runtime: eliminate externalthreadhandler

This function is no longer used.

Eliminating this actually fixes several problems:

- It made assumptions about what registers memclrNoHeapPointers would
  preserve. Besides being an abstraction violation and lurking
  maintenance issue, this actively became a problem for regabi because
  the call to memclrNoHeapPointers now happens through an ABI wrapper,
  which is generated by the compiler and hence we can't easily control
  what registers it clobbers.

- The amd64 implementation (at least), does not interact with the host
  ABI correctly. Notably, it doesn't save many of the registers that
  are callee-save in the host ABI but caller-save in the Go ABI.

- It interacts strangely with the NOSPLIT checker because it allocates
  an entire M and G on its stack. It worked around this on arm64, and
  happened to do things the NOSPLIT checker couldn't track on 386 and
  amd64, and happened to be *4 bytes* below the limit on arm (so any
  addition to the m or g structs would cause a NOSPLIT failure). See
  CL 309031 for a more complete explanation.

Fixes #45530.
Updates #40724.

Change-Id: Ic70d4d7e1c17f1d796575b3377b8529449e93576
Reviewed-on: https://go-review.googlesource.com/c/go/+/309634
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 0 insertions, 67 deletions

diff --git a/src/runtime/sys_windows_arm.s b/src/runtime/sys_windows_arm.s
index 8914a2688f..8073fc0198 100644
--- a/src/runtime/sys_windows_arm.s
+++ b/src/runtime/sys_windows_arm.s
@@ -233,73 +233,6 @@ TEXT runtime·lastcontinuetramp<ABIInternal>(SB),NOSPLIT|NOFRAME,$0
 	MOVW	$runtime·lastcontinuehandler(SB), R1
 	B	sigtramp<>(SB)
 
-// int32 externalthreadhandler(uint32 arg, int (*func)(uint32))
-// stack layout:
-//   +----------------+
-//   | callee-save    |
-//   | registers      |
-//   +----------------+
-//   | m              |
-//   +----------------+
-// 20| g              |
-//   +----------------+
-// 16| func ptr (r1)  |
-//   +----------------+
-// 12| argument (r0)  |
-//---+----------------+
-// 8 | param1         | (also return value for called Go function)
-//   +----------------+
-// 4 | param0         |
-//   +----------------+
-// 0 | slot for LR    |
-//   +----------------+
-//
-TEXT runtime·externalthreadhandler<ABIInternal>(SB),NOSPLIT|NOFRAME|TOPFRAME,$0
-	MOVM.DB.W [R4-R11, R14], (R13)		// push {r4-r11, lr}
-	SUB	$(m__size + g__size + 20), R13	// space for locals
-	MOVW	R14, 0(R13)			// push LR again for anything unwinding the stack
-	MOVW	R0, 12(R13)
-	MOVW	R1, 16(R13)
-
-	// zero out m and g structures
-	ADD	$20, R13, R0			// compute pointer to g
-	MOVW	R0, 4(R13)
-	MOVW	$(m__size + g__size), R0
-	MOVW	R0, 8(R13)
-	BL	runtime·memclrNoHeapPointers(SB)
-
-	// initialize m and g structures
-	ADD	$20, R13, R2			// R2 = g
-	ADD	$(20 + g__size), R13, R3	// R3 = m
-	MOVW	R2, m_g0(R3)			// m->g0 = g
-	MOVW	R3, g_m(R2)			// g->m = m
-	MOVW	R2, m_curg(R3)			// m->curg = g
-
-	MOVW	R2, g
-	BL	runtime·save_g(SB)
-
-	// set up stackguard stuff
-	MOVW	R13, R0
-	MOVW	R0, g_stack+stack_hi(g)
-	SUB	$(32*1024), R0
-	MOVW	R0, (g_stack+stack_lo)(g)
-	MOVW	R0, g_stackguard0(g)
-	MOVW	R0, g_stackguard1(g)
-
-	// move argument into position and call function
-	MOVW	12(R13), R0
-	MOVW	R0, 4(R13)
-	MOVW	16(R13), R1
-	BL	(R1)
-
-	// clear g
-	MOVW	$0, g
-	BL	runtime·save_g(SB)
-
-	MOVW	8(R13), R0			// load return value
-	ADD	$(m__size + g__size + 20), R13	// free locals
-	MOVM.IA.W (R13), [R4-R11, R15]		// pop {r4-r11, pc}
-
 GLOBL runtime·cbctxts(SB), NOPTR, $4
 
 TEXT runtime·callbackasm1<ABIInternal>(SB),NOSPLIT|NOFRAME,$0