rt: Add lots of documentation to __morestack
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Brian Anderson
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@ -1,9 +1,70 @@
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.text
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/*
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__morestack
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// __morestack
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//
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// LLVM generates a call to this to allocate more stack space in a function
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// prolog when we run out.
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This function implements stack growth using the mechanism
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devised by Ian Lance Taylor for gccgo, described here:
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http://gcc.gnu.org/wiki/SplitStacks
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The Rust stack is composed of a linked list of stack segments,
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and each stack segment contains two parts: the work area,
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where Rust functions are allowed to execute; and the red zone,
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where no Rust code can execute, but where short runtime
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functions (including __morestack), the dynamic linker, signal
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handlers, and the unwinder can run.
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Each Rust function contains an LLVM-generated prologue that
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compares the stack space required for the current function to
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the space space remaining in the current stack segment,
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maintained in a platform-specific TLS slot. The stack limit
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is strategically maintained by the Rust runtime so that it is
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always in place whenever a Rust function is running.
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When there is not enough room to run the function, the function
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prologue makes a call to __morestack to allocate a new stack
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segment, copy any stack-based arguments to it, switch stacks,
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then resume execution of the original function.
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-- The __morestack calling convention --
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For reasons of efficiency the __morestack calling convention
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is bizarre. The calling function does not attempt to align the
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stack for the call, and on x86_64 the arguments to __morestack
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are passed in scratch registers in order to preserve the
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original function's arguments.
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Once __morestack has switched to the new stack, instead of
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returning, it then calls into the original function, resuming
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execution at the instruction following the call to
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__morestack. Thus, when the original function returns it
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actually returns to __morestack, which then deallocates the
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stack and returns again to the original function's caller.
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-- Unwinding --
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All this trickery causes hell when it comes time for the
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unwinder to navigate it's way through this function. What
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will happen is the original function will be unwound first
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without any special effort, then the unwinder encounters
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the __morestack frame, which is sitting just above a
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tiny fraction of a frame (containing just a return pointer
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and, on 32-bit, the arguments to __morestack).
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We deal with this by claiming that that little bit of stack
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is actually part of the __morestack frame, encoded as
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DWARF call frame instructions (CFI) by .cfi assembler
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pseudo-ops.
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One final complication (that took me a week to figure out)
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is that OS X 10.6+ uses its own 'compact unwind info',
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an undocumented format generated by the linker from
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the DWARF CFI. This compact unwind info doesn't correctly
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capture the nuance of the __morestack frame, and as a
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result all of our linking on OS X uses the -no_compact_unwind
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flag.
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*/
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.text
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#if defined(__APPLE__)
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#define RUST_GET_TASK L_rust_get_task$stub
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@ -51,13 +112,31 @@ MORESTACK:
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.cfi_startproc
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#endif
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// This base pointer setup differs from most in that we are
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// telling the unwinder to consider the Canonical Frame
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// Address (CFA) for this frame to be the value of the stack
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// pointer prior to entry to the original function, whereas
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// the CFA would typically be the the value of the stack
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// pointer prior to entry to this function. This will allow
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// the unwinder to understand how to skip the tiny partial
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// frame that the original function created by calling
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// __morestack.
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// In practical terms, our CFA is 12 bytes greater than it
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// would normally be, accounting for the two arguments to
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// __morestack, and an extra return address.
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pushl %ebp
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#if defined(__linux__) || defined(__APPLE__)
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// The CFA is 20 bytes above the register that it is
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// associated with for this frame (which will be %ebp)
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.cfi_def_cfa_offset 20
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// %ebp is -20 bytes from the CFA
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.cfi_offset %ebp, -20
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#endif
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movl %esp, %ebp
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#if defined(__linux__) || defined(__APPLE__)
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// Calculate the CFA as an offset from %ebp
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.cfi_def_cfa_register %ebp
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#endif
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@ -81,17 +160,25 @@ MORESTACK:
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// Save the the correct %esp value for our grandparent frame,
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// for the unwinder
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// FIXME: This isn't used
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leal 20(%ebp), %eax
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movl %eax, -4(%ebp)
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// The arguments to rust_new_stack2
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movl 56(%esp),%eax // Size of stack arguments
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// The arguments to upcall_new_stack
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// The size of the stack arguments to copy to the new stack,
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// ane of the the arguments to __morestack
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movl 56(%esp),%eax
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movl %eax,20(%esp)
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leal 64(%esp),%eax // Address of stack arguments
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// The address of the stack arguments to the original function
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leal 64(%esp),%eax
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movl %eax,16(%esp)
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// The amount of stack needed for the original function,
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// the other argument to __morestack
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movl 52(%esp),%eax // The amount of stack needed
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movl %eax,12(%esp)
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movl $0, 8(%esp) // Out pointer
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// Out pointer to the new stack
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movl $0, 8(%esp)
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#ifdef __APPLE__
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call 1f
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@ -106,18 +193,22 @@ MORESTACK:
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movl %eax,(%esp)
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call UPCALL_CALL_C
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movl 48(%esp),%eax // Grab the return pointer.
