[cfe-dev] Clang PGO and LLVM PGO commandline interface

[Johan Engelen via cfe-dev]

Hi David and Pirama,     (first sent off-list, but resending to list)
  Thank you for your quick responses. I've now successfully added IR-based
PGO to LDC; it's almost trivial. The only thing needed (on LLVM side) was
```
// For generating pgo info
    builder.EnablePGOInstrGen = true;
    builder.PGOInstrGen = <outputfilename>;
```

```
// For using pgo profile
    builder.PGOInstrUse = <inputfilename>;
````
where `builder` is the PassManagerBuilder.

This works well on Linux and macOS, however for a Windows target triple, it
does not work. The "instrprof" pass fails in that case.
I have attached the failing testcase, that fails with "opt -instrprof
crash.ll":
```
>  opt crash.ll -instrprof

comdat global value has private linkage
[1 x i64]* @__profc_luke
comdat global value has private linkage
[1 x i64]* @__profc_cold
comdat global value has private linkage
[1 x i64]* @__profc_main
comdat global value has private linkage
[2 x i64]* @__profc__Dmain
LLVM ERROR: Broken module found, compilation aborted!
```
I've obtained the IR by compiling the D code with `-print-before-all`, and
crash.ll is the IR before the instrprof pass.

Now, before I file this as a bug, I want to make sure that I didn't do
anything wrong in setting up IR-based PGO. Are there any other steps
needed? FYI: we don't use a special pipeline setup, just the default what
LLVM gives us: https://github.com/ldc-developers/ldc/blob/
2645ae8645c70d4dc01988f85482d1b85f82fc14/gen/optimizer.cpp#L235-L331 .
(somehow "opt -instr-prof-gen" does work for Windows...)

As Pirama suggested, I tested with Clang and there it does work with a
Windows triple. (https://godbolt.org/g/h5CMJx)

Thanks a lot,
Kind regards,
  Johan

On Thu, Dec 28, 2017 at 7:02 AM, Xinliang David Li <xinliangli at gmail.com>
wrote:

> -fprofile-use and -fprofile-instr-use can actually be used interchangeably
> for front-end and IR based PGO.  The difference is that -fprofile-use=<>
> also takes a directory as the argument.
>
>
> David
>
> On Wed, Dec 27, 2017 at 7:46 PM, Pirama Arumuga Nainar via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> https://clang.llvm.org/docs/UsersManual.html#profile-guided-optimization
>> is the documentation for Clang's PGO.  It doesn't explicitly state which
>> flags map to frontend and IR-based instrumentation.
>>
>> * -fprofile-generate and -fprofile-use are for IR-based instrumentation
>> and profile use.
>> * -fprofile-instr-generate and -fprofile-instr-use are for frontend-based
>> instrumentation and profile use.
>> * -fprofile-sample-use is to use sparse profiles from
>> hardware-counter-based profiles.
>>
>>
>>
>> On Wed, Dec 27, 2017 at 2:37 PM, Johan Engelen via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> Hello all,
>>>   A while ago, I implemented frontend-based PGO in LDC (very similar to
>>> Clang's) and now I'd like to add IR-based PGO. I want the commandline
>>> interface to be the same (or very similar) to Clang's, but I am having
>>> trouble finding the current state of Clang's PGO commandline flags (and
>>> their meaning related to Clang/LLVM-IR PGO).
>>>
>>> Can you give me a pointer to where I can find this, or perhaps give me a
>>> brief overview of the current flags and how to select whether to use
>>> frontend-based or IR-based PGO ?
>>>
>>> Thanks a lot,
>>>   Johan
>>>
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>>
>>
>> _______________________________________________
>> cfe-dev mailing list
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>>
>>
>
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