runtime: simplify reflectcall write barriers

Currently reflectcall has a subtle dance with write barriers where the
assembly code copies the result values from the stack to the in-heap
argument frame without write barriers and then calls into the runtime
after the fact to invoke the necessary write barriers.

For the hybrid barrier (and for ROC), we need to switch to a
*pre*-write write barrier, which is very difficult to do with the
current setup. We could tie ourselves in knots of subtle reasoning
about why it's okay in this particular case to have a post-write write
barrier, but this commit instead takes a different approach. Rather
than making things more complex, this simplifies reflection calls so
that the argument copy is done in Go using normal bulk write barriers.

The one difficulty with this approach is that calling into Go requires
putting arguments on the stack, but the call* functions "donate" their
entire stack frame to the called function. We can get away with this
now because the copy avoids using the stack and has copied the results
out before we clobber the stack frame to call into the write barrier.
The solution in this CL is to call another function, passing arguments
in registers instead of on the stack, and let that other function
reserve more stack space and setup the arguments for the runtime.

This approach seemed to work out the best. I also tried making the
call* functions reserve 32 extra bytes of frame for the write barrier
arguments and adjust SP up by 32 bytes around the call. However, even
with the necessary changes to the assembler to correct the spdelta
table, the runtime was still having trouble with the frame layout (and
the changes to the assembler caused many other things that do strange
things with the SP to fail to assemble). The approach I took doesn't
require any funny business with the SP.

Updates #17503.

Change-Id: Ie2bb0084b24d6cff38b5afb218b9e0534ad2119e
Reviewed-on: https://go-review.googlesource.com/31655
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/reflect/value.go

@@ -447,7 +447,7 @@ func (v Value) call(op string, in []Value) []Value {
 		// because the Values returned by this function contain pointers to the args object,
 		// and will thus keep the args object alive indefinitely.
 		memclr(args, retOffset)
-		// Copy return values out of args.
+		// Wrap Values around return values in args.
 		ret = make([]Value, nout)
 		off = retOffset
 		for i := 0; i < nout; i++ {

src/runtime/asm_386.s

@@ -477,6 +477,7 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-20;		\
 	PCDATA  $PCDATA_StackMapIndex, $0;	\
 	CALL	AX;				\
 	/* copy return values back */		\
+	MOVL	argtype+0(FP), DX;		\
 	MOVL	argptr+8(FP), DI;		\
 	MOVL	argsize+12(FP), CX;		\
 	MOVL	retoffset+16(FP), BX;		\
@@ -484,17 +485,19 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-20;		\
 	ADDL	BX, DI;				\
 	ADDL	BX, SI;				\
 	SUBL	BX, CX;				\
-	REP;MOVSB;				\
-	/* execute write barrier updates */	\
-	MOVL	argtype+0(FP), DX;		\
-	MOVL	argptr+8(FP), DI;		\
-	MOVL	argsize+12(FP), CX;		\
-	MOVL	retoffset+16(FP), BX;		\
-	MOVL	DX, 0(SP);			\
-	MOVL	DI, 4(SP);			\
-	MOVL	CX, 8(SP);			\
-	MOVL	BX, 12(SP);			\
-	CALL	runtime·callwritebarrier(SB);	\
+	CALL	callRet<>(SB);			\
+	RET
+
+// callRet copies return values back at the end of call*. This is a
+// separate function so it can allocate stack space for the arguments
+// to reflectcallmove. It does not follow the Go ABI; it expects its
+// arguments in registers.
+TEXT callRet<>(SB), NOSPLIT, $16-0
+	MOVL	DX, 0(SP)
+	MOVL	DI, 4(SP)
+	MOVL	SI, 8(SP)
+	MOVL	CX, 12(SP)
+	CALL	runtime·reflectcallmove(SB)
 	RET
 
 CALLFN(·call16, 16)

