* Re: Functions using attribute `noreturn'...
@ 2000-02-02 11:56 Ralf Guetlein
2000-04-01 0:00 ` Ralf Guetlein
0 siblings, 1 reply; 6+ messages in thread
From: Ralf Guetlein @ 2000-02-02 11:56 UTC (permalink / raw)
To: gcc-help; +Cc: pmw
Paul wrote
>I'm currently in the process of porting GCC and am trying to optimise my code
>for function prologues and epilogues.
May we know what target you are working on?
>To minimise the amount of code output, I would like to check to see if the
>current function is declared with the __noreturn__ attribute. If it is, then
>I don't need to do output any of the function return code or save/restore any
>registers that otherwise would be.
Not exactly...
__attribute__((noreturn)) has 2 effects:
1) It allows the compiler to issue a warning if a 'noreturn' function
returns
(the code is apparently erroneous)
2) If a function calls a function with attribute `noreturn' in the main
path,
the compiler knows that the caller doesn't return either.
BUT:
The attribute `noreturn' is not involved in code generation directly.
Instead, the compiler is able to recognize if there is a return path or
not,
independently of the `noreturn' attribute (E.g. if there is an endless
loop).
You have to deal with that in your function_epilogue() in the target
support
file (target.c). Find out if the last non-note insn of the current
function
is a BARRIER (that means no path leads to the epilogue). If this is the
case,
you don't need to emit an epilogue.
Your code in function_epilogue() may look like this (see h8300.c or
m68k.c):
/* If the last insn was a BARRIER, we don't have to write any code.
*/
if (GET_CODE (insn) == NOTE)
insn = prev_nonnote_insn (insn);
if (insn && GET_CODE (insn) == BARRIER)
return;
>Is there any way I can find out whether the current function has been declared
>with this attribute? (Similarly, for other attributes, such as interrupt). This
>would be in the C support code, not in the machine description (.md) file. Is
>there a GCC support routine I can call, or is it more involved?
You have to distinguish: For your `noreturn' problem see the code
snippet above.
To recognize attributes, you may use code like the following
(also taken from h8300.c):
/* Return nonzero if FUNC is an interrupt function as specified
by the "interrupt" attribute. */
static int
h8300_interrupt_function_p (func)
tree func;
{
tree a;
if (TREE_CODE (func) != FUNCTION_DECL)
return 0;
a = lookup_attribute ("interrupt_handler", DECL_MACHINE_ATTRIBUTES
(func));
return a != NULL_TREE;
}
Enjoy,
Ralf
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Functions using attribute `noreturn'...
2000-02-02 11:56 Functions using attribute `noreturn' Ralf Guetlein
@ 2000-04-01 0:00 ` Ralf Guetlein
0 siblings, 0 replies; 6+ messages in thread
From: Ralf Guetlein @ 2000-04-01 0:00 UTC (permalink / raw)
To: gcc-help; +Cc: pmw
Paul wrote
>I'm currently in the process of porting GCC and am trying to optimise my code
>for function prologues and epilogues.
May we know what target you are working on?
>To minimise the amount of code output, I would like to check to see if the
>current function is declared with the __noreturn__ attribute. If it is, then
>I don't need to do output any of the function return code or save/restore any
>registers that otherwise would be.
Not exactly...
__attribute__((noreturn)) has 2 effects:
1) It allows the compiler to issue a warning if a 'noreturn' function
returns
(the code is apparently erroneous)
2) If a function calls a function with attribute `noreturn' in the main
path,
the compiler knows that the caller doesn't return either.
BUT:
The attribute `noreturn' is not involved in code generation directly.
Instead, the compiler is able to recognize if there is a return path or
not,
independently of the `noreturn' attribute (E.g. if there is an endless
loop).
You have to deal with that in your function_epilogue() in the target
support
file (target.c). Find out if the last non-note insn of the current
function
is a BARRIER (that means no path leads to the epilogue). If this is the
case,
you don't need to emit an epilogue.
Your code in function_epilogue() may look like this (see h8300.c or
m68k.c):
/* If the last insn was a BARRIER, we don't have to write any code.
*/
if (GET_CODE (insn) == NOTE)
insn = prev_nonnote_insn (insn);
if (insn && GET_CODE (insn) == BARRIER)
return;
>Is there any way I can find out whether the current function has been declared
>with this attribute? (Similarly, for other attributes, such as interrupt). This
>would be in the C support code, not in the machine description (.md) file. Is
>there a GCC support routine I can call, or is it more involved?
You have to distinguish: For your `noreturn' problem see the code
snippet above.
To recognize attributes, you may use code like the following
(also taken from h8300.c):
/* Return nonzero if FUNC is an interrupt function as specified
by the "interrupt" attribute. */
static int
h8300_interrupt_function_p (func)
tree func;
{
tree a;
if (TREE_CODE (func) != FUNCTION_DECL)
return 0;
a = lookup_attribute ("interrupt_handler", DECL_MACHINE_ATTRIBUTES
(func));
return a != NULL_TREE;
}
Enjoy,
Ralf
^ permalink raw reply [flat|nested] 6+ messages in thread
* Functions using attribute `noreturn'...
