# Re: [PATCH GIT 0.6] make use of register variables & size_t 2 messages from 2005-04-25 to 2005-04-26. Participants: Bodo Eggert . Thread: https://gitlist.dev/t/308 ## Bodo Eggert , 2005-04-25 17:36 Subject: Re: [PATCH GIT 0.6] make use of register variables & size_t Message-ID: URL: https://gitlist.dev/e/E1DQ7Vg-0003fq-DC%40be1.7eggert.dyndns.org In-Reply-To: <3X9X6-5JP-27@gated-at.bofh.it> ``` Matthias-Christian Ott wrote: > The "git" didn't try store small variables, which aren't referenced, in > the processor registers. It also didn't use the size_t type. I corrected > a C++ style comment too. I have compared functions using 'register' against functions not doing that. Not using register allowed better optimization on my x86. -- The most dangerous thing in the world is a second lieutenant with a map and a compass. ``` ## Bodo Eggert , 2005-04-26 18:18 Subject: Re: [PATCH GIT 0.6] make use of register variables & size_t Message-ID: URL: https://gitlist.dev/e/E1DQUdL-0000rx-P3%40be1.7eggert.dyndns.org In-Reply-To: <3XfpD-21C-11@gated-at.bofh.it> ``` Matthias-Christian Ott wrote: > And if you think "register" variables are outdated, please remove the > CONFIG_REGPARM option from the Kernel source. Register variables are outdated because they hinder the compiler from optimizing by taking away 1/7 of the usable registers on x86. Use six register variables and you're back to a accumulator-machine. Use -O2 or -Os instead, the compiler is smarter than you. The regparm calling convention will change the load value -> store value on stack -> call sequence into load value -> call -> store if needed Obviously the second form can be better optimized than the first one. Therefore it's a gain instead of a loss. If you really want to help the compiler, look for something like repeated pointer dereference or access to global variables and cache them in block- local variables (pointers, int and uint only, even for caching chars). Beware of volatile variables, and don't forget to look at the assembler code. -- The generation of random numbers is too important to be left to chance. ```