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SubjectRe: memcpy_toio on i386 using byte writes even when n%2==0
linux-os (Dick Johnson) wrote:
> If byte writes are used, they should always be last for any
> odd byte. I think you found a bug in spite of the fact that
> whoever made the revision to memcpy probably thinks they
> did something 'cool'. This is an example of cute code causing
> problems. The classic example of a proper memcpy() that uses
> the ix86 built-in macros runs like this:
>
> pushl %esi # Save precious registers
> pushl %edi
> movl COUNT(%esp),%ecx
> movl SOURCE(%esp),%esi
> movl DEST(%esp),%edi
> cld
> shrl $1,%ecx # Make WORDS, possibly set carry
> rep movsw # Copy the words
> adcl %ecx,%ecx # Any spare byte
> rep movsb # Copy any spare byte
> popl %edi # Restore precious registers
> popl %esi
>
> Note that there isn't any code for moving dwords because the
> chances of gaining anything are slim (alignment may hurt).

I'd say the chances of gaining something from executing half as many
instructions on copying a large block of memory are very good indeed..

--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

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