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Implement and document the new jit_indirect call.

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* include/lightning.h, lib/lightning.c: Add the new
	jit_indirect() call, that returns a special label node,
	and tells lightning that the label may be the target of
	an indirect jump.

	* doc/body.texi: Document the new jit_indirect() call, and
	add examples of different ways to create labels and branches.
This commit is contained in:
pcpa 2014-03-06 18:20:29 -03:00
parent c146f06793
commit a9433b5a2c
4 changed files with 88 additions and 2 deletions
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62
doc/body.texi
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@ -22,7 +22,7 @@ both retargetable and very fast.
@menu
* Overview:: What GNU lightning is
* Installation:: Configuring and installing GNU lightning
* The instruction set:: The RISC instruction set used i GNU lightning
* The instruction set:: The RISC instruction set used in GNU lightning
* GNU lightning examples:: GNU lightning's examples
* Reentrancy:: Re-entrant usage of GNU lightning
* Acknowledgements:: Acknowledgements for GNU lightning
@ -501,6 +501,66 @@ Like branch instruction, @code{jmpi} also returns a value which is to
be used to compile forward branches. @xref{Fibonacci, , Fibonacci
numbers}.
@item Labels
There are 3 @lightning{} instructions to create labels:
@example
label (not specified) @r{simple label}
forward (not specified) @r{forward label}
indirect (not specified) @r{special simple label}
@end example
@code{label} is normally used as @code{patch_at} argument for backward
jumps.
@example
jit_node_t *jump, *label;
label = jit_label();
...
jump = jit_beqr(JIT_R0, JIT_R1);
jit_patch_at(jump, label);
@end example
@code{forward} is used to patch code generation before the actual
position of the label is known.
@example
jit_node_t *jump, *label;
label = jit_forward();
jump = jit_beqr(JIT_R0, JIT_R1);
jit_patch_at(jump, label);
...
jit_link(label);
@end example
@code{indirect} is useful when creating jump tables, and tells
@lightning{} to not optimize out a label that is not the target of
any jump, because an indirect jump may land where it is defined.
@example
jit_node_t *jump, *label;
...
jmpr(JIT_R0); @rem{/* may jump to label */}
...
label = jit_indirect();
@end example
@code{indirect} is an special case of @code{note} and @code{name}
because it is a valid argument to @code{address}.
Note that the usual idiom to write the previous example is
@example
jit_node_t *addr, *jump;
addr = jit_movi(JIT_R0, 0); @rem{/* immediate is ignored */}
...
jmpr(JIT_R0);
...
jit_patch(addr); @rem{/* implicit label added */}
@end example
that automatically binds the implicit label added by @code{patch} with
the @code{movi}, but on some special conditions it is required to create
an "unbound" label.
@item Function prolog
These macros are used to set up a function prolog. The @code{allocai}