[release-branch.go1.15] cmd/link/internal: fix use of DynlinkingGo with ppc64le trampolines

When creating programs with large text sections on ppc64le,
trampolines are needed for calls that are too far; however
they are not created if the code is generated such that the TOC
register r2 is initialized and maintained in the code because
then the external linker can create the trampolines. Previously
the function DynlinkingGo was used to determine this but in the
case where plugins are used, this could return true even though
r2 is not valid.

To fix this problem I've added a new function r2Valid which returns
true when the build options indicate that the r2 is
initialized and maintained. Because of the ways that
DynlinkingGo is used I wanted to maintain its previous
behavior.

Fixes #46002

Change-Id: I6d902eba6ad41757aa6474948b79acdbd479cb38
Reviewed-on: https://go-review.googlesource.com/c/go/+/315289
Trust: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
(cherry picked from commit 9ed736a)
Reviewed-on: https://go-review.googlesource.com/c/go/+/317974
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Author: laboger

src/cmd/link/internal/ppc64/asm.go

@@ -658,14 +658,24 @@ func archrelocaddr(target *ld.Target, syms *ld.ArchSyms, r *sym.Reloc, s *sym.Sy
 	return int64(o2)<<32 | int64(o1)
 }
 
+// Determine if the code was compiled so that the TOC register R2 is initialized and maintained
+func r2Valid(ctxt *ld.Link) bool {
+	switch ctxt.BuildMode {
+	case ld.BuildModeCArchive, ld.BuildModeCShared, ld.BuildModePIE, ld.BuildModeShared, ld.BuildModePlugin:
+		return true
+	}
+	// -linkshared option
+	return ctxt.IsSharedGoLink()
+}
+
 // resolve direct jump relocation r in s, and add trampoline if necessary
 func trampoline(ctxt *ld.Link, ldr *loader.Loader, ri int, rs, s loader.Sym) {
 
 	// Trampolines are created if the branch offset is too large and the linker cannot insert a call stub to handle it.
 	// For internal linking, trampolines are always created for long calls.
 	// For external linking, the linker can insert a call stub to handle a long call, but depends on having the TOC address in
 	// r2.  For those build modes with external linking where the TOC address is not maintained in r2, trampolines must be created.
-	if ctxt.IsExternal() && (ctxt.DynlinkingGo() || ctxt.BuildMode == ld.BuildModeCArchive || ctxt.BuildMode == ld.BuildModeCShared || ctxt.BuildMode == ld.BuildModePIE) {
+	if ctxt.IsExternal() && r2Valid(ctxt) {
 		// No trampolines needed since r2 contains the TOC
 		return
 	}
@@ -719,7 +729,7 @@ func trampoline(ctxt *ld.Link, ldr *loader.Loader, ri int, rs, s loader.Sym) {
 				}
 			}
 			if ldr.SymType(tramp) == 0 {
-				if ctxt.DynlinkingGo() || ctxt.BuildMode == ld.BuildModeCArchive || ctxt.BuildMode == ld.BuildModeCShared || ctxt.BuildMode == ld.BuildModePIE {
+				if r2Valid(ctxt) {
 					// Should have returned for above cases
 					ctxt.Errorf(s, "unexpected trampoline for shared or dynamic linking")
 				} else {