cmd/link: generate PPC64 ABI register save/restore functions if needed

They are usually needed when internally linking gcc code
compiled with -Os. These are typically generated by ld
or gold, but are missing when linking internally.

The PPC64 ELF ABI describes a set of functions to save/restore
non-volatile, callee-save registers using R1/R0/R12:

 _savegpr0_n: Save Rn-R31 relative to R1, save LR (in R0), return
 _restgpr0_n: Restore Rn-R31 from R1, and return to saved LR
  _savefpr_n: Save Fn-F31 based on R1, and save LR (in R0), return
  _restfpr_n: Restore Fn-F31 from R1, and return to 16(R1)
 _savegpr1_n: Save Rn-R31 based on R12, return
 _restgpr1_n: Restore Rn-R31 based on R12, return
   _savevr_m: Save VRm-VR31 based on R0, R12 is scratch, return
   _restvr_m: Restore VRm-VR31 based on R0, R12 is scratch, return

 m is a value 20<=m<=31
 n is a value 14<=n<=31

Add several new functions similar to those suggested by the
PPC64 ELFv2 ABI. And update the linker to scan external relocs
for these calls, and redirect them to runtime.elf_<func>+offset
in runtime/asm_ppc64x.go.

Similarly, code which generates plt stubs is moved into
a dedicated function. This avoids an extra scan of relocs.

fixes #52336

Change-Id: I2f0f8b5b081a7b294dff5c92b4b1db8eba9a9400
Reviewed-on: https://go-review.googlesource.com/c/go/+/400796
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 139 insertions, 41 deletions

diff --git a/src/cmd/link/internal/ppc64/asm.go b/src/cmd/link/internal/ppc64/asm.go
index 73c2718a33..5d5fbe2a97 100644
--- a/src/cmd/link/internal/ppc64/asm.go
+++ b/src/cmd/link/internal/ppc64/asm.go
@@ -40,10 +40,11 @@ import (
 	"encoding/binary"
 	"fmt"
 	"log"
+	"strconv"
 	"strings"
 )
 
-func genplt(ctxt *ld.Link, ldr *loader.Loader) {
+func genpltstub(ctxt *ld.Link, ldr *loader.Loader, r loader.Reloc, s loader.Sym) (sym loader.Sym, firstUse bool) {
 	// The ppc64 ABI PLT has similar concepts to other
 	// architectures, but is laid out quite differently. When we
 	// see an R_PPC64_REL24 relocation to a dynamic symbol
@@ -92,53 +93,82 @@ func genplt(ctxt *ld.Link, ldr *loader.Loader) {
 	//
 	// This assumes "case 1" from the ABI, where the caller needs
 	// us to save and restore the TOC pointer.
-	var stubs []loader.Sym
-	for _, s := range ctxt.Textp {
-		relocs := ldr.Relocs(s)
-		for i := 0; i < relocs.Count(); i++ {
-			r := relocs.At(i)
-			if r.Type() != objabi.ElfRelocOffset+objabi.RelocType(elf.R_PPC64_REL24) || ldr.SymType(r.Sym()) != sym.SDYNIMPORT {
-				continue
-			}
 
-			// Reserve PLT entry and generate symbol
-			// resolver
-			addpltsym(ctxt, ldr, r.Sym())
+	// Reserve PLT entry and generate symbol
+	// resolver
+	addpltsym(ctxt, ldr, r.Sym())
 
-			// Generate call stub. Important to note that we're looking
-			// up the stub using the same version as the parent symbol (s),
-			// needed so that symtoc() will select the right .TOC. symbol
-			// when processing the stub.  In older versions of the linker
-			// this was done by setting stub.Outer to the parent, but
-			// if the stub has the right version initially this is not needed.
-			n := fmt.Sprintf("%s.%s", ldr.SymName(s), ldr.SymName(r.Sym()))
-			stub := ldr.CreateSymForUpdate(n, ldr.SymVersion(s))
-			if stub.Size() == 0 {
-				stubs = append(stubs, stub.Sym())
-				gencallstub(ctxt, ldr, 1, stub, r.Sym())
-			}
+	// Generate call stub. Important to note that we're looking
+	// up the stub using the same version as the parent symbol (s),
+	// needed so that symtoc() will select the right .TOC. symbol
+	// when processing the stub.  In older versions of the linker
+	// this was done by setting stub.Outer to the parent, but
+	// if the stub has the right version initially this is not needed.
+	n := fmt.Sprintf("%s.%s", ldr.SymName(s), ldr.SymName(r.Sym()))
+	stub := ldr.CreateSymForUpdate(n, ldr.SymVersion(s))
+	firstUse = stub.Size() == 0
+	if firstUse {
+		gencallstub(ctxt, ldr, 1, stub, r.Sym())
+	}
 
