From: Paul E. Murphy <murp@ibm.com>
Date: Tue, 10 Jan 2023 21:14:46 +0000 (-0600)
Subject: cmd/link/internal/ppc64: generate PCrel trampolines on P10
X-Git-Tag: go1.21rc1~1004
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=a3df6c0e81cd14aef0b4967721821d34b962b7be;p=gostls13.git

cmd/link/internal/ppc64: generate PCrel trampolines on P10

PCrelative trampolines have no dependence on the TOC pointer or build
mode, thus they are preferable to use when supported.

This is a step towards eliminating the need to use and maintain the
TOC pointer in R2 when PCrel is supported.

Change-Id: I1b1a7e16831cfd6732b31f7fad8df2a7c88c8f84
Reviewed-on: https://go-review.googlesource.com/c/go/+/461599
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
---

diff --git a/src/cmd/link/internal/ppc64/asm.go b/src/cmd/link/internal/ppc64/asm.go
index 9141d05ffb..333411a53d 100644
--- a/src/cmd/link/internal/ppc64/asm.go
+++ b/src/cmd/link/internal/ppc64/asm.go
@@ -39,11 +39,15 @@ import (
 	"debug/elf"
 	"encoding/binary"
 	"fmt"
+	"internal/buildcfg"
 	"log"
 	"strconv"
 	"strings"
 )
 
+// The build configuration supports PC-relative instructions and relocations.
+var hasPCrel = buildcfg.GOPPC64 >= 10 && buildcfg.GOOS == "linux" && buildcfg.GOARCH == "ppc64le"
+
 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
@@ -1027,6 +1031,13 @@ func gentramp(ctxt *ld.Link, ldr *loader.Loader, tramp *loader.SymbolBuilder, ta
 	P := make([]byte, tramp.Size())
 	var o1, o2 uint32
 
+	// 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))
+	}
+
 	if ctxt.IsAIX() {
 		// On AIX, the address is retrieved with a TOC symbol.
 		// For internal linking, the "Linux" way might still be used.
@@ -1044,6 +1055,17 @@ func gentramp(ctxt *ld.Link, ldr *loader.Loader, tramp *loader.SymbolBuilder, ta
 		r.SetOff(0)
 		r.SetSiz(8) // generates 2 relocations: HA + LO
 		r.SetSym(toctramp.Sym())
+	} else if hasPCrel {
+		// pla r12, addr (PCrel). This works for static or PIC, with or without a valid TOC pointer.
+		o1 = uint32(0x06100000)
+		o2 = uint32(0x39800000) // pla r12, addr
+
+		// The trampoline's position is not known yet, insert a relocation.
+		r, _ := tramp.AddRel(objabi.R_ADDRPOWER_PCREL34)
+		r.SetOff(0)
+		r.SetSiz(8) // This spans 2 words.
+		r.SetSym(target)
+		r.SetAdd(offset)
 	} else {
 		// Used for default build mode for an executable
 		// Address of the call target is generated using
@@ -1051,13 +1073,6 @@ 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