Author: Russell Reagan
Date: 09:09:26 03/26/04
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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.)
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