Author: Slater Wold
Date: 13:09:31 03/26/04
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On March 26, 2004 at 12:09:26, Russell Reagan wrote: >On March 26, 2004 at 09:15:12, Fabien Letouzey wrote: > >>Would there be a reason why, for instance, 16-bit integers would be slower in >>64-bit mode? > >Software Optimization Guide for AMD Athlon™ 64 and AMD Opteron™ > >2.22 Using Unsigned Integers for 32-Bit Array Indices > >Optimization >When using a 32-bit variable as an array index, declare the variable as an >unsigned integer instead of a signed integer. > >Application >This optimization applies to 64-bit software. > >Rationale >When performing 64-bit address arithmetic, the compiler must insert an >additional instruction into the object code to sign-extend a signed 32-bit >integer, which reduces performance; no additional instruction is necessary to >zero-extend an unsigned 32-bit integer because the processor performs >zero-extension automatically. (There is no performance penalty for using 64-bit >variables—either signed or unsigned—as array indices.) This is not factual, at all. And it was removed from the Guide.
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