src/runtime/asm_amd64.s

@@ -416,8 +416,6 @@ TEXT reflect·call(SB), NOSPLIT, $0-0
 
 TEXT ·reflectcall(SB), NOSPLIT, $0-32
 	MOVLQZX argsize+24(FP), CX
-	// NOTE(rsc): No call16, because CALLFN needs four words
-	// of argument space to invoke callwritebarrier.
 	DISPATCH(runtime·call32, 32)
 	DISPATCH(runtime·call64, 64)
 	DISPATCH(runtime·call128, 128)
@@ -460,6 +458,7 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-32;		\
 	PCDATA  $PCDATA_StackMapIndex, $0;	\
 	CALL	(DX);				\
 	/* copy return values back */		\
+	MOVQ	argtype+0(FP), DX;		\
 	MOVQ	argptr+16(FP), DI;		\
 	MOVLQZX	argsize+24(FP), CX;		\
 	MOVLQZX	retoffset+28(FP), BX;		\
@@ -467,17 +466,20 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-32;		\
 	ADDQ	BX, DI;				\
 	ADDQ	BX, SI;				\
 	SUBQ	BX, CX;				\
-	REP;MOVSB;				\
-	/* execute write barrier updates */	\
-	MOVQ	argtype+0(FP), DX;		\
-	MOVQ	argptr+16(FP), DI;		\
-	MOVLQZX	argsize+24(FP), CX;		\
-	MOVLQZX retoffset+28(FP), BX;		\
-	MOVQ	DX, 0(SP);			\
-	MOVQ	DI, 8(SP);			\
-	MOVQ	CX, 16(SP);			\
-	MOVQ	BX, 24(SP);			\
-	CALL	runtime·callwritebarrier(SB);	\
+	CALL	callRet<>(SB);			\
+	RET
+
+// callRet copies return values back at the end of call*. This is a
+// separate function so it can allocate stack space for the arguments
+// to reflectcallmove. It does not follow the Go ABI; it expects its
+// arguments in registers.
+TEXT callRet<>(SB), NOSPLIT, $32-0
+	NO_LOCAL_POINTERS
+	MOVQ	DX, 0(SP)
+	MOVQ	DI, 8(SP)
+	MOVQ	SI, 16(SP)
+	MOVQ	CX, 24(SP)
+	CALL	runtime·reflectcallmove(SB)
 	RET
 
 CALLFN(·call32, 32)

src/runtime/asm_amd64p32.s

@@ -371,6 +371,7 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-20;		\
 	MOVL	(DX), AX;			\
 	CALL	AX;				\
 	/* copy return values back */		\
+	MOVL	argtype+0(FP), DX;		\
 	MOVL	argptr+8(FP), DI;		\
 	MOVL	argsize+12(FP), CX;		\
 	MOVL	retoffset+16(FP), BX;		\
@@ -378,17 +379,19 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-20;		\
 	ADDL	BX, DI;				\
 	ADDL	BX, SI;				\
 	SUBL	BX, CX;				\
-	REP;MOVSB;				\
-	/* execute write barrier updates */	\
-	MOVL	argtype+0(FP), DX;		\
-	MOVL	argptr+8(FP), DI;		\
-	MOVL	argsize+12(FP), CX;		\
-	MOVL	retoffset+16(FP), BX;		\
-	MOVL	DX, 0(SP);			\
-	MOVL	DI, 4(SP);			\
-	MOVL	CX, 8(SP);			\
-	MOVL	BX, 12(SP);			\
-	CALL	runtime·callwritebarrier(SB);	\
+	CALL	callRet<>(SB);			\
+	RET
+
+// callRet copies return values back at the end of call*. This is a
+// separate function so it can allocate stack space for the arguments
+// to reflectcallmove. It does not follow the Go ABI; it expects its
+// arguments in registers.
+TEXT callRet<>(SB), NOSPLIT, $16-0
+	MOVL	DX, 0(SP)
+	MOVL	DI, 4(SP)
+	MOVL	SI, 8(SP)
+	MOVL	CX, 12(SP)
+	CALL	runtime·reflectcallmove(SB)
 	RET
 
 CALLFN(·call16, 16)

src/runtime/asm_arm.s

@@ -406,6 +406,7 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-20;		\
 	PCDATA  $PCDATA_StackMapIndex, $0;	\
 	BL	(R0);				\
 	/* copy return values back */		\
+	MOVW	argtype+0(FP), R4;		\
 	MOVW	argptr+8(FP), R0;		\
 	MOVW	argsize+12(FP), R2;		\
 	MOVW	retoffset+16(FP), R3;		\
@@ -413,24 +414,19 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-20;		\
 	ADD	R3, R1;				\
 	ADD	R3, R0;				\
 	SUB	R3, R2;				\
-loop:						\
-	CMP	$0, R2;				\
-	B.EQ	end;				\
-	MOVBU.P	1(R1), R5;			\
-	MOVBU.P R5, 1(R0);			\
-	SUB	$1, R2, R2;			\
-	B	loop;				\
-end:						\
-	/* execute write barrier updates */	\
-	MOVW	argtype+0(FP), R1;		\
-	MOVW	argptr+8(FP), R0;		\
-	MOVW	argsize+12(FP), R2;		\
-	MOVW	retoffset+16(FP), R3;		\
-	MOVW	R1, 4(R13);			\
-	MOVW	R0, 8(R13);			\
-	MOVW	R2, 12(R13);			\
-	MOVW	R3, 16(R13);			\
-	BL	runtime·callwritebarrier(SB);	\
+	BL	callRet<>(SB);			\
+	RET
+
+// callRet copies return values back at the end of call*. This is a
+// separate function so it can allocate stack space for the arguments
+// to reflectcallmove. It does not follow the Go ABI; it expects its
+// arguments in registers.
+TEXT callRet<>(SB), NOSPLIT, $16-0
+	MOVW	R4, 4(R13)
+	MOVW	R0, 8(R13)
+	MOVW	R1, 12(R13)
+	MOVW	R2, 16(R13)
+	BL	runtime·reflectcallmove(SB)
 	RET	
 
