cvs commit: src/include _ctype.h

Christoph Mallon christoph.mallon at
Mon Oct 29 14:15:59 PDT 2007

Andrey A. Chernov wrote:
> ache        2007-10-27 22:32:28 UTC
>   FreeBSD src repository
>   Modified files:
>     include              _ctype.h 
>   Log:
>   Micro-optimization of prev. commit, change
>   (_c < 0 || _c >= 128) to (_c & ~0x7F)
>   Revision  Changes    Path
>   1.33      +1 -1      src/include/_ctype.h

Actually this is rather a micro-pessimisation. Every compiler worth its 
money transforms the range check into single unsigned comparison. The 
latter test on the other hand on x86 gets probably transformed into a 
test instruction. This instruction has no form with sign extended 8bit 
immediate, but only with 32bit immediate. This results in a 
significantly longer opcode (three bytes more) than a single 
(unsigned)_c > 127, which a sane compiler produces. I suspect some RISC 
machines need one more instruction for the "micro-optimised" code, too.
In theory GCC could transform the _c & ~0x7F back into a (unsigned)_c > 
127, but it does not do this (the only compiler I found, which does this 
transformation, is LLVM).
Further IMO it is hard to decipher what _c & ~0x7F is supposed to do.


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