runtime: improve 386/amd64 systemstack

Minor improvements, noticed while investigating other things.

Shorten the prologue.

Make branch direction better for static branch prediction;
the most common case by far is switching stacks (g==curg).

Change-Id: Ib2211d3efecb60446355cda56194221ccb78057d
Reviewed-on: https://go-review.googlesource.com/97377
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/asm_386.s

@@ -424,24 +424,17 @@ TEXT runtime·systemstack(SB), NOSPLIT, $0-4
 	MOVL	g(CX), AX	// AX = g
 	MOVL	g_m(AX), BX	// BX = m
 
-	MOVL	m_gsignal(BX), DX	// DX = gsignal
-	CMPL	AX, DX
+	CMPL	AX, m_gsignal(BX)
 	JEQ	noswitch
 
 	MOVL	m_g0(BX), DX	// DX = g0
 	CMPL	AX, DX
 	JEQ	noswitch
 
-	MOVL	m_curg(BX), BP
-	CMPL	AX, BP
-	JEQ	switch
-	
-	// Bad: g is not gsignal, not g0, not curg. What is it?
-	// Hide call from linker nosplit analysis.
-	MOVL	$runtime·badsystemstack(SB), AX
-	CALL	AX
+	CMPL	AX, m_curg(BX)
+	JNE	bad
 
-switch:
+	// switch stacks
 	// save our state in g->sched. Pretend to
 	// be systemstack_switch if the G stack is scanned.
 	MOVL	$runtime·systemstack_switch(SB), (g_sched+gobuf_pc)(AX)
@@ -481,6 +474,12 @@ noswitch:
 	MOVL	0(DI), DI
 	JMP	DI
 
+bad:
+	// Bad: g is not gsignal, not g0, not curg. What is it?
+	// Hide call from linker nosplit analysis.
+	MOVL	$runtime·badsystemstack(SB), AX
+	CALL	AX
+
 /*
  * support for morestack
  */

src/runtime/asm_amd64.s

@@ -369,23 +369,17 @@ TEXT runtime·systemstack(SB), NOSPLIT, $0-8
 	MOVQ	g(CX), AX	// AX = g
 	MOVQ	g_m(AX), BX	// BX = m
 
-	MOVQ	m_gsignal(BX), DX	// DX = gsignal
-	CMPQ	AX, DX
+	CMPQ	AX, m_gsignal(BX)
 	JEQ	noswitch
 
 	MOVQ	m_g0(BX), DX	// DX = g0
 	CMPQ	AX, DX
 	JEQ	noswitch
 
-	MOVQ	m_curg(BX), R8
-	CMPQ	AX, R8
-	JEQ	switch
-	
-	// Bad: g is not gsignal, not g0, not curg. What is it?
-	MOVQ	$runtime·badsystemstack(SB), AX
-	CALL	AX
+	CMPQ	AX, m_curg(BX)
+	JNE	bad
 
-switch:
+	// switch stacks
 	// save our state in g->sched. Pretend to
 	// be systemstack_switch if the G stack is scanned.
 	MOVQ	$runtime·systemstack_switch(SB), SI
@@ -426,6 +420,12 @@ noswitch:
 	MOVQ	0(DI), DI
 	JMP	DI
 
+bad:
+	// Bad: g is not gsignal, not g0, not curg. What is it?
+	MOVQ	$runtime·badsystemstack(SB), AX
+	CALL	AX
+
+
 /*
  * support for morestack
  */

src/runtime/asm_amd64p32.s

@@ -260,24 +260,17 @@ TEXT runtime·systemstack(SB), NOSPLIT, $0-4
 	MOVL	g(CX), AX	// AX = g
 	MOVL	g_m(AX), BX	// BX = m
 
-	MOVL	m_gsignal(BX), DX	// DX = gsignal
-	CMPL	AX, DX
+	CMPL	AX, m_gsignal(BX)
 	JEQ	noswitch
 
 	MOVL	m_g0(BX), DX	// DX = g0
 	CMPL	AX, DX
 	JEQ	noswitch
 
-	MOVL	m_curg(BX), R8
-	CMPL	AX, R8
-	JEQ	switch
-	
-	// Not g0, not curg. Must be gsignal, but that's not allowed.
-	// Hide call from linker nosplit analysis.
-	MOVL	$runtime·badsystemstack(SB), AX
-	CALL	AX
+	CMPL	AX, m_curg(BX)
+	JNE	bad
 
-switch:
+	// switch stacks
 	// save our state in g->sched. Pretend to
 	// be systemstack_switch if the G stack is scanned.
 	MOVL	$runtime·systemstack_switch(SB), SI
@@ -312,6 +305,12 @@ noswitch:
 	MOVL	0(DI), DI
 	JMP	DI
 
+bad:
+	// Not g0, not curg. Must be gsignal, but that's not allowed.
+	// Hide call from linker nosplit analysis.
+	MOVL	$runtime·badsystemstack(SB), AX
+	CALL	AX
+
 /*
  * support for morestack
  */