Author: Robert Hyatt
Date: 08:58:46 03/28/04
Go up one level in this thread
On March 27, 2004 at 20:12:27, Slater Wold wrote: >On March 27, 2004 at 17:11:31, Robert Hyatt wrote: > >>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... > >Because it does not apply 100% of the time. There are times when using a signed >int will be faster. > >Generally speaking, using an unsigned int is faster. But not always. I am talking about one specific case: Using a signed anything (less than 64 bits) as an array index. It will never be faster there, because anything less than 64 bits has to be extended to 64 bits as all addresses are 64 bits on that processor in 64 bit mode... I can't imagine any case where unsigned is slower, period, however... The same instruction is used...
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