-			// Update the relocation to use the call stub
-			r.SetSym(stub.Sym())
+	// Update the relocation to use the call stub
+	r.SetSym(stub.Sym())
 
-			// Make the symbol writeable so we can fixup toc.
-			su := ldr.MakeSymbolUpdater(s)
-			su.MakeWritable()
-			p := su.Data()
+	// Make the symbol writeable so we can fixup toc.
+	su := ldr.MakeSymbolUpdater(s)
+	su.MakeWritable()
+	p := su.Data()
 
-			// Check for toc restore slot (a nop), and replace with toc restore.
-			var nop uint32
-			if len(p) >= int(r.Off()+8) {
-				nop = ctxt.Arch.ByteOrder.Uint32(p[r.Off()+4:])
-			}
-			if nop != 0x60000000 {
-				ldr.Errorf(s, "Symbol %s is missing toc restoration slot at offset %d", ldr.SymName(s), r.Off()+4)
+	// Check for toc restore slot (a nop), and replace with toc restore.
+	var nop uint32
+	if len(p) >= int(r.Off()+8) {
+		nop = ctxt.Arch.ByteOrder.Uint32(p[r.Off()+4:])
+	}
+	if nop != 0x60000000 {
+		ldr.Errorf(s, "Symbol %s is missing toc restoration slot at offset %d", ldr.SymName(s), r.Off()+4)
+	}
+	const o1 = 0xe8410018 // ld r2,24(r1)
+	ctxt.Arch.ByteOrder.PutUint32(p[r.Off()+4:], o1)
+
+	return stub.Sym(), firstUse
+}
+
+// Scan relocs and generate PLT stubs and generate/fixup ABI defined functions created by the linker
+func genstubs(ctxt *ld.Link, ldr *loader.Loader) {
+	var stubs []loader.Sym
+	var abifuncs []loader.Sym
+	for _, s := range ctxt.Textp {
+		relocs := ldr.Relocs(s)
+		for i := 0; i < relocs.Count(); i++ {
+			if r := relocs.At(i); r.Type() == objabi.ElfRelocOffset+objabi.RelocType(elf.R_PPC64_REL24) {
+				switch ldr.SymType(r.Sym()) {
+				case sym.SDYNIMPORT:
+					// This call goes throught the PLT, generate and call through a PLT stub.
+					if sym, firstUse := genpltstub(ctxt, ldr, r, s); firstUse {
+						stubs = append(stubs, sym)
+					}
+
+				case sym.SXREF:
+					// Is this an ELF ABI defined function which is (in practice)
+					// generated by the linker to save/restore callee save registers?
+					// These are defined similarly for both PPC64 ELF and ELFv2.
+					targName := ldr.SymName(r.Sym())
+					if strings.HasPrefix(targName, "_save") || strings.HasPrefix(targName, "_rest") {
+						if sym, firstUse := rewriteABIFuncReloc(ctxt, ldr, targName, r); firstUse {
+							abifuncs = append(abifuncs, sym)
+						}
+					}
+				}
 			}
-			const o1 = 0xe8410018 // ld r2,24(r1)
-			ctxt.Arch.ByteOrder.PutUint32(p[r.Off()+4:], o1)
 		}
 	}
-	// Put call stubs at the beginning (instead of the end).
+
+	// Append any usage of the go versions of ELF save/restore
+	// functions to the end of the callstub list to minimize
+	// chances a trampoline might be needed.
+	stubs = append(stubs, abifuncs...)
+
+	// Put stubs at the beginning (instead of the end).
 	// So when resolving the relocations to calls to the stubs,
 	// the addresses are known and trampolines can be inserted
 	// when necessary.
@@ -202,13 +232,74 @@ func genaddmoduledata(ctxt *ld.Link, ldr *loader.Loader) {
 	o(0x4e800020)
 }
 
