[llvm-dev] ModulePass cannot be registered as EarlyAsPossible

Son Tuan VU via llvm-dev llvm-dev at lists.llvm.org
Wed Jun 13 07:07:52 PDT 2018

Hi Bevin,

Thank you for your reply. Unfortunately, I was using
EP_ModuleOptimizerEarly and I just realized that it is not early enough,
since the FunctionPassManager does apply some basic optimization on the
functions before any Module optimizations...

So there's no way to insert a Module pass earlier than
ModuleOptimizerEarly? I will try to hack it as Björn pointed out then.

Thanks for your help,

Son Tuan Vu

On Wed, Jun 13, 2018 at 4:01 PM, Bevin Hansson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
> EP_EarlyAsPossible only works with FunctionPasses, yes. If you look at how
> it's used in PassManagerBuilder, it is only invoked from
> populateFunctionPassManager.
> The earliest you can add ModulePasses is with EP_ModuleOptimizerEarly.
> However, those passes are not added on O0. If the OptLevel is 0, you can
> instead add the passes to EP_EnabledOnOptLevel0.
> This might not be early enough for you, but those are the points that are
> available for module passes as far as I know.
> Regards,
> Bevin
> On 2018-06-12 15:46, Son Tuan VU via llvm-dev wrote:
> Hello all,
> I've followed the example in https://github.com/
> CompilerTeaching/SimplePass/blob/master/SimplePass.cc in order to create
> a custom pass.
> The pass needs to be added before any transformation, so I used
> EP_EarlyAsPossible extension point to register it.  Furthermore, I need to
> access to every GlobalVariable in the IR, so my pass has to be a
> ModulePass, like this:
>   struct MyPass : public ModulePass {
>      static char ID;
>      MyPass(): ModulePass(ID) {}
>      virtual bool runOnModule(Module &M) {...}
>   ...
>   }
> However, every time I try to access to the Module object M inside
> runOnModule(), clang just crashes.  Even a debug message like *outs() <<
> M.getName() << '\n';* would cause a segfault. So am I doing something
> wrong, like EP_EarlyAsPossible is really not to be used with ModulePasses,
> or is this rather a bug?
> In case this is not a bug, what would be the best way to manipulate an IR
> Module as it is coming right out of the frontend?
> Thanks for your help,
> Son Tuan Vu
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