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 9141d05ffb24e548daf9665ee3f8894633fa1fa1..333411a53d9cd89f2a6905f1e9b5bcf9764bedab 100644 (file)
--- 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