cmd/internal/obj/ppc64: avoid calling morestack via a PLT when dynamically linking

Change-Id: Ie79f72786b1d7154f1910e717a0faf354b913b89
Reviewed-on: https://go-review.googlesource.com/15970Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/obj/ppc64/obj9.go

@@ -940,19 +940,62 @@ func stacksplit(ctxt *obj.Link, p *obj.Prog, framesize int32) *obj.Prog {
 		q.Pcond = p
 	}
 
-	// BL	runtime.morestack(SB)
-	p = obj.Appendp(ctxt, p)
-
-	p.As = ABL
-	p.To.Type = obj.TYPE_BRANCH
+	var morestacksym *obj.LSym
 	if ctxt.Cursym.Cfunc != 0 {
-		p.To.Sym = obj.Linklookup(ctxt, "runtime.morestackc", 0)
+		morestacksym = obj.Linklookup(ctxt, "runtime.morestackc", 0)
 	} else if ctxt.Cursym.Text.From3.Offset&obj.NEEDCTXT == 0 {
-		p.To.Sym = obj.Linklookup(ctxt, "runtime.morestack_noctxt", 0)
+		morestacksym = obj.Linklookup(ctxt, "runtime.morestack_noctxt", 0)
 	} else {
-		p.To.Sym = obj.Linklookup(ctxt, "runtime.morestack", 0)
+		morestacksym = obj.Linklookup(ctxt, "runtime.morestack", 0)
 	}
 
+	if ctxt.Flag_dynlink {
+		// Avoid calling morestack via a PLT when dynamically linking.  The
+		// PLT stubs generated by the system linker on ppc64le when "std r2,
+		// 24(r1)" to save the TOC pointer in their callers stack
+		// frame. Unfortunately (and necessarily) morestack is called before
+		// the function that calls it sets up its frame and so the PLT ends
+		// up smashing the saved TOC pointer for its caller's caller.
+		//
+		// According to the ABI documentation there is a mechanism to avoid
+		// the TOC save that the PLT stub does (put a R_PPC64_TOCSAVE
+		// relocation on the nop after the call to morestack) but at the time
+		// of writing it is not supported at all by gold and my attempt to
+		// use it with ld.bfd caused an internal linker error. So this hack
+		// seems preferable.
+
+		// MOVD $runtime.morestack(SB), R12
+		p = obj.Appendp(ctxt, p)
+		p.As = AMOVD
+		p.From.Type = obj.TYPE_MEM
+		p.From.Sym = morestacksym
+		p.From.Name = obj.NAME_GOTREF
+		p.To.Type = obj.TYPE_REG
+		p.To.Reg = REG_R12
+
+		// MOVD R12, CTR
+		p = obj.Appendp(ctxt, p)
+		p.As = AMOVD
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = REG_R12
+		p.To.Type = obj.TYPE_REG
+		p.To.Reg = REG_CTR
+
+		// BL CTR
+		p = obj.Appendp(ctxt, p)
+		p.As = obj.ACALL
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = REG_R12
+		p.To.Type = obj.TYPE_REG
+		p.To.Reg = REG_CTR
+	} else {
+		// BL	runtime.morestack(SB)
+		p = obj.Appendp(ctxt, p)
+
+		p.As = ABL
+		p.To.Type = obj.TYPE_BRANCH
+		p.To.Sym = morestacksym
+	}
 	// BR	start
 	p = obj.Appendp(ctxt, p)