Author: Robert Hyatt
Date: 14:11:31 03/27/04
Go up one level in this thread
On March 26, 2004 at 16:09:31, Slater Wold wrote: >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. What is not factual. Signed 32 bit indices definitely cause a problem. you can look at the asm output to see why...
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