morestack.S

// Mark stack as non-executable
#if defined(__linux__) && defined(__ELF__)
.section	.note.GNU-stack, "", @progbits
#endif

/*
	__morestack

	This function implements stack growth using the mechanism
	devised by Ian Lance Taylor for gccgo, described here:

	http://gcc.gnu.org/wiki/SplitStacks

	The Rust stack is composed of a linked list of stack segments,
	and each stack segment contains two parts: the work area,
	where Rust functions are allowed to execute; and the red zone,
	where no Rust code can execute, but where short runtime
	functions (including __morestack), the dynamic linker, signal
	handlers, and the unwinder can run.

	Each Rust function contains an LLVM-generated prologue that
	compares the stack space required for the current function to
	the space remaining in the current stack segment,
	maintained in a platform-specific TLS slot.  The stack limit
	is strategically maintained by the Rust runtime so that it is
	always in place whenever a Rust function is running.

	When there is not enough room to run the function, the function
	prologue makes a call to __morestack to allocate a new stack
	segment, copy any stack-based arguments to it, switch stacks,
	then resume execution of the original function.

	-- The __morestack calling convention --

	For reasons of efficiency the __morestack calling convention
	is bizarre. The calling function does not attempt to align the
	stack for the call, and on x86_64 the arguments to __morestack
	are passed in scratch registers in order to preserve the
	original function's arguments.

	Once __morestack has switched to the new stack, instead of
	returning, it then calls into the original function, resuming
	execution at the instruction following the call to
	__morestack. Thus, when the original function returns it
	actually returns to __morestack, which then deallocates the
	stack and returns again to the original function's caller.

	-- Unwinding --

	All this trickery causes hell when it comes time for the
	unwinder to navigate it's way through this function. What
	will happen is the original function will be unwound first
	without any special effort, then the unwinder encounters
	the __morestack frame, which is sitting just above a
	tiny fraction of a frame (containing just a return pointer
	and, on 32-bit, the arguments to __morestack).

	We deal with this by claiming that little bit of stack
	is actually part of the __morestack frame, encoded as
	DWARF call frame instructions (CFI) by .cfi assembler
	pseudo-ops.

	One final complication (that took me a week to figure out)
	is that OS X 10.6+ uses its own 'compact unwind info',
	an undocumented format generated by the linker from
	the DWARF CFI. This compact unwind info doesn't correctly
	capture the nuance of the __morestack frame, and as a
	result all of our linking on OS X uses the -no_compact_unwind
	flag.
*/

.text

#if defined(__APPLE__)
#define RUST_GET_TASK           L_rust_get_task$stub
#define UPCALL_NEW_STACK        L_upcall_new_stack$stub
#define UPCALL_DEL_STACK        L_upcall_del_stack$stub
#define MORESTACK               ___morestack
#else
#if defined(__linux__) || defined(__FreeBSD__)
#define UPCALL_NEW_STACK        upcall_new_stack
#define UPCALL_DEL_STACK        upcall_del_stack
#define RUST_GET_TASK           rust_get_task
#define MORESTACK               __morestack
#else
#define UPCALL_NEW_STACK        _upcall_new_stack
#define UPCALL_DEL_STACK        _upcall_del_stack
#define RUST_GET_TASK           _rust_get_task
#define MORESTACK               ___morestack
#endif
#endif

#ifndef __APPLE__
.globl UPCALL_NEW_STACK
.globl UPCALL_DEL_STACK
.globl RUST_GET_TASK
#endif
.globl MORESTACK

// FIXME: What about __WIN32__?
#if defined(__linux__) || defined(__FreeBSD__)
	.hidden MORESTACK
#else
#if defined(__APPLE__)
	.private_extern MORESTACK
#endif
#endif

#ifdef __ELF__
	.type MORESTACK,@function
#endif

MORESTACK:
#if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || defined(__WIN32__)
	.cfi_startproc
#endif

	// This base pointer setup differs from most in that we are
	// telling the unwinder to consider the Canonical Frame
	// Address (CFA) for this frame to be the value of the stack
	// pointer prior to entry to the original function, whereas
	// the CFA would typically be the value of the stack
	// pointer prior to entry to this function. This will allow
	// the unwinder to understand how to skip the tiny partial
	// frame that the original function created by calling
	// __morestack.

