cmd/internal/obj: make morestack cutoff the same on all architectures

There is always 128 bytes available below the stackguard. Allow functions
with medium-sized stack frames to use this, potentially allowing them to
avoid growing the stack.

This change makes all architectures use the same calculation as x86.

Change-Id: I2afb1a7c686ae5a933e50903b31ea4106e4cd0a0
Reviewed-on: https://go-review.googlesource.com/38734Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/internal/obj/arm/obj5.go

@@ -747,14 +747,14 @@ func stacksplit(ctxt *obj.Link, p *obj.Prog, framesize int32) *obj.Prog {
 		p.Reg = REGSP
 	} else if framesize <= obj.StackBig {
 		// large stack: SP-framesize < stackguard-StackSmall
-		//	MOVW $-framesize(SP), R2
+		//	MOVW $-(framesize-StackSmall)(SP), R2
 		//	CMP stackguard, R2
 		p = obj.Appendp(ctxt, p)
 
 		p.As = AMOVW
 		p.From.Type = obj.TYPE_ADDR
 		p.From.Reg = REGSP
-		p.From.Offset = int64(-framesize)
+		p.From.Offset = -(int64(framesize) - obj.StackSmall)
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REG_R2
 

src/cmd/internal/obj/arm64/obj7.go

@@ -82,13 +82,13 @@ func stacksplit(ctxt *obj.Link, p *obj.Prog, framesize int32) *obj.Prog {
 		p.Reg = REG_R2
 	} else if framesize <= obj.StackBig {
 		// large stack: SP-framesize < stackguard-StackSmall
-		//	SUB	$framesize, SP, R2
+		//	SUB	$(framesize-StackSmall), SP, R2
 		//	CMP	stackguard, R2
 		p = obj.Appendp(ctxt, p)
 
 		p.As = ASUB
 		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = int64(framesize)
+		p.From.Offset = int64(framesize) - obj.StackSmall
 		p.Reg = REGSP
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REG_R2

src/cmd/internal/obj/mips/obj0.go

@@ -658,13 +658,13 @@ func stacksplit(ctxt *obj.Link, p *obj.Prog, framesize int32) *obj.Prog {
 		p.To.Reg = REG_R1
 	} else if framesize <= obj.StackBig {
 		// large stack: SP-framesize < stackguard-StackSmall
-		//	ADD	$-framesize, SP, R2
+		//	ADD	$-(framesize-StackSmall), SP, R2
 		//	SGTU	R2, stackguard, R1
 		p = obj.Appendp(ctxt, p)
 
 		p.As = add
 		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = int64(-framesize)
+		p.From.Offset = -(int64(framesize) - obj.StackSmall)
 		p.Reg = REGSP
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REG_R2

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

@@ -868,13 +868,13 @@ func stacksplit(ctxt *obj.Link, p *obj.Prog, framesize int32) *obj.Prog {
 		p.To.Reg = REGSP
 	} else if framesize <= obj.StackBig {
 		// large stack: SP-framesize < stackguard-StackSmall
-		//	ADD $-framesize, SP, R4
+		//	ADD $-(framesize-StackSmall), SP, R4
 		//	CMP stackguard, R4
 		p = obj.Appendp(ctxt, p)
 
 		p.As = AADD
 		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = int64(-framesize)
+		p.From.Offset = -(int64(framesize) - obj.StackSmall)
 		p.Reg = REGSP
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REG_R4

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

@@ -586,13 +586,13 @@ func stacksplitPre(ctxt *obj.Link, p *obj.Prog, framesize int32) (*obj.Prog, *ob
 
 	} else if framesize <= obj.StackBig {
 		// large stack: SP-framesize < stackguard-StackSmall
-		//	ADD $-framesize, SP, R4
+		//	ADD $-(framesize-StackSmall), SP, R4
 		//	CMP stackguard, R4
 		p = obj.Appendp(ctxt, p)
 
 		p.As = AADD
 		p.From.Type = obj.TYPE_CONST
-		p.From.Offset = int64(-framesize)
+		p.From.Offset = -(int64(framesize) - obj.StackSmall)
 		p.Reg = REGSP
 		p.To.Type = obj.TYPE_REG
 		p.To.Reg = REG_R4