RE: FUNCTION_OK_FOR_SIBCALL vs INITIAL_FRAME_POINTER_OFFSET

["Dave Korn" <dave dot korn at artimi dot com>]

On 20 March 2006 15:14, Richard Henderson wrote:

> On Mon, Mar 20, 2006 at 02:56:00PM -0000, Dave Korn wrote:
>>   If the stack frame size is >32kB, I need to use a temporary register in
>> the epilogue to assemble the lo/hi parts of the frame size before adding
>> it to the SP.  In the non-sibcall version of the epilogue[*] it uses one
>> of the arg-passing volatiles as a scratch register, but of course in a
>> sibcall epilogue that register might have been pre-loaded with an argument
>> for the sibcall which we don't want to trash.  So rather than get hairy
>> with trying to allocate scratch regs, I was just going to refuse sibcalls
>> for functions with huge stack frames.  Hence my curiosity.
> 
> Ah, interesting.  In this case I'd deny sibcalls to functions that
> use all of the available scratch registers for arguments.  

  Heh, of course, lateral thinking, attack the conflict from the opposite
direction!

> We do
> something similar for i386 indirect sibcalls -- deny unless there's
> a free register for the jump.

  Mmmf.  In that case, of course, you absolutely /have/ to have a register
available and no two ways about it.  But in the direct-sibcall case, this test
will generate false positives since if the stackframe is small enough I don't
need a register at all and the sibcall should be ok.  And I expect it to be
the case that 99% of all functions /will/ have small stack frames, and so this
will deny every sibcall to a function using the full set of arg passing regs
when they'll almost all be ok.

  So I guess if I want to make the most of the opportunities for sibcalls, I
still need hope that get_frame_size() is valid at F_O_F_S time and make a
best-guess at the size of the frame, and if it goes from < 32k to > 32k by the
time I actually come to emit the sibcall epilogue I'll just have to abort.  Do
you happen to know off the top of your head when get_frame_size() becomes
valid?


    cheers,
      DaveK
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