runtime: unify asmcgocall and systemstack traceback setup

Both asmcgocall and systemstack need to save the calling Go code's
context for use by traceback, but they do it differently.
Systemstack's appraoch is better, because it doesn't require a
special case in traceback.
So make them both use that.

While we are here, the fake mstart caller in systemstack is
no longer needed and can be removed.
(traceback knows to stop in systemstack because of the writes to SP.)

Also remove the fake mstarts in sys_windows_*.s.

And while we are there, fix the control flow guard code in sys_windows_arm.s.
The current code is using pointers to a stack frame that technically is gone
once we hit the RET instruction. Clearly it's working OK, but better not to depend
on data below SP being preserved, even for just a few instructions.
Store the value we need in other registers instead.
(This code is only used for pushing a sigpanic call, which does not
actually return to the site of the fault and therefore doesn't need to
preserve any of the registers.)

This CL is part of a stack adding windows/arm64
support (#36439), intended to land in the Go 1.17 cycle.
This CL is, however, not windows/arm64-specific.
It is cleanup meant to make the port (and future ports) easier.

Change-Id: Id1e3ef5e54f7ad786e4b87043f2626eba7c3bbd9
Reviewed-on: https://go-review.googlesource.com/c/go/+/288799
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 23 insertions, 27 deletions

diff --git a/src/runtime/sys_windows_arm.s b/src/runtime/sys_windows_arm.s
index d2bdc50e3b..7d134dd3f4 100644
--- a/src/runtime/sys_windows_arm.s
+++ b/src/runtime/sys_windows_arm.s
@@ -6,6 +6,8 @@
 #include "go_tls.h"
 #include "textflag.h"
 
+// Note: For system ABI, R0-R3 are args, R4-R11 are callee-save.
+
 // void runtime·asmstdcall(void *c);
 TEXT runtime·asmstdcall(SB),NOSPLIT|NOFRAME,$0
 	MOVM.DB.W [R4, R5, R14], (R13)	// push {r4, r5, lr}
@@ -139,11 +141,10 @@ TEXT sigtramp<>(SB),NOSPLIT|NOFRAME,$0
 	MOVW	(g_sched+gobuf_sp)(g), R3	// R3 = g->gobuf.sp
 	BL      runtime·save_g(SB)
 
-	// traceback will think that we've done PUSH and SUB
-	// on this stack, so subtract them here to match.
-	// (we need room for sighandler arguments anyway).
+	// make room for sighandler arguments
 	// and re-save old SP for restoring later.
-	SUB	$(40+8+20), R3
+	// (note that the 24(R3) here must match the 24(R13) above.)
+	SUB	$40, R3
 	MOVW	R13, 24(R3)		// save old stack pointer
 	MOVW	R3, R13			// switch stack
 
@@ -151,22 +152,14 @@ g0:
 	MOVW	0(R6), R2	// R2 = ExceptionPointers->ExceptionRecord
 	MOVW	4(R6), R3	// R3 = ExceptionPointers->ContextRecord
 
-	// make it look like mstart called us on g0, to stop traceback
-	MOVW    $runtime·mstart(SB), R4
-
-	MOVW	R4, 0(R13)	// Save link register for traceback
+	MOVW	$0, R4
+	MOVW	R4, 0(R13)	// No saved link register.
 	MOVW	R2, 4(R13)	// Move arg0 (ExceptionRecord) into position
 	MOVW	R3, 8(R13)	// Move arg1 (ContextRecord) into position
 	MOVW	R5, 12(R13)	// Move arg2 (original g) into position
 	BL	(R7)		// Call the go routine
 	MOVW	16(R13), R4	// Fetch return value from stack
 
-	// Compute the value of the g0 stack pointer after deallocating
-	// this frame, then allocating 8 bytes. We may need to store
-	// the resume SP and PC on the g0 stack to work around
-	// control flow guard when we resume from the exception.
-	ADD	$(40+20), R13, R12
-
 	// switch back to original stack and g
 	MOVW	24(R13), R13
 	MOVW	20(R13), g
@@ -192,33 +185,36 @@ done:
 	// If returntramp has already been set up by a previous exception
 	// handler, don't clobber the stored SP and PC on the stack.
 	MOVW	4(R3), R3			// PEXCEPTION_POINTERS->Context
-	MOVW	0x40(R3), R2			// load PC from context record
+	MOVW	context_pc(R3), R2		// load PC from context record
 	MOVW	$returntramp<>(SB), R1
 	CMP	R1, R2
 	B.EQ	return				// do not clobber saved SP/PC
 
-	// Save resume SP and PC on g0 stack
-	MOVW	0x38(R3), R2			// load SP from context record
-	MOVW	R2, 0(R12)			// Store resume SP on g0 stack
-	MOVW	0x40(R3), R2			// load PC from context record
-	MOVW	R2, 4(R12)			// Store resume PC on g0 stack
+	// Save resume SP and PC into R0, R1.
+	MOVW	context_spr(R3), R2
+	MOVW	R2, context_r0(R3)
+	MOVW	context_pc(R3), R2
+	MOVW	R2, context_r1(R3)
 
 	// Set up context record to return to returntramp on g0 stack
-	MOVW	R12, 0x38(R3)			// save g0 stack pointer
-						// in context record
-	MOVW	$returntramp<>(SB), R2	// save resume address
-	MOVW	R2, 0x40(R3)			// in context record
+	MOVW	R12, context_spr(R3)
+	MOVW	$returntramp<>(SB), R2
+	MOVW	R2, context_pc(R3)
 
 return:
 	B	(R14)				// return
 
-//
 // Trampoline to resume execution from exception handler.
 // This is part of the control flow guard workaround.
 // It switches stacks and jumps to the continuation address.
-//
+// R0 and R1 are set above at the end of sigtramp<>
+// in the context that starts executing at returntramp<>.
 TEXT returntramp<>(SB),NOSPLIT|NOFRAME,$0
-	MOVM.IA	(R13), [R13, R15]		// ldm sp, [sp, pc]
+	// Important: do not smash LR,
+	// which is set to a live value when handling
+	// a signal by pushing a call to sigpanic onto the stack.
+	MOVW	R0, R13
+	B	(R1)
 
 TEXT runtime·exceptiontramp(SB),NOSPLIT|NOFRAME,$0
 	MOVW	$runtime·exceptionhandler(SB), R1