	// In practical terms, our CFA is 12 bytes greater than it
	// would normally be, accounting for the two arguments to
	// __morestack, and an extra return address.

	pushl %ebp
#if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || defined(__WIN32__)
	// The CFA is 20 bytes above the register that it is
	// associated with for this frame (which will be %ebp)
	.cfi_def_cfa_offset 20
	// %ebp is -20 bytes from the CFA
	.cfi_offset %ebp, -20
#endif
	movl %esp, %ebp
#if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || defined(__WIN32__)
	// Calculate the CFA as an offset from %ebp
	.cfi_def_cfa_register %ebp
#endif

	// NB: This can be called with the fastcc convention so we
	// have to preserve any argument registers

	// NB: __morestack is called misaligned by 4 bytes, i.e.
	// subl $4, %esp would get us to a normal alignment

	subl $28,%esp

	// Save fastcc arguments
	movl %ecx, 16(%esp)
	movl %edx, 12(%esp)

	// FIXME (1388): it's possible we also need to save/restore some
	// SSE2 registers here, if floats-go-in-regs on x86+SSE2. Unclear.

	// FIXME (1226): main is compiled with the split-stack prologue,
	// causing it to call __morestack, so we have to jump back out
	calll RUST_GET_TASK
	testl %eax,%eax
	jz .L$bail

	// The arguments to upcall_new_stack

	// The size of the stack arguments to copy to the new stack,
	// and of the arguments to __morestack
	movl 40(%esp),%eax
	movl %eax,8(%esp)
	// The address of the stack arguments to the original function
	leal 48(%esp),%eax
	movl %eax,4(%esp)
	// The amount of stack needed for the original function,
	// the other argument to __morestack
	movl 36(%esp),%eax // The amount of stack needed
	movl %eax,(%esp)

	call UPCALL_NEW_STACK

	// Save the address of the new stack
	movl %eax, (%esp)

	// Grab the __morestack return pointer
	movl 32(%esp),%eax
	// Skip past the ret instruction in the parent fn
	inc  %eax

	// Restore the fastcc arguments to the original function
	movl 16(%esp), %ecx
	movl 12(%esp), %edx

        // Switch stacks
	movl (%esp),%esp
        // Re-enter the function that called us
	call *%eax

	// Now the function that called us has returned, so we need to
	// delete the old stack space

	// Switch back to the rust stack
	movl %ebp, %esp

	// Realign stack - remember that __morestack was called misaligned
	subl $12, %esp

	// Save the return value of the function we allocated space for
	movl %edx, 4(%esp)
	movl %eax, (%esp)

	call UPCALL_DEL_STACK

	// And restore it
	movl (%esp), %eax
	movl 4(%esp), %edx

	addl $12,%esp

	popl %ebp

	retl $8

.L$bail:
	movl 32(%esp),%eax
	inc %eax

	addl $44, %esp
	popl %ebp
	addl $4+8,%esp

	jmpl *%eax

#if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || defined(__WIN32__)
	.cfi_endproc
#endif

#ifdef __APPLE__

.section __IMPORT,__jump_table,symbol_stubs,pure_instructions+self_modifying_code,5

	// Linker will replace the hlts (the ascii) with jmp
L_rust_get_task$stub:
	.indirect_symbol _rust_get_task
	.ascii	 "\364\364\364\364\364"

L_upcall_new_stack$stub:
	.indirect_symbol _upcall_new_stack
	.ascii	 "\364\364\364\364\364"

L_upcall_del_stack$stub:
	.indirect_symbol _upcall_del_stack
	.ascii	 "\364\364\364\364\364"

	.subsections_via_symbols
#endif