#include "programs.h"
\f
-/*
- * 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. */
+
+
+\f
/* This structure maps to the contents of a VM stack frame. It can
alias a frame directly. */
{
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)
(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))
\f
HCoop / bpt / guile.git / blobdiff