[llvm-dev] Debugify and Verify-each mode

[Son Tuan VU via llvm-dev]

Hi Vedant,

Thank you for your reply. I think I can make this debugify-each mode, but I
guess this is reserved for your GSoC project ?

However, if I understand correctly, we do not want to take the output of
the first check-debugify (I mean the .ll file with potentially all the
WARNINGs and ERRORs after the first pass) as input for the second debugify.
What we need is to take the fresh output of *clang -Xclang -emit-llvm
-disable-O0-optnone -S *and iteratively test each optimization. Am I right?

Cheers,


Son Tuan Vu

On Thu, Mar 15, 2018 at 4:05 AM, Vedant Kumar <vsk at apple.com> wrote:

> 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
>
>
>
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