[Bug target/29524] [4.2/4.3 Regression] Too much RAM used: __clz_tab[] linked

["anrp at mit dot edu" <gcc-bugzilla@gcc.gnu.org>]

------- 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
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                 CC|                            |anrp at mit dot edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29524