildMode == BuildModePIE || ctxt.DynlinkingGo() {
-			got := ctxt.Syms.Lookup("_GLOBAL_OFFSET_TABLE_", 0)
-			got.Type = sym.SDYNIMPORT
-			got.Attr |= sym.AttrReachable
-		}
 	}
 
 	importcycles()
@@ -686,18 +576,129 @@ func (ctxt *Link) loadlib() {
 		}
 		ctxt.Textp = textp
 	}
+}
 
-	// Resolve ABI aliases in the list of cgo-exported functions.
-	// This is necessary because we load the ABI0 symbol for all
-	// cgo exports.
-	for i, s := range dynexp {
-		if s.Type != sym.SABIALIAS {
-			continue
+// Set up flags and special symbols depending on the platform build mode.
+func (ctxt *Link) linksetup() {
+	switch ctxt.BuildMode {
+	case BuildModeCShared, BuildModePlugin:
+		s := ctxt.Syms.Lookup("runtime.islibrary", 0)
+		s.Type = sym.SNOPTRDATA
+		s.Attr |= sym.AttrDuplicateOK
+		s.AddUint8(1)
+	case BuildModeCArchive:
+		s := ctxt.Syms.Lookup("runtime.isarchive", 0)
+		s.Type = sym.SNOPTRDATA
+		s.Attr |= sym.AttrDuplicateOK
+		s.AddUint8(1)
+	}
+
+	// Recalculate pe parameters now that we have ctxt.LinkMode set.
+	if ctxt.HeadType == objabi.Hwindows {
+		Peinit(ctxt)
+	}
+
+	if ctxt.HeadType == objabi.Hdarwin && ctxt.LinkMode == LinkExternal {
+		*FlagTextAddr = 0
+	}
+
+	// If there are no dynamic libraries needed, gcc disables dynamic linking.
+	// Because of this, glibc's dynamic ELF loader occasionally (like in version 2.13)
+	// assumes that a dynamic binary always refers to at least one dynamic library.
+	// Rather than be a source of test cases for glibc, disable dynamic linking
+	// the same way that gcc would.
+	//
+	// Exception: on OS X, programs such as Shark only work with dynamic
+	// binaries, so leave it enabled on OS X (Mach-O) binaries.
+	// Also leave it enabled on Solaris which doesn't support
+	// statically linked binaries.
+	if ctxt.BuildMode == BuildModeExe {
+		if havedynamic == 0 && ctxt.HeadType != objabi.Hdarwin && ctxt.HeadType != objabi.Hsolaris {
+			*FlagD = true
+		}
+	}
+
+	if ctxt.LinkMode == LinkExternal && ctxt.Arch.Family == sys.PPC64 && objabi.GOOS != "aix" {
+		toc := ctxt.Syms.Lookup(".TOC.", 0)
+		toc.Type = sym.SDYNIMPORT
+	}
+
+	// The Android Q linker started to complain about underalignment of the our TLS
+	// section. We don't actually use the section on android, so dont't
+	// generate it.
+	if objabi.GOOS != "android" {
+		tlsg := ctxt.Syms.Lookup("runtime.tlsg", 0)
+
+		// runtime.tlsg is used for external linking on platforms that do not define
+		// a variable to hold g in assembly (currently only intel).
+		if tlsg.Type == 0 {
+			tlsg.Type = sym.STLSBSS
+			tlsg.Size = int64(ctxt.Arch.PtrSize)
+		} else if tlsg.Type != sym.SDYNIMPORT {
+			Errorf(nil, "runtime declared tlsg variable %v", tlsg.Type)
+		}
+		tlsg.Attr |= sym.AttrReachable
+		ctxt.Tlsg = tlsg
+	}
+
+	var moduledata *sym.Symbol
+	if ctxt.BuildMode == BuildModePlugin {
+		moduledata = ctxt.Syms.Lookup("local.pluginmoduledata", 0)
+		moduledata.Attr |= sym.AttrLocal
+	} else {
+		moduledata = ctxt.Syms.Lookup("runtime.firstmoduledata", 0)
+	}
+	if moduledata.Type != 0 && moduledata.Type != sym.SDYNIMPORT {
+		// If the module (toolchain-speak for "executable or shared
+		// library") we are linking contains the runtime package, it
+		// will define the runtime.firstmoduledata symbol and we
+		// truncate it back to 0 bytes so we can define its entire
+		// contents in symtab.go:symtab().
+		moduledata.Size = 0
+
+		// In addition, on ARM, the runtime depends on the linker
+		// recording the value of GOARM.
+		if ctxt.Arch.Family == sys.ARM {
+			s := ctxt.Syms.Lookup("runtime.goarm", 0)
+			s.Type = sym.SDATA
+			s.Size = 0
+			s.AddUint8(uint8(objabi.GOARM))
+		}
+
+		if objabi.Framepointer_enabled(objabi.GOOS, objabi.GOARCH) {
+			s := ctxt.Syms.Lookup("runtime.framepointer_enabled", 0)
+			s.Type = sym.SDATA
+			s.Size = 0
+			s.AddUint8(1)
+		}
+	} else {
+		// If OTOH the module does not contain the runtime package,
+		// create a local symbol for the moduledata.
+		moduledata = ctxt.Syms.Lookup("local.moduledata", 0)
+		moduledata.Attr |= sym.AttrLocal
+	}
+	// In all cases way we mark the moduledata as noptrdata to hide it from
+	// the GC.
+	moduledata.Type = sym.SNOPTRDATA
+	moduledata.Attr |= sym.AttrReachable
+	ctxt.Moduledata = moduledata
+
+	// If package versioning is required, generate a hash of the
+	// packages used in the link.
+	if ctxt.BuildMode == BuildModeShared || ctxt.BuildMode == BuildModePlugin || ctxt.CanUsePlugins() {
+		for _, lib := range ctxt.Library {
+			if lib.Shlib == "" {
+				genhash(ctxt, lib)
+			}
+		}
+	}
+
+	if ctxt.Arch == sys.Arch386 && ctxt.HeadType != objabi.Hwindows {
+		if (ctxt.BuildMode == BuildModeCArchive && ctxt.IsELF) || ctxt.BuildMode == BuildModeCShared || ctxt.BuildMode == BuildModePIE || ctxt.DynlinkingGo() {
+			got := ctxt.Syms.Lookup("_GLOBAL_OFFSET_TABLE_", 0)
+			got.Type = sym.SDYNIMPORT
+			got.Attr |= sym.AttrReachable
 		}
-		t := resolveABIAlias(s)
-		t.Attr |= s.Attr
-		t.SetExtname(s.Extname())
-		dynexp[i] = t
 	}
 }
 

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

@@ -208,9 +208,10 @@ func Main(arch *sys.Arch, theArch Arch) {
 	}
 	ctxt.loadlib()
 
-	ctxt.dostrdata()
 	deadcode(ctxt)
-	//ctxt.dumpsyms() // XXX
+	ctxt.linksetup()
+	ctxt.dostrdata()
+
 	dwarfGenerateDebugInfo(ctxt)
 	if objabi.Fieldtrack_enabled != 0 {
 		fieldtrack(ctxt)