Re: Patch to implement rotates by zero

[cgd at broadcom dot com]

At Sat, 17 Aug 2002 17:10:55 +0000 (UTC), "Richard Sandiford" wrote:
> Noticed while reading the code that:
> 
>         ror $2,$4,0
> 
> expands to nothing at all.  And the testsuite actually requires this
> behaviour ;).
> 
> This patch implements double-rotates by zero using "dsll", so that they
> trap in the same circumstances that non-zero rotates would.

specifically, you mean, so they trap when 64-bit operations aren't
enabled?

This seems in one way desirable, but in another ... not.  if you're
generating a macro, why generate one which is in fact a no-op?

(of course, then, why emit the no-op instruction to begin with.  8-)


> Likewise
> "rol" and "ror" will use "sll" to give the correct sign extension
> on 64-bit targets.

Uh, this one makes less sense to me.

AFAIK, 'sll' w/ a shift of 0 is only guaranteed to sign-extend by
MIPS64.  I've seen no previous documentation that defined that it must
work that way.

So, I really don't see much point in this except from a "code
generation is the same" POV, and since you're emitting special-case
code there it's still not "code generation is the same".


I dunno, from a consistency standpoint, it would make sense to simply
remove the check for the count being non-zero.  i.e. if you ask for
rol/rot/etc. and get the macro, you always end up with a 3-instruction
sequence being generated.



cgd