[llvm-dev] Debugify and Verify-each mode

[Vedant Kumar via llvm-dev]

Hi Son Tuan,

Thanks for taking a look at this :). Responses inline --

> On Mar 14, 2018, at 8:11 AM, Son Tuan VU <sontuan.vu119 at gmail.com> wrote:
> 
> Hi Vedant, hi all,
> 
> My goal is to measure debug info loss of each optimization pass in LLVM. I am trying to create a debugify-each mode in opt, inspired by verify-each mode which is supposed to already work.

+ Anastasis, who's interested in working on this as well. There's definitely enough work to go around: once we can measure debug info loss after each pass, we'll need a testing harness.


> However, if I understand correctly, the verify-each mode (triggered by -verify-each option in opt) only works when we provide a pass list or a pass pipeline.

Yes, you're correct.


> Is this intended? I mean, why do not let people verify each pass in -O{1,2,3} pipeline?

That's a good question! Like you, I assumed -verify-each "does the right thing" when you pass -O1/-O2/etc. to opt.

I'm not sure if the current behavior is intended (hopefully others will chime in about this :). If no one does, please file a bug.

I imagine this is pretty simple to fix. You can just define and use custom pass managers within opt which inject debugify passes as needed:

>> // In opt.cpp:
>> 
>> class DebugifyEachFunctionPassManager : public legacy::FunctionPassManager {
>> public:
>>   explicit DebugifyEachFunctionPassManager(Module *M)
>>       : FunctionPassManager(M) {}
>> 
>>   void add(Pass *P) override {
>>     // FunctionPassManager::add(<debugify>)
>>     FunctionPassManager::add(P);
>>   }
>> };


> My second question is more about debugify: what should be the best way to debugify each pass? Adding a debugify-each mode would make the output unreadable!

The intermediate output is all irrelevant. I think it'd be best to simply throw it away. What really matters are the debug info loss statistics: we should capture these stats after each pass and dump them as JSON, at the end of the pipeline.

vedant


> Maybe writing a script that collects all optimization options (like -mem2reg or -constmerge), then pass each one of them to opt with -enable-debugify so that we have 1 output file for each debugified pass?
> 
> Thank you for your help,
> 
> Son Tuan Vu

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180314/5459d9ae/attachment.html>