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.
Christoph
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