2000-02-02 9:08 Paul Webster
2000-02-02 14:23 ` Jeffrey A Law
@ 2000-04-01 0:00 ` Paul Webster
1 sibling, 0 replies; 6+ messages in thread
From: Paul Webster @ 2000-04-01 0:00 UTC (permalink / raw)
To: gcc-help
I'm currently in the process of porting GCC and am trying to optimise my code
for function prologues and epilogues.
To minimise the amount of code output, I would like to check to see if the
current function is declared with the __noreturn__ attribute. If it is, then
I don't need to do output any of the function return code or save/restore any
registers that otherwise would be.
Is there any way I can find out whether the current function has been declared
with this attribute? (Similarly, for other attributes, such as interrupt). This
would be in the C support code, not in the machine description (.md) file. Is
there a GCC support routine I can call, or is it more involved?
Thanks in advance for any help.
Paul
==============================================================================
AT&T Laboratories Cambridge
------------------------------------------------------------------------------
Dr. Paul Webster, 24a Trumpington Street, Cambridge, CB2 1QA, England.
Tel: +44 (0)1223 343 218 Fax: +44 (0)1223 313542
==============================================================================
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Functions using attribute `noreturn'...
2000-02-02 14:23 ` Jeffrey A Law
@ 2000-04-01 0:00 ` Jeffrey A Law
0 siblings, 0 replies; 6+ messages in thread
From: Jeffrey A Law @ 2000-04-01 0:00 UTC (permalink / raw)
To: Paul Webster; +Cc: gcc-help
In message < 200002021707.RAA14128@rocoto >you write:
> To minimise the amount of code output, I would like to check to see if the
> current function is declared with the __noreturn__ attribute. If it is,
> then I don't need to do output any of the function return code or
> save/restore any registers that otherwise would be.
This will happen automatically if you are using RTL prologues and epilogues.
The code which emits the epilogue will notice that the exit block has no
true predecessors and thus the epilogue code is unnecessary and never added
to the insn stream.
jeff
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Functions using attribute `noreturn'...
2000-02-02 9:08 Paul Webster
@ 2000-02-02 14:23 ` Jeffrey A Law
2000-04-01 0:00 ` Jeffrey A Law
2000-04-01 0:00 ` Paul Webster
1 sibling, 1 reply; 6+ messages in thread
From: Jeffrey A Law @ 2000-02-02 14:23 UTC (permalink / raw)
To: Paul Webster; +Cc: gcc-help
In message < 200002021707.RAA14128@rocoto >you write:
> To minimise the amount of code output, I would like to check to see if the
> current function is declared with the __noreturn__ attribute. If it is,
> then I don't need to do output any of the function return code or
> save/restore any registers that otherwise would be.
This will happen automatically if you are using RTL prologues and epilogues.
The code which emits the epilogue will notice that the exit block has no
true predecessors and thus the epilogue code is unnecessary and never added
to the insn stream.
jeff
^ permalink raw reply [flat|nested] 6+ messages in thread
* Functions using attribute `noreturn'...
@ 2000-02-02 9:08 Paul Webster
2000-02-02 14:23 ` Jeffrey A Law
2000-04-01 0:00 ` Paul Webster
0 siblings, 2 replies; 6+ messages in thread
From: Paul Webster @ 2000-02-02 9:08 UTC (permalink / raw)
To: gcc-help
I'm currently in the process of porting GCC and am trying to optimise my code
for function prologues and epilogues.
To minimise the amount of code output, I would like to check to see if the
current function is declared with the __noreturn__ attribute. If it is, then
I don't need to do output any of the function return code or save/restore any
registers that otherwise would be.
Is there any way I can find out whether the current function has been declared
with this attribute? (Similarly, for other attributes, such as interrupt). This
would be in the C support code, not in the machine description (.md) file. Is
there a GCC support routine I can call, or is it more involved?
Thanks in advance for any help.
Paul
==============================================================================
AT&T Laboratories Cambridge
------------------------------------------------------------------------------
Dr. Paul Webster, 24a Trumpington Street, Cambridge, CB2 1QA, England.
Tel: +44 (0)1223 343 218 Fax: +44 (0)1223 313542
==============================================================================
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2000-04-01 0:00 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2000-02-02 11:56 Functions using attribute `noreturn' Ralf Guetlein
2000-04-01 0:00 ` Ralf Guetlein
-- strict thread matches above, loose matches on Subject: below --
2000-02-02 9:08 Paul Webster
2000-02-02 14:23 ` Jeffrey A Law
2000-04-01 0:00 ` Jeffrey A Law
2000-04-01 0:00 ` Paul Webster
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).