runtime: rework asmcgocall on ppc64x

On some platforms asmcgocall can be called with a nil g. Additionally, it
can be called when already on a the system (g0) stack or on a signal stack.
In these cases we do not need to switch (and/or cannot switch) to the
system stack and as a result, do not need to save the g.

Rework asmcgocall on ppc64x to follow the pattern used on other architectures,
such as amd64 and arm64, where a separate nosave path is called in the above
cases. The nil g case will be needed to support openbsd/ppc64.

Updates #56001

Change-Id: I431d4200bcbc4aaddeb617aefe18590165ff2927
Reviewed-on: https://go-review.googlesource.com/c/go/+/478775
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Paul Murphy <murp@ibm.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/runtime/asm_ppc64x.s

@@ -554,6 +554,8 @@ TEXT ·asmcgocall(SB),NOSPLIT,$0-20
 	MOVD	arg+8(FP), R4
 
 	MOVD	R1, R7		// save original stack pointer
+	CMP	$0, g
+	BEQ	nosave
 	MOVD	g, R5
 
 	// Figure out if we need to switch to m->g0 stack.
@@ -563,29 +565,29 @@ TEXT ·asmcgocall(SB),NOSPLIT,$0-20
 	MOVD	g_m(g), R8
 	MOVD	m_gsignal(R8), R6
 	CMP	R6, g
-	BEQ	g0
+	BEQ	nosave
 	MOVD	m_g0(R8), R6
 	CMP	R6, g
-	BEQ	g0
+	BEQ	nosave
+
 	BL	gosave_systemstack_switch<>(SB)
 	MOVD	R6, g
 	BL	runtime·save_g(SB)
 	MOVD	(g_sched+gobuf_sp)(g), R1
 
 	// Now on a scheduling stack (a pthread-created stack).
-g0:
 #ifdef GOOS_aix
 	// Create a fake LR to improve backtrace.
 	MOVD	$runtime·asmcgocall(SB), R6
 	MOVD	R6, 16(R1)
-	// AIX also save one argument on the stack.
+	// AIX also saves one argument on the stack.
-	SUB $8, R1
+	SUB	$8, R1
 #endif
 	// Save room for two of our pointers, plus the callee
 	// save area that lives on the caller stack.
 	SUB	$(asmcgocallSaveOffset+16), R1
 	RLDCR	$0, R1, $~15, R1	// 16-byte alignment for gcc ABI
-	MOVD	R5, (asmcgocallSaveOffset+8)(R1)// save old g on stack
+	MOVD	R5, (asmcgocallSaveOffset+8)(R1)	// save old g on stack
 	MOVD	(g_stack+stack_hi)(R5), R5
 	SUB	R7, R5
 	MOVD	R5, asmcgocallSaveOffset(R1)    // save depth in old g stack (can't just save SP, as stack might be copied during a callback)
@@ -605,9 +607,10 @@ g0:
 	MOVD	R12, CTR
 	MOVD	R4, R3		// arg in r3
 	BL	(CTR)
-	// C code can clobber R0, so set it back to 0. F27-F31 are
+
-	// callee save, so we don't need to recover those.
+	// Reinitialise zero value register.
 	XOR	R0, R0
+
 	// Restore g, stack pointer, toc pointer.
 	// R3 is errno, so don't touch it
 	MOVD	(asmcgocallSaveOffset+8)(R1), g
@@ -623,6 +626,41 @@ g0:
 	MOVW	R3, ret+16(FP)
 	RET
 
+nosave:
+	// Running on a system stack, perhaps even without a g.
+	// Having no g can happen during thread creation or thread teardown.
+	// This code is like the above sequence but without saving/restoring g
+	// and without worrying about the stack moving out from under us
+	// (because we're on a system stack, not a goroutine stack).
+	// The above code could be used directly if already on a system stack,
+	// but then the only path through this code would be a rare case.
+	// Using this code for all "already on system stack" calls exercises it more,
+	// which should help keep it correct.
+
+	SUB	$(asmcgocallSaveOffset+8), R1
+	RLDCR	$0, R1, $~15, R1		// 16-byte alignment for gcc ABI
+	MOVD	R7, asmcgocallSaveOffset(R1)	// Save original stack pointer.
+
+	MOVD	R3, R12		// fn
+#ifdef GO_PPC64X_HAS_FUNCDESC
+	// Load the real entry address from the first slot of the function descriptor.
+	MOVD	8(R12), R2
+	MOVD	(R12), R12
+#endif
+	MOVD	R12, CTR
+	MOVD	R4, R3		// arg
+	BL	(CTR)
+
+	// Reinitialise zero value register.
+	XOR	R0, R0
+
+	MOVD	asmcgocallSaveOffset(R1), R1	// Restore stack pointer.
+#ifndef GOOS_aix
+	MOVD	24(R1), R2
+#endif
+	MOVW	R3, ret+16(FP)
+	RET
+
 // func cgocallback(fn, frame unsafe.Pointer, ctxt uintptr)
 // See cgocall.go for more details.
 TEXT ·cgocallback(SB),NOSPLIT,$24-24