Author: Matt Taylor
Date: 11:14:32 12/07/02
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On December 06, 2002 at 22:50:40, Walter Faxon wrote: >On December 06, 2002 at 05:33:42, Matt Taylor wrote: > ><snip> >> >>It's also a tad strange that the code loads dl and then copies edx into eax. >It would be more direct to simply store the table value in eax. >> ><snip> > >I get the feeling that, for the compiler in question at least, once it decides >that a register is going to be used as an address or offset, it loses or ignores >its knowledge of the register's bitwise mapping. It preps edx to receive the >byte in dl, uses eax to load dl, then copies the whole thing to eax. If it used >eax to write to al directly, the compiler would think it still needs to mask out >the (already zeroed) rest of eax afterwards. So it does it this way because the >reg-reg copy is faster and the edx prep can be overlapped with other work. >Anyway, that's a possible explanation. One would need detailed knowledge of the >compiler to know for sure. (And don't get mad at the compiler writers; writing >good compilers is hard work!) > >-- Walter Yeah, but the x86 architecture has a movzx instruction for that very purpose. AMD manuals actually advise that it is faster to use movzx than the equivalent sequence... And yeah, I know compiler writing is very difficult. I have actually been working on one for various reasons. The difference is that I have a human optimizer. :-) Actually I was working on an optimizer that takes machine code and produces more optimal machine code (which is what I will spit my compiler output through when it's done). -Matt
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