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inc %eax // Skip past the ret instruction in the parent fn
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// Grab the __morestack return pointer
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movl 48(%esp),%eax
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// Skip past the ret instruction in the parent fn
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inc %eax
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// Restore fastcc arguments
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// Restore the fastcc arguments to the original function
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movl 28(%esp), %ecx
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movl 24(%esp), %edx
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movl 8(%esp),%esp // Switch stacks.
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call *%eax // Re-enter the function that called us.
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// Switch stacks
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movl 8(%esp),%esp
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// Re-enter the function that called us
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call *%eax
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// Now the function that called us has returned, so we need to delete the
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// old stack space.
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// Now the function that called us has returned, so we need to
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// delete the old stack space
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// Switch back to the rust stack
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movl %ebp, %esp
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@ -127,8 +218,8 @@ MORESTACK:
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subl $4, %esp
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// Now that we're on the return path we want to avoid
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// stomping on %eax. FIXME: Need to save and restore
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// eax to actually preserve it across the call to delete the stack
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// stomping on %eax. FIXME: Need to save and restore %eax to
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// actually preserve it across the call to delete the stack
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#ifdef __APPLE__
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call 1f
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1: popl %ecx
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@ -144,8 +235,14 @@ MORESTACK:
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addl $12,%esp
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popl %ebp
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// FIXME: I don't think these rules are necessary
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// since the unwinder should never encounter an instruction
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// pointer pointing here.
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#if defined(__linux__) || defined(__APPLE__)
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// Restore the rule for how to find %ebp
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.cfi_restore %ebp
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// Tell the unwinder how to find the CFA in terms of %esp
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.cfi_def_cfa %esp, 16
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#endif
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retl $8
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@ -1,9 +1,10 @@
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.text
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/*
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__morestack
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// __morestack
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//
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// LLVM generates a call to this to allocate more stack space in a functiono
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// prolog when we run out.
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See i386/morestack.S for the lengthy, general explanation.
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*/
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.text
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#if defined(__APPLE__) || defined(_WIN32)
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#define UPCALL_NEW_STACK _upcall_new_stack
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@ -15,19 +16,6 @@
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#define UPCALL_DEL_STACK upcall_del_stack
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#define UPCALL_CALL_C upcall_call_shim_on_c_stack
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#define MORESTACK __morestack
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#endif
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// Naturally, nobody can agree as to
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// which arguments should go in which
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// registers:
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#if defined(_WIN32)
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# define ARG0 %rcx
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# define ARG1 %rdx
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# define ARG2 %r8
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#else
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# define ARG0 %rdi
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# define ARG1 %rsi
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# define ARG2 %rdx
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#endif
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.globl UPCALL_NEW_STACK
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@ -35,7 +23,6 @@
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.globl UPCALL_CALL_C
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.globl MORESTACK
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// FIXME: What about _WIN32?
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#if defined(__linux__)
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.hidden MORESTACK
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#else
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@ -48,24 +35,31 @@
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.type MORESTACK,@function
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#endif
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#if defined(__linux__) || defined(__APPLE__)
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MORESTACK:
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.cfi_startproc
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// Set up a normal backtrace
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pushq %rbp
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// The CFA is 24 bytes above the register that it will
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// be associated with for this frame (%rbp). That is 8
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// bytes greater than a normal frame, to allow the unwinder
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// to skip the partial frame of the original function.
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.cfi_def_cfa_offset 24
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// %rbp is -24 bytes from the CFA
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.cfi_offset %rbp, -24
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movq %rsp, %rbp
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// Calculate the CFA as on offset from %ebp
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.cfi_def_cfa_register %rbp
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// Save the grandparent stack pointer for the unwinder
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// FIXME: This isn't used
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leaq 24(%rbp), %rax
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pushq %rax
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// FIXME: libgcc also saves rax. not sure if we need to
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// Save argument registers
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// Save argument registers of the original function
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pushq %rdi
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pushq %rsi
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pushq %rdx
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@ -79,6 +73,8 @@ MORESTACK:
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movq %rbp, %rcx
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addq $24, %rcx // Base pointer, return address x2
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// The arguments to __morestack are passed in %r10 & %r11
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pushq %r11 // Size of stack arguments
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pushq %rcx // Address of stack arguments
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pushq %r10 // The amount of stack needed
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@ -119,7 +115,8 @@ MORESTACK:
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// Align the stack again
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pushq $0
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// FIXME: Should preserve %rax here
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movq UPCALL_DEL_STACK@GOTPCREL(%rip), %rsi
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movq $0, %rdi
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#ifdef __APPLE__
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@ -131,6 +128,9 @@ MORESTACK:
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addq $8, %rsp
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popq %rbp
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// FIXME: I don't think these rules are necessary
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// since the unwinder should never encounter an instruction
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// pointer pointing here.
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.cfi_restore %rbp
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.cfi_def_cfa %rsp, 16
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ret
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