bugAVR C Runtime Library - Bugs: bug #35496, _FFS: Macro should be obsoleted/no...


bug #35496: _FFS: Macro should be obsoleted/no more recommended

Submitted by:  Georg-Johann Lay <gjlayde>
Submitted on:  Thu 09 Feb 2012 09:43:52 PM UTC  
Category: LibrarySeverity: 3 - Normal
Priority: 9 - ImmediateItem Group: libc code
Status: Wont FixPercent Complete: 0%
Assigned to: NoneOpen/Closed: Closed
Release: 1.8.0Fixed Release: None

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Fri 16 Nov 2012 08:57:21 PM UTC, comment #2:

GCC's optimization for ffs depends on -std=, and users will use ffs and not __builtin_ffs.

I'd say close this issue

...because it is tedious to work out in configure whether or not avr-gcc comes with optimized ffs insns and if it's appropriate that libc maps ffs to __builtin_ffs or not.

Georg-Johann Lay <gjlayde>
Fri 16 Nov 2012 06:44:09 PM UTC, comment #1:

Johann, which file needs to be changed?

Eric Weddington <arcanum>
Project Administrator
Thu 09 Feb 2012 09:43:52 PM UTC, original submission:

GCC's builtin machinery can fold ffs* with constant arguments.

If a no-constant is passed to ffs resp. ffsl then avr-gcc provides efficient assembler implementations, see __ffshi2 and __ffssi2 in libgcc:


Notice that avr-gcc is aware of the reduces register footprint of these libgcc functions.

Georg-Johann Lay <gjlayde>


(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

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    Follow 3 latest changes.

    Date Changed By Updated Field Previous Value => Replaced By
    Fri 16 Nov 2012 09:11:19 PM UTCarcanumStatusNone=>Wont Fix
    Fri 16 Nov 2012 06:44:09 PM UTCarcanumPriority5 - Normal=>9 - Immediate

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