cmd/link, runtime: don't store text start in pcHeader

The textStart field requires a relocation, the only relocation in pclntab.
And nothing uses it. So remove it. Replace it with a zero,
which can itself be removed at some point in coordination with Delve.

For #76038

Change-Id: I35675c0868c5d957bb375e40b804c516ae0300ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/717240
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

Author: ianlancetaylor

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

@@ -243,7 +243,6 @@ func makeInlSyms(ctxt *Link, funcs []loader.Sym, nameOffsets map[loader.Sym]uint
 // generator to fill in its data later.
 func (state *pclntab) generatePCHeader(ctxt *Link) {
 	ldr := ctxt.loader
-	textStartOff := int64(8 + 2*ctxt.Arch.PtrSize)
 	size := int64(8 + 8*ctxt.Arch.PtrSize)
 	writeHeader := func(ctxt *Link, s loader.Sym) {
 		header := ctxt.loader.MakeSymbolUpdater(s)
@@ -264,10 +263,7 @@ func (state *pclntab) generatePCHeader(ctxt *Link) {
 		header.SetUint8(ctxt.Arch, 7, uint8(ctxt.Arch.PtrSize))
 		off := header.SetUint(ctxt.Arch, 8, uint64(state.nfunc))
 		off = header.SetUint(ctxt.Arch, off, uint64(state.nfiles))
-		if off != textStartOff {
-			panic(fmt.Sprintf("pcHeader textStartOff: %d != %d", off, textStartOff))
-		}
-		off += int64(ctxt.Arch.PtrSize) // skip runtimeText relocation
+		off = header.SetUintptr(ctxt.Arch, off, 0) // unused
 		off = writeSymOffset(off, state.funcnametab)
 		off = writeSymOffset(off, state.cutab)
 		off = writeSymOffset(off, state.filetab)
@@ -279,9 +275,6 @@ func (state *pclntab) generatePCHeader(ctxt *Link) {
 	}
 
 	state.pcheader = state.addGeneratedSym(ctxt, "runtime.pcheader", size, writeHeader)
-	// Create the runtimeText relocation.
-	sb := ldr.MakeSymbolUpdater(state.pcheader)
-	sb.SetAddr(ctxt.Arch, textStartOff, ldr.Lookup("runtime.text", 0))
 }
 
 // walkFuncs iterates over the funcs, calling a function for each unique

src/runtime/runtime2.go

@@ -1009,7 +1009,7 @@ const (
 type _func struct {
 	sys.NotInHeap // Only in static data
 
-	entryOff uint32 // start pc, as offset from moduledata.text/pcHeader.textStart
+	entryOff uint32 // start pc, as offset from moduledata.text
 	nameOff  int32  // function name, as index into moduledata.funcnametab.
 
 	args        int32  // in/out args size

src/runtime/symtab.go

@@ -374,13 +374,19 @@ func (f *_func) funcInfo() funcInfo {
 
 // pcHeader holds data used by the pclntab lookups.
 type pcHeader struct {
-	magic          uint32  // 0xFFFFFFF1
-	pad1, pad2     uint8   // 0,0
-	minLC          uint8   // min instruction size
-	ptrSize        uint8   // size of a ptr in bytes
-	nfunc          int     // number of functions in the module
-	nfiles         uint    // number of entries in the file tab
-	textStart      uintptr // base for function entry PC offsets in this module, equal to moduledata.text
+	magic      uint32 // 0xFFFFFFF1
+	pad1, pad2 uint8  // 0,0
+	minLC      uint8  // min instruction size
+	ptrSize    uint8  // size of a ptr in bytes
+	nfunc      int    // number of functions in the module
+	nfiles     uint   // number of entries in the file tab
+
+	// The next field used to be textStart. This is no longer stored
+	// as it requires a relocation. Code should use the moduledata text
+	// field instead. This unused field can be removed in coordination
+	// with Delve.
+	_ uintptr
+
 	funcnameOffset uintptr // offset to the funcnametab variable from pcHeader
 	cuOffset       uintptr // offset to the cutab variable from pcHeader
 	filetabOffset  uintptr // offset to the filetab variable from pcHeader
@@ -618,10 +624,9 @@ func moduledataverify1(datap *moduledata) {
 	// Check that the pclntab's format is valid.
 	hdr := datap.pcHeader
 	if hdr.magic != 0xfffffff1 || hdr.pad1 != 0 || hdr.pad2 != 0 ||
-		hdr.minLC != sys.PCQuantum || hdr.ptrSize != goarch.PtrSize || hdr.textStart != datap.text {
+		hdr.minLC != sys.PCQuantum || hdr.ptrSize != goarch.PtrSize {
 		println("runtime: pcHeader: magic=", hex(hdr.magic), "pad1=", hdr.pad1, "pad2=", hdr.pad2,
-			"minLC=", hdr.minLC, "ptrSize=", hdr.ptrSize, "pcHeader.textStart=", hex(hdr.textStart),
-			"text=", hex(datap.text), "pluginpath=", datap.pluginpath)
+			"minLC=", hdr.minLC, "ptrSize=", hdr.ptrSize, "pluginpath=", datap.pluginpath)
 		throw("invalid function symbol table")
 	}