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

CL 307010 for mips.

Change-Id: Ie9addea728548315fdbb8e46f75da4010a9796bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/307051
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/mips/obj0.go

@@ -648,16 +648,14 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 }
 
 func (c *ctxt0) stacksplit(p *obj.Prog, framesize int32) *obj.Prog {
-	var mov, add, sub obj.As
+	var mov, add obj.As
 
 	if c.ctxt.Arch.Family == sys.MIPS64 {
 		add = AADDV
 		mov = AMOVV
-		sub = ASUBVU
 	} else {
 		add = AADDU
 		mov = AMOVW
-		sub = ASUBU
 	}
 
 	// MOV	g_stackguard(g), R1
@@ -691,81 +689,49 @@ func (c *ctxt0) stacksplit(p *obj.Prog, framesize int32) *obj.Prog {
 		p.Reg = REG_R1
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REG_R1
-	} else if framesize <= objabi.StackBig {
+	} else {
 		// large stack: SP-framesize < stackguard-StackSmall
+		offset := int64(framesize) - objabi.StackSmall
+		if framesize > objabi.StackBig {
+			// Such a large stack we need to protect against underflow.
+			// The runtime guarantees SP > objabi.StackBig, but
+			// framesize is large enough that SP-framesize may
+			// underflow, causing a direct comparison with the
+			// stack guard to incorrectly succeed. We explicitly
+			// guard against underflow.
+			//
+			//	SGTU	$(framesize-StackSmall), SP, R2
+			//	BNE	R2, label-of-call-to-morestack
+
+			p = obj.Appendp(p, c.newprog)
+			p.As = ASGTU
+			p.From.Type = obj.TYPE_CONST
+			p.From.Offset = offset
+			p.Reg = REGSP
+			p.To.Type = obj.TYPE_REG
+			p.To.Reg = REG_R2
+
+			p = obj.Appendp(p, c.newprog)
+			q = p
+			p.As = ABNE
+			p.From.Type = obj.TYPE_REG
+			p.From.Reg = REG_R2
+			p.To.Type = obj.TYPE_BRANCH
+			p.Mark |= BRANCH
+		}
+
+		// Check against the stack guard. We've ensured this won't underflow.
 		//	ADD	$-(framesize-StackSmall), SP, R2
 		//	SGTU	R2, stackguard, R1
 		p = obj.Appendp(p, c.newprog)
 
 		p.As = add
 		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = -(int64(framesize) - objabi.StackSmall)
+		p.From.Offset = -offset
 		p.Reg = REGSP
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REG_R2
 
-		p = obj.Appendp(p, c.newprog)
-		p.As = ASGTU
-		p.From.Type = obj.TYPE_REG
-		p.From.Reg = REG_R2
-		p.Reg = REG_R1
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R1
-	} 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.
-		// That breaks the math above, so we have to check for that explicitly.
-		//	// stackguard is R1
-		//	MOV	$StackPreempt, R2
-		//	BEQ	R1, R2, label-of-call-to-morestack
-		//	ADD	$StackGuard, SP, R2
-		//	SUB	R1, R2
-		//	MOV	$(framesize+(StackGuard-StackSmall)), R1
-		//	SGTU	R2, R1, R1
-		p = obj.Appendp(p, c.newprog)
-
-		p.As = mov
-		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = objabi.StackPreempt
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R2
-
-		p = obj.Appendp(p, c.newprog)
-		q = p
-		p.As = ABEQ
-		p.From.Type = obj.TYPE_REG
-		p.From.Reg = REG_R1
-		p.Reg = REG_R2
-		p.To.Type = obj.TYPE_BRANCH
-		p.Mark |= BRANCH
-
-		p = obj.Appendp(p, c.newprog)
-		p.As = add
-		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_R2
-
-		p = obj.Appendp(p, c.newprog)
-		p.As = sub
-		p.From.Type = obj.TYPE_REG
-		p.From.Reg = REG_R1
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R2
-
-		p = obj.Appendp(p, c.newprog)
-		p.As = mov
-		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = int64(framesize) + int64(objabi.StackGuard) - objabi.StackSmall
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R1
-
 		p = obj.Appendp(p, c.newprog)
 		p.As = ASGTU
 		p.From.Type = obj.TYPE_REG