[llvm-dev] Suggestion / Help regarding new calling convention

John Criswell via llvm-dev
Tue, 21 Jun 2016 20:56:22 -0000

On 6/21/16 11:27 AM, vivek pandya wrote:
>
>
> On Tue, Jun 21, 2016 at 8:58 PM, John Criswell <jtcriswel at gmail.com
> <mailto:jtcriswel at gmail.com>> wrote:
>
>     On 6/20/16 11:29 PM, Mehdi Amini wrote:
>>
>>>     On Jun 20, 2016, at 11:12 AM, John Criswell via llvm-dev
>>>     <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>>>
>>>     On 6/20/16 9:39 AM, vivek pandya via llvm-dev wrote:
>>>>     Dear Community,
>>>>
>>>>     To improve current interprocedural register allocation
(IPRA) ,
>>>>     we have planned to set callee saved registers to none for
local
>>>>     functions, currently I am doing it in following way:
>>>>
>>>>     if (F->hasLocalLinkage() &&
!F->hasAddressTaken()) {
>>>
>>>     As an aside, you might want to analyze how many functions have
>>>     both local linkage and are not address taken.  I recall that
>>>     many functions returned false for hasAddressTaken() because
some
>>>     direct calls casted the function to a different function type
>>>     before calling it.  Such functions are still not address taken,
>>>     but the simple hasAddressTaken() method can't determine it.
>>
>>     Looks like hasAddressTaken could be updated to handle these
>>     simple case maybe?
>
>     That might make sense if it has not been fixed already.  Another
>     approach (if in-tree LLVM passes are frequently checking for
>     indirect calls) would be to write a simple analysis pass that
>     lazily computes the information on demand.  That way, if multiple
>     passes are checking the same function repeatedly, it gets cached
>     in the analysis pass instead of being recomputed (so long as the
>     analysis pass is not invalidated by a transform).
>
> Addition of new pass will require other passes to be modified. So it
> will be good have strong reason for adding new pass. Other wise I am
> in favor to modify hasAddressTaken().

Addition of a new pass means that existing passes would not get the
benefit of the new pass until they are updated to use it.  You would not
need to update existing passes though it would probably make sense to do
so if using a separate analysis pass is better.

Also keep in mind that the two approaches are not mutually exclusive.
Updating hasAddressTaken() to take casting into account would probably
not increase its run-time much.  The point of using a pass is to cache
the results of hasAddressTaken(); the pass makes sense if the
hasAddressTaken() queries are being performed repeatedly on the same
functions by multiple passes.

Regards,

John Criswell

>
> -Vivek
>
>     Regards,
>
>     John Criswell
>
>
>     --
>     John Criswell
>     Assistant Professor
>     Department of Computer Science, University of Rochester
>     http://www.cs.rochester.edu/u/criswell
>
>


--
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

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