 CALLFN(·call16, 16)

src/runtime/asm_arm64.s

@@ -335,8 +335,6 @@ TEXT reflect·call(SB), NOSPLIT, $0-0
 
 TEXT ·reflectcall(SB), NOSPLIT, $-8-32
 	MOVWU argsize+24(FP), R16
-	// NOTE(rsc): No call16, because CALLFN needs four words
-	// of argument space to invoke callwritebarrier.
 	DISPATCH(runtime·call32, 32)
 	DISPATCH(runtime·call64, 64)
 	DISPATCH(runtime·call128, 128)
@@ -387,33 +385,27 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-24;		\
 	PCDATA  $PCDATA_StackMapIndex, $0;	\
 	BL	(R0);				\
 	/* copy return values back */		\
+	MOVD	argtype+0(FP), R7;		\
 	MOVD	arg+16(FP), R3;			\
 	MOVWU	n+24(FP), R4;			\
 	MOVWU	retoffset+28(FP), R6;		\
-	MOVD	RSP, R5;				\
+	ADD	$8, RSP, R5;			\
 	ADD	R6, R5; 			\
 	ADD	R6, R3;				\
 	SUB	R6, R4;				\
-	ADD	$(8-1), R5;			\
-	SUB	$1, R3;				\
-	ADD	R5, R4;				\
-loop:						\
-	CMP	R5, R4;				\
-	BEQ	end;				\
-	MOVBU.W	1(R5), R6;			\
-	MOVBU.W	R6, 1(R3);			\
-	B	loop;				\
-end:						\
-	/* execute write barrier updates */	\
-	MOVD	argtype+0(FP), R7;		\
-	MOVD	arg+16(FP), R3;			\
-	MOVWU	n+24(FP), R4;			\
-	MOVWU	retoffset+28(FP), R6;		\
-	MOVD	R7, 8(RSP);			\
-	MOVD	R3, 16(RSP);			\
-	MOVD	R4, 24(RSP);			\
-	MOVD	R6, 32(RSP);			\
-	BL	runtime·callwritebarrier(SB);	\
+	BL	callRet<>(SB);			\
+	RET
+
+// callRet copies return values back at the end of call*. This is a
+// separate function so it can allocate stack space for the arguments
+// to reflectcallmove. It does not follow the Go ABI; it expects its
+// arguments in registers.
+TEXT callRet<>(SB), NOSPLIT, $40-0
+	MOVD	R7, 8(RSP)
+	MOVD	R3, 16(RSP)
+	MOVD	R5, 24(RSP)
+	MOVD	R4, 32(RSP)
+	BL	runtime·reflectcallmove(SB)
 	RET
 
