cmd/internal/obj/s390x: simplify huge frame prologue

CL 307010 for s390x.

Change-Id: I43e1f93dd01c814417f8ef7480aa82c05b2b6b66
Reviewed-on: https://go-review.googlesource.com/c/go/+/307151
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/s390x/objz.go

@@ -568,7 +568,6 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 }
 
 func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (*obj.Prog, *obj.Prog) {
-	var q *obj.Prog
 
 	// MOVD	g_stackguard(g), R3
 	p = obj.Appendp(p, c.newprog)
@@ -589,97 +588,68 @@ func (c *ctxtz) stacksplitPre(p *obj.Prog, framesize int32) (*obj.Prog, *obj.Pro
 	// unnecessarily. See issue #35470.
 	p = c.ctxt.StartUnsafePoint(p, c.newprog)
 
-	q = nil
 	if framesize <= objabi.StackSmall {
 		// small stack: SP < stackguard
 		//	CMPUBGE	stackguard, SP, label-of-call-to-morestack
 
 		p = obj.Appendp(p, c.newprog)
-		//q1 = p
 		p.From.Type = obj.TYPE_REG
 		p.From.Reg = REG_R3
 		p.Reg = REGSP
 		p.As = ACMPUBGE
 		p.To.Type = obj.TYPE_BRANCH
 
-	} else if framesize <= objabi.StackBig {
+		return p, nil
+	}
+
 	// large stack: SP-framesize < stackguard-StackSmall
-		//	ADD $-(framesize-StackSmall), SP, R4
-		//	CMPUBGE stackguard, R4, label-of-call-to-morestack
-		p = obj.Appendp(p, c.newprog)
 
-		p.As = AADD
+	var q *obj.Prog
-		p.From.Type = obj.TYPE_CONST
+	offset := int64(framesize) - objabi.StackSmall
-		p.From.Offset = -(int64(framesize) - objabi.StackSmall)
+	if framesize > objabi.StackBig {
-		p.Reg = REGSP
+		// Such a large stack we need to protect against underflow.
-		p.To.Type = obj.TYPE_REG
+		// The runtime guarantees SP > objabi.StackBig, but
-		p.To.Reg = REG_R4
+		// framesize is large enough that SP-framesize may
-
+		// underflow, causing a direct comparison with the
-		p = obj.Appendp(p, c.newprog)
+		// stack guard to incorrectly succeed. We explicitly
-		p.From.Type = obj.TYPE_REG
+		// guard against underflow.
-		p.From.Reg = REG_R3
-		p.Reg = REG_R4
-		p.As = ACMPUBGE
-		p.To.Type = obj.TYPE_BRANCH
-
-	} else {
-		// Such a large stack we need to protect against wraparound.
-		// If SP is close to zero:
-		//	SP-stackguard+StackGuard <= framesize + (StackGuard-StackSmall)
-		// The +StackGuard on both sides is required to keep the left side positive:
-		// SP is allowed to be slightly below stackguard. See stack.h.
 		//
-		// Preemption sets stackguard to StackPreempt, a very large value.
+		//	MOVD	$(framesize-StackSmall), R4
-		// That breaks the math above, so we have to check for that explicitly.
+		//	CMPUBLT	SP, R4, label-of-call-to-morestack
-		//	// stackguard is R3
-		//	CMP	R3, $StackPreempt
-		//	BEQ	label-of-call-to-morestack
-		//	ADD	$StackGuard, SP, R4
-		//	SUB	R3, R4
-		//	MOVD	$(framesize+(StackGuard-StackSmall)), TEMP
-		//	CMPUBGE	TEMP, R4, label-of-call-to-morestack
-		p = obj.Appendp(p, c.newprog)
-
-		p.As = ACMP
-		p.From.Type = obj.TYPE_REG
-		p.From.Reg = REG_R3
-		p.To.Type = obj.TYPE_CONST
-		p.To.Offset = objabi.StackPreempt
-
-		p = obj.Appendp(p, c.newprog)
-		q = p
-		p.As = ABEQ
-		p.To.Type = obj.TYPE_BRANCH
-
-		p = obj.Appendp(p, c.newprog)
-		p.As = AADD
-		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = int64(objabi.StackGuard)
-		p.Reg = REGSP
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R4
-
-		p = obj.Appendp(p, c.newprog)
-		p.As = ASUB
-		p.From.Type = obj.TYPE_REG
-		p.From.Reg = REG_R3
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R4
 
 		p = obj.Appendp(p, c.newprog)
 		p.As = AMOVD
 		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = int64(framesize) + int64(objabi.StackGuard) - objabi.StackSmall
+		p.From.Offset = offset
 		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REGTMP
+		p.To.Reg = REG_R4
+
+		p = obj.Appendp(p, c.newprog)
+		q = p
+		p.As = ACMPUBLT
+		p.From.Type = obj.TYPE_REG
+		p.From.Reg = REGSP
+		p.Reg = REG_R4
+		p.To.Type = obj.TYPE_BRANCH
+	}
+
+	// Check against the stack guard. We've ensured this won't underflow.
+	//	ADD $-(framesize-StackSmall), SP, R4
+	//	CMPUBGE stackguard, R4, label-of-call-to-morestack
+	p = obj.Appendp(p, c.newprog)
+	p.As = AADD
+	p.From.Type = obj.TYPE_CONST
+	p.From.Offset = -offset
+	p.Reg = REGSP
+	p.To.Type = obj.TYPE_REG
+	p.To.Reg = REG_R4
 
 	p = obj.Appendp(p, c.newprog)
 	p.From.Type = obj.TYPE_REG
-		p.From.Reg = REGTMP
+	p.From.Reg = REG_R3
 	p.Reg = REG_R4
 	p.As = ACMPUBGE
 	p.To.Type = obj.TYPE_BRANCH
-	}
 
 	return p, q
 }