micro-optimizing loops (was Help with casts)

[Tim Roberts]

In article <10250 at dog.ee.lbl.gov> torek at elf.ee.lbl.gov (Chris Torek) writes:
>(with any luck this will die its own death after this...)

Sorry.  I just had to comment.

In article <344 at smds.UUCP> rh at smds.UUCP (Richard Harter) writes:
>For reasons that are not clear to me many optimizing compilers will not
>collapse the two machine instructions
>		dec r1
>		bge 1$
>into the available single instruction to do the same thing.  Perhaps
>some of our compiler writers can explain this to us.

>From the Intel i486 Programmers Reference Manual and the Department of
Counterintuitive Advice:

  o The LOOP instruction takes two more clock cycles to execute than the
    equivalent decrement and conditional jump instructions.

Even more strange:

  o The JECXZ (jump on ECX=0) instruction takes one more clock cycle to 
    execute than the equivalent compare and conditional jump instructions.

-- 
timr at gssc.gss.com	Tim N Roberts, CCP	Graphic Software Systems
						Beaverton, OR

This is a very long palindrome. .emordnilap gnol yrev a si sihT