diff options
Diffstat (limited to 'libguile/frames.h')
-rw-r--r-- | libguile/frames.h | 126 |
1 files changed, 79 insertions, 47 deletions
diff --git a/libguile/frames.h b/libguile/frames.h index f3bb9b046..bc5216568 100644 --- a/libguile/frames.h +++ b/libguile/frames.h @@ -23,47 +23,62 @@ #include "programs.h" -/* - * VM frames - */ +/* Stack frames + ------------ -/* - * It's a little confusing, but there are two representations of frames in this - * file: frame pointers and Scheme objects wrapping those frame pointers. The - * former uses the SCM_FRAME_... macro prefix, the latter SCM_VM_FRAME_.. - * prefix. - * - * The confusing thing is that only Scheme frame objects have functions that use - * them, and they use the scm_frame_.. prefix. Hysterical raisins. - */ + It's a little confusing, but there are two representations of frames + in this file: frame pointers, and Scheme objects wrapping those frame + pointers. The former uses the SCM_FRAME macro prefix, the latter + SCM_VM_FRAME prefix. + + The confusing thing is that only Scheme frame objects have functions + that use them, and they use the lower-case scm_frame prefix. -/* VM Frame Layout - --------------- + Stack frame layout + ------------------ + + /------------------\ + | Local N-1 | <- sp | ... | - | Intermed. val. 0 | <- fp + nargs + nlocs - +------------------+ - | Local variable 1 | - | Local variable 0 | <- fp + nargs - | Argument 1 | - | Argument 0 | <- fp = SCM_FRAME_STACK_ADDRESS (fp) - | Program | <- fp - 1 + | Local 1 | + | Local 0 | <- fp = SCM_FRAME_LOCALS_ADDRESS (fp) +==================+ - | Return address | <- SCM_FRAME_UPPER_ADDRESS (fp) - | Dynamic link | <- fp - 3 = SCM_FRAME_DATA_ADDRESS (fp) = SCM_FRAME_LOWER_ADDRESS (fp) + | Return address | + | Dynamic link | <- fp - 2 = SCM_FRAME_LOWER_ADDRESS (fp) +==================+ - | | + | | <- fp - 3 = SCM_FRAME_PREVIOUS_SP (fp) - As can be inferred from this drawing, it is assumed that - `sizeof (SCM *) == sizeof (SCM)', since pointers (the `link' parts) are - assumed to be as long as SCM objects. + The calling convention is that a caller prepares a stack frame + consisting of the saved FP and the return address, followed by the + procedure and then the arguments to the call, in order. Thus in the + beginning of a call, the procedure being called is in slot 0, the + first argument is in slot 1, and the SP points to the last argument. + The number of arguments, including the procedure, is thus SP - FP + + 1. - When a program returns multiple values, it will shuffle them down to - start contiguously from slot 1, as for a tail call. This means that - when the caller goes to access them, there are 2 or 3 empty words - between the top of the caller stack and the bottom of the values, - corresponding to the frame that was just popped. -*/ + After ensuring that the correct number of arguments have been passed, + a function will set the stack pointer to point to the last local + slot. This lets a function allocate the temporary space that it + needs once in the beginning of the call, instead of pushing and + popping the stack pointer during the call's extent. + + When a program returns, it returns its values in the slots starting + from local 1, as if the values were arguments to a tail call. We + start from 1 instead of 0 for the convenience of the "values" builtin + function, which can just leave its arguments in place. + + The callee resets the stack pointer to point to the last value. In + this way the caller knows how many values there are: it's the number + of words between the stack pointer and the slot at which the caller + placed the procedure. + + After checking that the number of values returned is appropriate, the + caller shuffles the values around (if needed), and resets the stack + pointer back to its original value from before the call. */ + + + /* This structure maps to the contents of a VM stack frame. It can alias a frame directly. */ @@ -71,20 +86,15 @@ struct scm_vm_frame { SCM *dynamic_link; scm_t_uint8 *return_address; - SCM program; - SCM stack[1]; /* Variable-length */ + SCM locals[1]; /* Variable-length */ }; +#define SCM_FRAME_LOWER_ADDRESS(fp) (((SCM *) (fp)) - 2) #define SCM_FRAME_STRUCT(fp) \ - ((struct scm_vm_frame *) SCM_FRAME_DATA_ADDRESS (fp)) - -#define SCM_FRAME_DATA_ADDRESS(fp) (((SCM *) (fp)) - 3) -#define SCM_FRAME_STACK_ADDRESS(fp) (SCM_FRAME_STRUCT (fp)->stack) -#define SCM_FRAME_UPPER_ADDRESS(fp) ((SCM*)&SCM_FRAME_STRUCT (fp)->return_address) -#define SCM_FRAME_LOWER_ADDRESS(fp) ((SCM*)SCM_FRAME_STRUCT (fp)) + ((struct scm_vm_frame *) SCM_FRAME_LOWER_ADDRESS (fp)) +#define SCM_FRAME_LOCALS_ADDRESS(fp) (SCM_FRAME_STRUCT (fp)->locals) -#define SCM_FRAME_BYTE_CAST(x) ((scm_t_uint8 *) SCM_UNPACK (x)) -#define SCM_FRAME_STACK_CAST(x) ((SCM *) SCM_UNPACK (x)) +#define SCM_FRAME_PREVIOUS_SP(fp) (((SCM *) (fp)) - 3) #define SCM_FRAME_RETURN_ADDRESS(fp) \ (SCM_FRAME_STRUCT (fp)->return_address) @@ -94,10 +104,32 @@ struct scm_vm_frame (SCM_FRAME_STRUCT (fp)->dynamic_link) #define SCM_FRAME_SET_DYNAMIC_LINK(fp, dl) \ SCM_FRAME_DYNAMIC_LINK (fp) = (dl) -#define SCM_FRAME_VARIABLE(fp,i) \ - (SCM_FRAME_STRUCT (fp)->stack[i]) -#define SCM_FRAME_PROGRAM(fp) \ - (SCM_FRAME_STRUCT (fp)->program) +#define SCM_FRAME_LOCAL(fp,i) \ + (SCM_FRAME_STRUCT (fp)->locals[i]) + +#define SCM_FRAME_NUM_LOCALS(fp, sp) \ + ((sp) + 1 - &SCM_FRAME_LOCAL (fp, 0)) + +/* Currently (November 2013) we keep the procedure and arguments in + their slots for the duration of the procedure call, regardless of + whether the values are live or not. This allows for backtraces that + show the closure and arguments. We may allow the compiler to relax + this restriction in the future, if the user so desires. This would + conserve stack space and make GC more precise. We would need better + debugging information to do that, however. + + Even now there is an exception to the rule that slot 0 holds the + procedure, which is in the case of tail calls. The compiler will + emit code that shuffles the new procedure and arguments into position + before performing the tail call, so there is a window in which + SCM_FRAME_PROGRAM does not correspond to the program being executed. + + The moral of the story is to use the IP in a frame to determine what + procedure is being called. It is only appropriate to use + SCM_FRAME_PROGRAM in the prologue of a procedure call, when you know + it must be there. */ + +#define SCM_FRAME_PROGRAM(fp) (SCM_FRAME_LOCAL (fp, 0)) |