> From: owner-openssl-...@openssl.org On Behalf Of Florian Weimer > Sent: Monday, 26 August, 2013 10:58 > To: openssl-dev@openssl.org > Cc: PMHager > Subject: Re: UTF8 decoding, unneeded byte masking > > On 08/25/2013 12:23 PM, PMHager wrote: > > If your intention is performance optimization you could even replace > > > > if((*p & 0x80) == 0) > > > > with > > > > if((signed char)(*p) >= 0) > > > > as you cannot assume that all compilers will do it > correctly themselves. > > Actually, this proposed change relies on a GCC extension. If *p is > unsigned, compilers are allowed to assume that (signed > char)(*p) >= 0 is > always true (because signed overflow is undefined). > Not quite. Overflow in signed integer computation is Undefined Behavior, which can be even worse than using the wrong value; in particular sometimes it causes a trap/interrupt/abort/etc. But this case is out-of-range conversion to signed integer -- and that is *implementation-defined*: the compiled code must give either a documented value or a documented signal, but this need not be the same as other systems, and need not be the value you want (and if a nonresumable signal, not any value at all).
The (oldish) versions of gcc I have conveniently to hand do document unsigned-to-signed as the common and intuitive just-use-the-bits, which means the "fix" works. The gcc maintainers could conceivably decide to change, but IME over the years they MOSTLY avoid changing basic things like this that would upset a lot of people. But that's not a real guarantee, and gcc is not the only compiler. Nit: but only if the char types are 8 bits. The Standard allows more (not less) and in that case the conversion from u-char 0x80:0xFF to s-char is guaranteed never negative, so the "fix" always fails. But I don't think OpenSSL supports any platforms with CHAR_BIT > 8; the known ones mentioned on the C newsgroup don't do IP, making libssl pretty useless, though maybe parts of libcrypto could still be workable. ______________________________________________________________________ OpenSSL Project http://www.openssl.org Development Mailing List openssl-dev@openssl.org Automated List Manager majord...@openssl.org
