[LLVMdev] Landing my new development on the trunk ...

[Eli Friedman]

On Fri, Oct 29, 2010 at 11:18 AM, Brian West <bnwest at rice.edu> wrote:
> Eli Friedman <eli.friedman <at> gmail.com> writes:
>> >> > I did not mention in the original email (and should have) that OSR
>> >> > needs
>> >> > -instcombine to be run after it for cleanup. Also -licm,
>> >> > -reassociate, -gvn
>> >> > and -sccp can be enabling optimizations for OSR.
>> >>
>> >> Hmm... perhaps that could be partially fixed using the InstSimplify
>> >> infrastructure.
>> >
>> > OSR can produce induction variables that are not used.  -instcombine
>> > finds the
>> > unused induction variables and deletes them (which is super cool btw
>> > :)).  I am
>> > unsure if InstructionSimplify can fix that.
>>
>> Oh... no, you would need instcombine for that.  However, how hard
>> would it be to just avoid creating unused induction variables?
>> instcombine doesn't normally run after LSR.
>
>
> Short answer is no.
>
> The OSR adds new induction variables based on existing variables.  That way,
> we
> can go from induction variable i to induction variable &a[long(i)] (aka, a +
> long(i)).   There may be some intermediate induction variables that assist
> in i
> reaching (a + long(i)), like long(i), e.g.
>
> i -> long(i) -> (a + long(i))
>
> after creating (a + long(i)), induction variable long(i) may be unused,
> depending on whether long(i) can participate in creating an another
> induction
> variable. Thus, it is not known until the algorithm is done, which of the
> created induction variables are unused.

Sure, but you know which induction variables you created; you can just
zap the unused ones at the end of the pass, no?

> FWIW I noticed that other optimizations (as seen in StandardPasses.h) are
> followed by -instcombine for cleanup. I thought requiring or suggesting
> running
> -instcombine after -osr would be SOP.

Strength reduction is sort of a special case; it runs extremely late
because it interferes with other optimizations.

> FWIW2 our intent in creating OSR was not to replace LSR, just provide an
> alternative strength reduction optimization.

Sure, but if it can't be used as an alternative to LSR, how would it be used?

-Eli