Date: Wed, 27 Feb 2013 22:38:34 +0100 From: Christoph Mallon <christoph.mallon@gmx.de> To: Pawel Jakub Dawidek <pjd@FreeBSD.org> Cc: =?UTF-8?B?RGFnLUVybGluZyBTbcO4cmdyYXY=?= <des@des.no>, freebsd-arch@FreeBSD.org Subject: Re: Large Capsicum patch for review. Message-ID: <512E7CDA.2020206@gmx.de> In-Reply-To: <20130226121345.GB1341@garage.freebsd.pl> References: <20130213025547.GA2025@garage.freebsd.pl> <20130213230221.GB1375@garage.freebsd.pl> <20130223221116.GR1377@garage.freebsd.pl> <5129ADC5.5040306@gmx.de> <512A2CA0.2050509@gmx.de> <20130224235936.GX1377@garage.freebsd.pl> <512B3DBB.4080909@gmx.de> <20130225215004.GA1375@garage.freebsd.pl> <867glvbbjz.fsf@ds4.des.no> <20130226121345.GB1341@garage.freebsd.pl>
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On 26.02.2013 13:13, Pawel Jakub Dawidek wrote: > On Tue, Feb 26, 2013 at 11:43:12AM +0100, Dag-Erling Smørgrav wrote: >> Pawel Jakub Dawidek <pjd@FreeBSD.org> writes: >>> I use size_t as this is preferred type for size, but I don't need size_t >>> for iterator, because I know the value will never need more than 16 >>> bits, so I use int as a more CPU-friendly type. >> >> Using int as an iterator can lead to warnings about signedness mismatch >> in loop conditions etc., so you should at the very least use unsigned >> int. [...] > > I use 'int' for 'ssize_t' and 'unsigned int' for 'size_t'. > >> [...] Besides, size_t is equal to unsigned long on all platforms, so >> "CPU-friendly" is not a valid argument. > > Is long more CPU-friendly than int? Interestingly, yes: 32 bit ints or uints are stored in the lower half of a 64 bit register on AMD64. All 32 bit operations automatically zero extend their result to 64 bits, i.e. the upper half is set to 0. Comparing an uint with size_t can therefore done directly with a 64 bit comparison. But to compare an int with an ssize_t, the int value has to be sign extended to 64 bits. This takes another instruction (movsx) and an extra register to temporarily hold the sign-extended value. On MIPS it's the other way round: 32 bit operations sign extend their result to 64 bits. So comparing int and ssize_t can be done directly and uint+size_t costs another instruction. In summary, mismatched types make the code harder to read and have the potential to force the compiler to produce worse code. Christophhome | help
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