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>

src/cmd/internal/objabi/funcid.go

@@ -29,7 +29,6 @@ const (
 	FuncID_asyncPreempt
 	FuncID_cgocallback
 	FuncID_debugCallV1
-	FuncID_externalthreadhandler
 	FuncID_gcBgMarkWorker
 	FuncID_goexit
 	FuncID_gogo
@@ -55,7 +54,6 @@ var funcIDs = map[string]FuncID{
 	"asyncPreempt":     FuncID_asyncPreempt,
 	"cgocallback":      FuncID_cgocallback,
 	"debugCallV1":      FuncID_debugCallV1,
-	"externalthreadhandler": FuncID_externalthreadhandler,
 	"gcBgMarkWorker":   FuncID_gcBgMarkWorker,
 	"go":               FuncID_rt0_go,
 	"goexit":           FuncID_goexit,

src/cmd/link/internal/ld/pe.go

@@ -947,9 +947,7 @@ func (f *peFile) writeOptionalHeader(ctxt *Link) {
 	// calls that may need more stack than we think.
 	//
 	// The default stack reserve size directly affects only the main
-	// thread and threads that enter in externalthreadhandler.
+	// thread.
-	// For this, it must be greater than the stack size assumed by
-	// externalthreadhandler.
 	//
 	// For other threads, the runtime explicitly asks the kernel
 	// to use the default stack size so that all stacks are

src/runtime/memclr_386.s

@@ -7,8 +7,6 @@
 #include "go_asm.h"
 #include "textflag.h"
 
-// NOTE: Windows externalthreadhandler expects memclr to preserve DX.
-
 // See memclrNoHeapPointers Go doc for important implementation constraints.
 
 // func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)

src/runtime/memclr_amd64.s

@@ -7,8 +7,6 @@
 #include "go_asm.h"
 #include "textflag.h"
 
-// NOTE: Windows externalthreadhandler expects memclr to preserve DX.
-
 // See memclrNoHeapPointers Go doc for important implementation constraints.
 
 // func memclrNoHeapPointers(ptr unsafe.Pointer, n uintptr)

src/runtime/os_windows.go

@@ -386,7 +386,6 @@ const (
 )
 
 // in sys_windows_386.s and sys_windows_amd64.s:
-func externalthreadhandler()
 func getlasterror() uint32
 
 // When loading DLLs, we prefer to use LoadLibraryEx with

src/runtime/symtab.go

@@ -317,7 +317,6 @@ const (
 	funcID_asyncPreempt
 	funcID_cgocallback
 	funcID_debugCallV1
-	funcID_externalthreadhandler
 	funcID_gcBgMarkWorker
 	funcID_goexit
 	funcID_gogo

src/runtime/sys_windows_386.s

@@ -156,57 +156,6 @@ TEXT runtime·lastcontinuetramp<ABIInternal>(SB),NOSPLIT,$0-0
 	MOVL	$runtime·lastcontinuehandler(SB), AX
 	JMP	sigtramp<>(SB)
 
-TEXT runtime·externalthreadhandler<ABIInternal>(SB),NOSPLIT|TOPFRAME,$0
-	PUSHL	BP
-	MOVL	SP, BP
-	PUSHL	BX
-	PUSHL	SI
-	PUSHL	DI
-	PUSHL	0x14(FS)
-	MOVL	SP, DX
-
-	// setup dummy m, g
-	SUBL	$m__size, SP		// space for M
-	MOVL	SP, 0(SP)
-	MOVL	$m__size, 4(SP)
-	CALL	runtime·memclrNoHeapPointers(SB)	// smashes AX,BX,CX
-
-	LEAL	m_tls(SP), CX
-	MOVL	CX, 0x14(FS)
-	MOVL	SP, BX
-	SUBL	$g__size, SP		// space for G
-	MOVL	SP, g(CX)
-	MOVL	SP, m_g0(BX)
-
-	MOVL	SP, 0(SP)
-	MOVL	$g__size, 4(SP)
-	CALL	runtime·memclrNoHeapPointers(SB)	// smashes AX,BX,CX
-	LEAL	g__size(SP), BX
-	MOVL	BX, g_m(SP)
-
-	LEAL	-32768(SP), CX		// must be less than SizeOfStackReserve set by linker
-	MOVL	CX, (g_stack+stack_lo)(SP)
-	ADDL	$const__StackGuard, CX
-	MOVL	CX, g_stackguard0(SP)
-	MOVL	CX, g_stackguard1(SP)
-	MOVL	DX, (g_stack+stack_hi)(SP)
-
-	PUSHL	AX			// room for return value
-	PUSHL	16(BP)			// arg for handler
-	CALL	8(BP)
-	POPL	CX
-	POPL	AX			// pass return value to Windows in AX
-
-	get_tls(CX)
-	MOVL	g(CX), CX
-	MOVL	(g_stack+stack_hi)(CX), SP
-	POPL	0x14(FS)
-	POPL	DI
-	POPL	SI
-	POPL	BX
-	POPL	BP
-	RET
-
 GLOBL runtime·cbctxts(SB), NOPTR, $4
 
 TEXT runtime·callbackasm1<ABIInternal>(SB),NOSPLIT,$0

src/runtime/sys_windows_amd64.s

@@ -202,57 +202,6 @@ TEXT runtime·lastcontinuetramp<ABIInternal>(SB),NOSPLIT|NOFRAME,$0-0
 	MOVQ	$runtime·lastcontinuehandler(SB), AX
 	JMP	sigtramp<>(SB)
 
