------- Comment #10 from anrp at mit dot edu 2007-09-08 21:44 -------
(In reply to comment #9)
> Here's what I see:
>
> The array __clz_tab is used in a macro, count_leading_zeros, which is called
> in
> the function __clzSI2 in libgcc2.c, which (AFAICT) gets compiled to the
> function __clzsi2 and aggregated in libgcc. The __clzsi2 function is called
> from the function clzusi() (in fp-bit.c) which is also included in libgcc. The
> clzusi() function is called from si_to_float() and usi_to_float() (also in
> fp-bit.c and included in libgcc). AFAICT, these two functions are used to
> convert an int or unsigned int to float.
>
> The test case does exactly this type of conversion in main() in comment #5.
> Testing shows that with gcc 4.2.1, and all int-to-float conversions removed,
> that __clz_tab is correctly not linked into the application.
>
> The clzusi() function was created in revision 107345, on Nov 22, 2005:
> http://gcc.gnu.org/viewcvs?view=rev&revision=107345
>
> This seems like it was an intended change. However, it is unfortunate that a
> 256-byte array is used in the count_leading_zeros macro. While using a table
> is
> fast and the size is neglible on larger platforms, using up 256 bytes is very
> significant on the AVR where 4K, 2K or even 1K of RAM is common. What is
> really
> needed is an alternative implementation (non-array) that is perhaps specific
> to
> the AVR.
Here's an untested (I'm going to try to figure out how to get it to build into
the AVR build) function that replaces the definition of clz_tab with a 6
instruction bit of code:
; r2 in, r3 out
; r2 clobbered
; Z, C, N. V clobbered
clz_compute:
ldi r3, 0x09 ; preload output
clc ; clear C (guarentees termination with 8 loops)
clz_compute_loop1:
rol r2 ; push MSB into C
dec r3 ; dec output
brcs clz_end ; if C is set (msb was set), we're done
rjmp clz_compute_loop1 ; otherwise, repeat
clz_end:
--
anrp at mit dot edu changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |anrp at mit dot edu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29524