 // These have 8 added to make the overall frame size a multiple of 16,

src/runtime/asm_mips64x.s

@@ -309,8 +309,6 @@ TEXT reflect·call(SB), NOSPLIT, $0-0
 
 TEXT ·reflectcall(SB), NOSPLIT, $-8-32
 	MOVWU argsize+24(FP), R1
-	// NOTE(rsc): No call16, because CALLFN needs four words
-	// of argument space to invoke callwritebarrier.
 	DISPATCH(runtime·call32, 32)
 	DISPATCH(runtime·call64, 64)
 	DISPATCH(runtime·call128, 128)
@@ -361,33 +359,27 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-24;		\
 	PCDATA  $PCDATA_StackMapIndex, $0;	\
 	JAL	(R4);				\
 	/* copy return values back */		\
+	MOVV	argtype+0(FP), R5;		\
 	MOVV	arg+16(FP), R1;			\
 	MOVWU	n+24(FP), R2;			\
 	MOVWU	retoffset+28(FP), R4;		\
-	MOVV	R29, R3;				\
+	ADDV	$8, R29, R3;				\
 	ADDV	R4, R3; 			\
 	ADDV	R4, R1;				\
 	SUBVU	R4, R2;				\
-	ADDV	$8, R3;			\
-	ADDV	R3, R2;				\
-loop:						\
-	BEQ	R3, R2, end;				\
-	MOVBU	(R3), R4;			\
-	ADDV	$1, R3;			\
-	MOVBU	R4, (R1);			\
-	ADDV	$1, R1;			\
-	JMP	loop;				\
-end:						\
-	/* execute write barrier updates */	\
-	MOVV	argtype+0(FP), R5;		\
-	MOVV	arg+16(FP), R1;			\
-	MOVWU	n+24(FP), R2;			\
-	MOVWU	retoffset+28(FP), R4;		\
-	MOVV	R5, 8(R29);			\
-	MOVV	R1, 16(R29);			\
-	MOVV	R2, 24(R29);			\
-	MOVV	R4, 32(R29);			\
-	JAL	runtime·callwritebarrier(SB);	\
+	JAL	callRet<>(SB);			\
+	RET
+
+// callRet copies return values back at the end of call*. This is a
+// separate function so it can allocate stack space for the arguments
+// to reflectcallmove. It does not follow the Go ABI; it expects its
+// arguments in registers.
+TEXT callRet<>(SB), NOSPLIT, $32-0
+	MOVV	R5, 8(R29)
+	MOVV	R1, 16(R29)
+	MOVV	R3, 24(R29)
+	MOVV	R2, 32(R29)
+	JAL	runtime·reflectcallmove(SB)
 	RET
 
 CALLFN(·call16, 16)

src/runtime/asm_ppc64x.s

@@ -363,8 +363,6 @@ TEXT reflect·call(SB), NOSPLIT, $0-0
 
 TEXT ·reflectcall(SB), NOSPLIT|NOFRAME, $0-32
 	MOVWZ argsize+24(FP), R3
-	// NOTE(rsc): No call16, because CALLFN needs four words
-	// of argument space to invoke callwritebarrier.
 	DISPATCH(runtime·call32, 32)
 	DISPATCH(runtime·call64, 64)
 	DISPATCH(runtime·call128, 128)
@@ -418,33 +416,27 @@ TEXT NAME(SB), WRAPPER, $MAXSIZE-24;		\
 	BL	(CTR);				\
 	MOVD	24(R1), R2;			\
 	/* copy return values back */		\
+	MOVD	argtype+0(FP), R7;		\
 	MOVD	arg+16(FP), R3;			\
 	MOVWZ	n+24(FP), R4;			\
 	MOVWZ	retoffset+28(FP), R6;		\
-	MOVD	R1, R5;				\
+	ADD	$FIXED_FRAME, R1, R5;		\
 	ADD	R6, R5; 			\
 	ADD	R6, R3;				\
 	SUB	R6, R4;				\
-	ADD	$(FIXED_FRAME-1), R5;			\
-	SUB	$1, R3;				\
-	ADD	R5, R4;				\
-loop:						\
-	CMP	R5, R4;				\
-	BEQ	end;				\
-	MOVBZU	1(R5), R6;			\
-	MOVBZU	R6, 1(R3);			\
-	BR	loop;				\
-end:						\
-	/* execute write barrier updates */	\
-	MOVD	argtype+0(FP), R7;		\
-	MOVD	arg+16(FP), R3;			\
-	MOVWZ	n+24(FP), R4;			\
-	MOVWZ	retoffset+28(FP), R6;		\
-	MOVD	R7, FIXED_FRAME+0(R1);			\
-	MOVD	R3, FIXED_FRAME+8(R1);			\
-	MOVD	R4, FIXED_FRAME+16(R1);			\
-	MOVD	R6, FIXED_FRAME+24(R1);			\
-	BL	runtime·callwritebarrier(SB);	\
+	BL	callRet<>(SB);			\
+	RET
+
+// callRet copies return values back at the end of call*. This is a
+// separate function so it can allocate stack space for the arguments
+// to reflectcallmove. It does not follow the Go ABI; it expects its
+// arguments in registers.
+TEXT callRet<>(SB), NOSPLIT, $32-0
+	MOVD	R7, FIXED_FRAME+0(R1)
+	MOVD	R3, FIXED_FRAME+8(R1)
+	MOVD	R5, FIXED_FRAME+16(R1)
+	MOVD	R4, FIXED_FRAME+24(R1)
+	BL	runtime·reflectcallmove(SB)
 	RET
 