-TEXT runtime·externalthreadhandler<ABIInternal>(SB),NOSPLIT|NOFRAME|TOPFRAME,$0
-	PUSHQ	BP
-	MOVQ	SP, BP
-	PUSHQ	BX
-	PUSHQ	SI
-	PUSHQ	DI
-	PUSHQ	0x28(GS)
-	MOVQ	SP, DX
-
-	// setup dummy m, g
-	SUBQ	$m__size, SP		// space for M
-	MOVQ	SP, 0(SP)
-	MOVQ	$m__size, 8(SP)
-	CALL	runtime·memclrNoHeapPointers(SB)	// smashes AX,BX,CX, maybe BP
-
-	LEAQ	m_tls(SP), CX
-	MOVQ	CX, 0x28(GS)
-	MOVQ	SP, BX
-	SUBQ	$g__size, SP		// space for G
-	MOVQ	SP, g(CX)
-	MOVQ	SP, m_g0(BX)
-
-	MOVQ	SP, 0(SP)
-	MOVQ	$g__size, 8(SP)
-	CALL	runtime·memclrNoHeapPointers(SB)	// smashes AX,BX,CX, maybe BP
-	LEAQ	g__size(SP), BX
-	MOVQ	BX, g_m(SP)
-
-	LEAQ	-32768(SP), CX		// must be less than SizeOfStackReserve set by linker
-	MOVQ	CX, (g_stack+stack_lo)(SP)
-	ADDQ	$const__StackGuard, CX
-	MOVQ	CX, g_stackguard0(SP)
-	MOVQ	CX, g_stackguard1(SP)
-	MOVQ	DX, (g_stack+stack_hi)(SP)
-
-	PUSHQ	AX			// room for return value
-	PUSHQ	32(BP)			// arg for handler
-	CALL	16(BP)
-	POPQ	CX
-	POPQ	AX			// pass return value to Windows in AX
-
-	get_tls(CX)
-	MOVQ	g(CX), CX
-	MOVQ	(g_stack+stack_hi)(CX), SP
-	POPQ	0x28(GS)
-	POPQ	DI
-	POPQ	SI
-	POPQ	BX
-	POPQ	BP
-	RET
-
 GLOBL runtime·cbctxts(SB), NOPTR, $8
 
 TEXT runtime·callbackasm1(SB),NOSPLIT,$0

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

src/runtime/sys_windows_arm64.s

@@ -299,66 +299,6 @@ TEXT runtime·lastcontinuetramp(SB),NOSPLIT|NOFRAME,$0
 	MOVD	$runtime·lastcontinuehandler<ABIInternal>(SB), R1
 	B	sigtramp<>(SB)
 
-// externalthreadhander called with R0 = uint32 arg, R1 = Go function f.
-// Need to call f(arg), which returns a uint32, and return it in R0.
-TEXT runtime·externalthreadhandler<ABIInternal>(SB),NOSPLIT|TOPFRAME,$96-0
-	NO_LOCAL_POINTERS
-
-	// Push C callee-save registers R19-R28. LR, FP already saved.
-	SAVE_R19_TO_R28(-10*8)
-
-	// Allocate space for args, saved R0+R1, g, and m structures.
-	// Hide from nosplit check.
-	#define extra ((64+g__size+m__size+15)&~15)
-	SUB	$extra, RSP, R2	// hide from nosplit overflow check
-	MOVD	R2, RSP
-
-	// Save R0 and R1 (our args).
-	MOVD	R0, 32(RSP)
-	MOVD	R1, 40(RSP)
-
-	// Zero out m and g structures.
-	MOVD	$64(RSP), R0
-	MOVD	R0, 8(RSP)
-	MOVD	$(m__size + g__size), R0
-	MOVD	R0, 16(RSP)
-	MOVD	$0, 0(RSP)	// not-saved LR
-	BL	runtime·memclrNoHeapPointers(SB)
-
-	// Initialize m and g structures.
-	MOVD	$64(RSP), g
-	MOVD	$g__size(g), R3		// m
-	MOVD	R3, g_m(g)		// g->m = m
-	MOVD	g, m_g0(R3)		// m->g0 = g
-	MOVD	g, m_curg(R3)		// m->curg = g
-	MOVD	RSP, R0
-	MOVD	R0, g_stack+stack_hi(g)
-	SUB	$(32*1024), R0
-	MOVD	R0, (g_stack+stack_lo)(g)
-	MOVD	R0, g_stackguard0(g)
-	MOVD	R0, g_stackguard1(g)
-	BL	runtime·save_g(SB)
-
-	// Call function.
-	MOVD	32(RSP), R0
-	MOVD	40(RSP), R1
-	MOVW	R0, 8(RSP)
-	BL	(R1)
-
-	// Clear g.
-	MOVD	$0, g
-	BL	runtime·save_g(SB)
-
-	// Load return value (save_g would have smashed)
-	MOVW	(2*8)(RSP), R0
-
-	ADD	$extra, RSP, R2
-	MOVD	R2, RSP
-	#undef extra
-
-	RESTORE_R19_TO_R28(-10*8)
-	RET
-
 GLOBL runtime·cbctxts(SB), NOPTR, $4
 
 TEXT runtime·callbackasm1<ABIInternal>(SB),NOSPLIT,$208-0