+// Rewrite ELF (v1 or v2) calls to _savegpr0_n, _savegpr1_n, _savefpr_n, _restfpr_n, _savevr_m, or
+// _restvr_m (14<=n<=31, 20<=m<=31). Redirect them to runtime.elf_restgpr0+(n-14)*4,
+// runtime.elf_restvr+(m-20)*8, and similar.
+//
+// These functions are defined in the ELFv2 ABI (generated when using gcc -Os option) to save and
+// restore callee-saved registers (as defined in the PPC64 ELF ABIs) from registers n or m to 31 of
+// the named type. R12 and R0 are sometimes used in exceptional ways described in the ABI.
+//
+// Final note, this is only needed when linking internally. The external linker will generate these
+// functions if they are used.
+func rewriteABIFuncReloc(ctxt *ld.Link, ldr *loader.Loader, tname string, r loader.Reloc) (sym loader.Sym, firstUse bool) {
+	s := strings.Split(tname, "_")
+	// A valid call will split like {"", "savegpr0", "20"}
+	if len(s) != 3 {
+		return 0, false // Not an abi func.
+	}
+	minReg := 14 // _savegpr0_{n}, _savegpr1_{n}, _savefpr_{n}, 14 <= n <= 31
+	offMul := 4  // 1 instruction per register op.
+	switch s[1] {
+	case "savegpr0", "savegpr1", "savefpr":
+	case "restgpr0", "restgpr1", "restfpr":
+	case "savevr", "restvr":
+		minReg = 20 // _savevr_{n} or _restvr_{n}, 20 <= n <= 31
+		offMul = 8  // 2 instructions per register op.
+	default:
+		return 0, false // Not an abi func
+	}
+	n, e := strconv.Atoi(s[2])
+	if e != nil || n < minReg || n > 31 || r.Add() != 0 {
+		return 0, false // Invalid register number, or non-zero addend. Not an abi func.
+	}
+
+	// tname is a valid relocation to an ABI defined register save/restore function. Re-relocate
+	// them to a go version of these functions in runtime/asm_ppc64x.s
+	ts := ldr.LookupOrCreateSym("runtime.elf_"+s[1], 0)
+	r.SetSym(ts)
+	r.SetAdd(int64((n - minReg) * offMul))
+	firstUse = !ldr.AttrReachable(ts)
+	if firstUse {
+		ldr.SetAttrReachable(ts, true)
+		// This function only becomes reachable now. It has been dropped from
+		// the text section (it was unreachable until now), it needs included.
+		//
+		// Similarly, TOC regeneration should not happen for these functions,
+		// remove it from this save/restore function.
+		if ldr.AttrShared(ts) {
+			sb := ldr.MakeSymbolUpdater(ts)
+			sb.SetData(sb.Data()[8:])
+			sb.SetSize(sb.Size() - 8)
+			relocs := sb.Relocs()
+			// Only one PCREL reloc to .TOC. should be present.
+			if relocs.Count() != 1 {
+				log.Fatalf("Unexpected number of relocs in %s\n", ldr.SymName(ts))
+			}
+			sb.ResetRelocs()
+
+		}
+	}
+	return ts, firstUse
+}
+
 func gentext(ctxt *ld.Link, ldr *loader.Loader) {
 	if ctxt.DynlinkingGo() {
 		genaddmoduledata(ctxt, ldr)
 	}
 
 	if ctxt.LinkMode == ld.LinkInternal {
-		genplt(ctxt, ldr)
+		genstubs(ctxt, ldr)
 	}
 }
 
@@ -863,6 +954,13 @@ func gentramp(ctxt *ld.Link, ldr *loader.Loader, tramp *loader.SymbolBuilder, ta
 		o1 = uint32(0x3c000000) | 12<<21          // lis  r12,targetaddr hi
 		o2 = uint32(0x38000000) | 12<<21 | 12<<16 // addi r12,r12,targetaddr lo
 
+		// ELFv2 save/restore functions use R0/R12 in special ways, therefore trampolines
+		// as generated here will not always work correctly.
+		if strings.HasPrefix(ldr.SymName(target), "runtime.elf_") {
+			log.Fatalf("Internal linker does not support trampolines to ELFv2 ABI"+
+				" register save/restore function %s", ldr.SymName(target))
+		}
+
 		t := ldr.SymValue(target)
 		if t == 0 || r2Valid(ctxt) || ctxt.IsExternal() {
 			// Target address is unknown, generate relocations