 CALLFN(·call32, 32)

src/runtime/asm_s390x.s

@@ -320,8 +320,6 @@ TEXT reflect·call(SB), NOSPLIT, $0-0
 
 TEXT ·reflectcall(SB), NOSPLIT, $-8-32
 	MOVWZ argsize+24(FP), R3
-	// NOTE(rsc): No call16, because CALLFN needs four words
-	// of argument space to invoke callwritebarrier.
 	DISPATCH(runtime·call32, 32)
 	DISPATCH(runtime·call64, 64)
 	DISPATCH(runtime·call128, 128)
@@ -377,6 +375,7 @@ callFunction:					\
 	PCDATA  $PCDATA_StackMapIndex, $0;	\
 	BL	(R8);				\
 	/* copy return values back */		\
+	MOVD	argtype+0(FP), R7;		\
 	MOVD	arg+16(FP), R6;			\
 	MOVWZ	n+24(FP), R5;			\
 	MOVD	$stack-MAXSIZE(SP), R4;		\
@@ -384,27 +383,19 @@ callFunction:					\
 	ADD	R1, R4;				\
 	ADD	R1, R6;				\
 	SUB	R1, R5;				\
-loopRets: /* copy 256 bytes at a time */	\
-	CMP	R5, $256;			\
-	BLT	tailRets;			\
-	SUB	$256, R5;			\
-	MVC	$256, 0(R4), 0(R6);		\
-	MOVD	$256(R4), R4;			\
-	MOVD	$256(R6), R6;			\
-	BR	loopRets;			\
-tailRets: /* copy remaining bytes */		\
-	CMP	R5, $0;				\
-	BEQ	writeBarrierUpdates;		\
-	SUB	$1, R5;				\
-	EXRL	$callfnMVC<>(SB), R5;		\
-writeBarrierUpdates:				\
-	/* execute write barrier updates */	\
-	MOVD	argtype+0(FP), R1;		\
-	MOVD	arg+16(FP), R2;			\
-	MOVWZ	n+24(FP), R3;			\
-	MOVWZ	retoffset+28(FP), R4;		\
-	STMG	R1, R4, stack-MAXSIZE(SP);	\
-	BL	runtime·callwritebarrier(SB);	\
+	BL	callRet<>(SB);			\
+	RET
+
+// callRet copies return values back at the end of call*. This is a
+// separate function so it can allocate stack space for the arguments
+// to reflectcallmove. It does not follow the Go ABI; it expects its
+// arguments in registers.
+TEXT callRet<>(SB), NOSPLIT, $32-0
+	MOVD	R7, 8(R15)
+	MOVD	R6, 16(R15)
+	MOVD	R4, 24(R15)
+	MOVD	R5, 32(R15)
+	BL	runtime·reflectcallmove(SB)
 	RET
 
 CALLFN(·call32, 32)

src/runtime/mbarrier.go

@@ -215,19 +215,21 @@ func reflect_typedmemmovepartial(typ *_type, dst, src unsafe.Pointer, off, size
 	heapBitsBulkBarrier(uintptr(dst), size&^(sys.PtrSize-1))
 }
 
-// callwritebarrier is invoked at the end of reflectcall, to execute
-// write barrier operations to record the fact that a call's return
-// values have just been copied to frame, starting at retoffset
-// and continuing to framesize. The entire frame (not just the return
-// values) is described by typ. Because the copy has already
-// happened, we call writebarrierptr_nostore, and this is nosplit so
-// the copy and write barrier appear atomic to GC.
+// reflectcallmove is invoked by reflectcall to copy the return values
+// out of the stack and into the heap, invoking the necessary write
+// barriers. dst, src, and size describe the return value area to
+// copy. typ describes the entire frame (not just the return values).
+// typ may be nil, which indicates write barriers are not needed.
+//
+// It must be nosplit and must only call nosplit functions because the
+// stack map of reflectcall is wrong.
+//
 //go:nosplit
-func callwritebarrier(typ *_type, frame unsafe.Pointer, framesize, retoffset uintptr) {
-	if !writeBarrier.needed || typ == nil || typ.kind&kindNoPointers != 0 || framesize-retoffset < sys.PtrSize {
-		return
+func reflectcallmove(typ *_type, dst, src unsafe.Pointer, size uintptr) {
+	memmove(dst, src, size)
+	if writeBarrier.needed && typ != nil && typ.kind&kindNoPointers == 0 && size >= sys.PtrSize {
+		heapBitsBulkBarrier(uintptr(dst), size)
 	}
-	heapBitsBulkBarrier(uintptr(add(frame, retoffset)), framesize-retoffset)
 }
 
 //go:nosplit