cmd/internal/obj/ppc64: simplify huge frame prologue

CL 307010 for ppc64.

I spent a long time trying to figure out how to use the carry bit from
ADDCCC to further simplify this (like what we do on arm64), but gave
up after I couldn't figure out how to access the carry bit without
just adding more instructions.

Change-Id: I6cad51b93616865b203cb16554f16121375aabbc
Reviewed-on: https://go-review.googlesource.com/c/go/+/307149
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -1081,80 +1081,65 @@ func (c *ctxt9) stacksplit(p *obj.Prog, framesize int32) *obj.Prog {
 		p.From.Reg = REG_R3
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REGSP
-	} else if framesize <= objabi.StackBig {
-		// large stack: SP-framesize < stackguard-StackSmall
-		//	ADD $-(framesize-StackSmall), SP, R4
-		//	CMP stackguard, R4
-		p = obj.Appendp(p, c.newprog)
-
-		p.As = AADD
-		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = -(int64(framesize) - objabi.StackSmall)
-		p.Reg = REGSP
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R4
-
-		p = obj.Appendp(p, c.newprog)
-		p.As = ACMPU
-		p.From.Type = obj.TYPE_REG
-		p.From.Reg = REG_R3
-		p.To.Type = obj.TYPE_REG
-		p.To.Reg = REG_R4
 	} else {
-		// Such a large stack we need to protect against wraparound.
+		// large stack: SP-framesize < stackguard-StackSmall
-		// If SP is close to zero:
+		offset := int64(framesize) - objabi.StackSmall
-		//	SP-stackguard+StackGuard <= framesize + (StackGuard-StackSmall)
+		if framesize > objabi.StackBig {
-		// The +StackGuard on both sides is required to keep the left side positive:
+			// Such a large stack we need to protect against underflow.
-		// SP is allowed to be slightly below stackguard. See stack.h.
+			// The runtime guarantees SP > objabi.StackBig, but
-		//
+			// framesize is large enough that SP-framesize may
-		// Preemption sets stackguard to StackPreempt, a very large value.
+			// underflow, causing a direct comparison with the
-		// That breaks the math above, so we have to check for that explicitly.
+			// stack guard to incorrectly succeed. We explicitly
-		//	// stackguard is R3
+			// guard against underflow.
-		//	CMP	R3, $StackPreempt
+			//
-		//	BEQ	label-of-call-to-morestack
+			//	CMPU	SP, $(framesize-StackSmall)
-		//	ADD	$StackGuard, SP, R4
+			//	BLT	label-of-call-to-morestack
-		//	SUB	R3, R4
+			if offset <= 0xffff {
-		//	MOVD	$(framesize+(StackGuard-StackSmall)), R31
+				p = obj.Appendp(p, c.newprog)
-		//	CMPU	R31, R4
+				p.As = ACMPU
+				p.From.Type = obj.TYPE_REG
+				p.From.Reg = REGSP
+				p.To.Type = obj.TYPE_CONST
+				p.To.Offset = offset
+			} else {
+				// Constant is too big for CMPU.
+				p = obj.Appendp(p, c.newprog)
+				p.As = AMOVD
+				p.From.Type = obj.TYPE_CONST
+				p.From.Offset = offset
+				p.To.Type = obj.TYPE_REG
+				p.To.Reg = REG_R4
+
+				p = obj.Appendp(p, c.newprog)
+				p.As = ACMPU
+				p.From.Type = obj.TYPE_REG
+				p.From.Reg = REGSP
+				p.To.Type = obj.TYPE_REG
+				p.To.Reg = REG_R4
+			}
+
+			p = obj.Appendp(p, c.newprog)
+			q = p
+			p.As = ABLT
+			p.To.Type = obj.TYPE_BRANCH
+		}
+
+		// Check against the stack guard. We've ensured this won't underflow.
+		//	ADD  $-(framesize-StackSmall), SP, R4
+		//	CMPU stackguard, R4
 		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.From.Offset = -offset
 		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.To.Type = obj.TYPE_REG
-		p.To.Reg = REGTMP
-
 		p = obj.Appendp(p, c.newprog)
 		p.As = ACMPU
 		p.From.Type = obj.TYPE_REG
-		p.From.Reg = REGTMP
+		p.From.Reg = REG_R3
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REG_R4
 	}