[LLVMdev] Proposal to merge SimplifyLibCalls into InstCombiner

[Meador Inge]

On 08/02/2012 11:11 AM, Chris Lattner wrote:

>> A new self-contained `LibCallSimplifier` class will be created.  An instance
>> of the class will be instantiated when running the `InstCombiner` pass.  It's
>> folding functionality will be invoked from `InstCombiner::tryOptimizeCall` and
>> the implementation will be table-driven like `SimplifyLibCalls`.  All of the
>> existing fortified library call simplifiers will be migrated to this new class
>> at this step.
> 
> Ok, the idea is that other general parts of the compiler can use the LibCallSimplifier
> interface as well?  That makes sense to me.

Yup, that's it.

>> An option for enabling/disabling library call simplification in `InstCombiner`
>> will be available.  For backwards compatibility perhaps it should remain
>> '-simplify-libcalls'.  The `NumSimplified` and `NumAnnotated` statistics shall
>> be added to `InstCombiner`.
> 
> There is no need to preserver the -simplify-libcalls flag.  It's an internal
> implementation detail of the compiler, one that is better left in the past :)

Do you think we still need some way to disable just the library call
simplification now that it is a part of the instruction combiner?

>> 1. What should be done about the `SimplifyFortifiedLibCalls` use in
>>   `CodeGenPrepare`?
> 
> I think it stays where it is.  CGP runs right before the code generator
> kicks in.  The idea here is that we want to leave the fortified libcalls
> in the IR as long as possible on the off chance that instcombine or something
> else can expose range information about underlying buffers... however, if we
> get all the way to code gen, then we failed to find a length, so we remove them.

Thanks for the explanation.  As a part of the merger I really want
`SimplifyFortifiedLibCalls` to go away as well, since the fortified
library call folding is being rolled into `LibCallSimplifier`.  I will
look at applying `LibCallSimplifier` to the CGP case as well (the conditions
for when it is OK to fold is slightly different for CGP).

Thanks for the feedback!

-- 
Meador Inge
CodeSourcery / Mentor Embedded
http://www.mentor.com/embedded-software