[llvm-dev] Pass manager extensions (follow-up from another thread)

[Krzysztof Parzyszek via llvm-dev]

Hello,
In the thread about target-specific AA, Hal proposed some ideas 
regarding the interface connecting target-specific functionality with 
the target-independent pass manager. His post did not get any replies on 
the list, but I think it's interesting enough to get a second look. The 
copy of it is at the bottom of this email.

I am particularly interested in the possibility of adding 
target-specific passes to the optimization pipeline via the extension 
mechanism.

Does anyone have any comments or suggestions (or objections?) regarding 
the approach described below?

-Krzysztof


On 8/12/2015 4:53 PM, Hal Finkel via llvm-dev wrote:
 >
 > I've cc'd Chandler so he can comment here.
 >
 > Currently, TargetMachine can optionally return a TargetIRAnalysis 
object; this is then provided a function for which it can produce the 
actual TargetTransformInfo-derived object. 
TargetTransformInfoWrapperPass is the actual pass, and it just holds a 
pointer to the TargetIRAnalysis object.
 >
 > In our case, we'd like to provide the target a way to add some AA 
pass to take advantage of target-specific knowledge regarding address 
spaces, intrinsics, and other IR constructs whose meaning is completely 
target specific. I'm not sure how well it fits into this model.
 >
 > In addition, we have other use cases where targets want to add passes 
to handle target-specific things into the pipeline 
(http://reviews.llvm.org/D11782, for example, for doing target-specific 
loop-idiom recognition).
 >
 > Plus, we already have a mechanism for allowing extension of the 
optimization pipeline, and I think the best way of approaching this 
(which is also useful for other use cases), is to provide the targets 
the ability to use this mechanism. I understand that this could be ripe 
for abuse (so we'd need to be vigilant in watching the targets), but 
nevertheless, how about this:
 >
 >   1. Add a function to TargetMachine called, for example, 
registerPassManagerBuilderExtensions(PassManagerBuilder *PMB). This 
would need to be called, for example, by the code in EmitAssemblyHelper 
in Clang in tools/clang/lib/CodeGen/BackendUtil.cpp (just as it 
currently calls TM->getTargetIRAnalysis() and so that it can call 
createTargetTransformInfoWrapperPass) and also registers other 
extensions itself.
 >
 >   2. registerPassManagerExtensions's job will be to call 
PassManagerBuilder::addExtension (or addGlobalExtension) as appropriate.
 >
 >   3. Create a new extension point for AA passes, and call 
addExtensionsToPM inside PassManagerBuilder::addInitialAliasAnalysisPasses
 >
 > This will automatically cover a wide variety of use cases, including 
this one. Thoughts?
 >
 >   -Hal


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