[llvm-dev] [RFC] Interprocedural MIR-level outlining pass

[Violeta Vukobrat via llvm-dev]

Hi Jessica,

Thank you for your quick reply.

I have a few more questions.

1. In the results section of your original post, I suppose that by 'llvm 
test suite' you meant the SingleSource, MultiSource tests. Is that correct?

2. I am trying to run these tests with the -mno-red-zone option, however 
I am running into issues, and I hope that you can help me.

2.1. The first problem is that I get a segfault when trying to e.g. 
create an .o file. Example of building a test from the test-suite:

clang++ 
-I<path-to-results-dir>/MultiSource/Benchmarks/Prolangs-C++/simul 
-I<path-to-test-suite>/MultiSource/Benchmarks/Prolangs-C++/simul 
-I<path-to-test-suite>/include -I../../../../include -D_GNU_SOURCE 
-D__STDC_LIMIT_MACROS -DNDEBUG -DSMALL_PROBLEM_SIZE -O3 -mno-red-zone 
-mllvm -enable-machine-outliner -m64 -fomit-frame-pointer -c 
<path-to-test-suite>/MultiSource/Benchmarks/Prolangs-C++/simul/simulate.cpp 
-o simulate.o

The problem occurs in AsmPrinter::EmitFunctionBody(), during running 
'X86 Assembly / Object Emitter' pass on an outlined function.
I haven't found what causes it, but the problematic bit seems to be the 
value of MI in EmitFunctionBody() in 
lib/CodeGen/AsmPrinter/AsmPrinter.cpp around line 854:
  ....

     // Print a label for the basic block.
     EmitBasicBlockStart(MBB);
     for (auto &MI : MBB) {    <---- line 854

       // Print the assembly for the instruction.
       if (!MI.isPosition() && !MI.isImplicitDef() && !MI.isKill() &&
           !MI.isDebugValue()) {
....

I haven't debugged this any further, so I don't really know what is the 
cause of the problem here.
What makes it strange is that this segfault doesn't occur always (but 
does majority of the times), and also, it doesn't always happen for the 
same outlined function.
I was wondering if you have come across a similar issue?


2.2. When I try to build a simple example with '-mno-red-zone' and 
'-enable-machine-outliner' options, I get an 'undefined reference' to an 
outlined function, e.g. "undefined reference to `l_OUTLINED_FUNCTION0'". 
The .s file shows that the call that gets inserted is a call to 
`l_OUTLINED_FUNCTION0', and the outlined function has the label with a 
different prefix - `.LOUTLINED_FUNCTION0'.
Did you have the same problem?

Note: I have cloned your repository from 
https://github.com/ornata/llvm.git and for clang, I used commit 
aa5fc8f0161578a00cb5e61a19cb6c6429ff85e3.


I hope that you can help me resolve these issues, or tell me if I am 
doing something wrong here. :)

Violeta


On 29.09.2016. 09:38, Jessica Paquette wrote:
> Hi Violeta,
>
> I compiled with
>
> clang -Oz and
> clang -Oz -mno-red-zone for comparisons against Oz
>
> and
>
> clang -O0 and
> clang -O0 -mno-red-zone for comparisons against a default clang.
>
> I unfortunately don’t have the clang commit I worked with on my home laptop, and don’t have access to the computer I was using at Apple, so I can’t help you there.
>
>
> Jessica
>
>> On Sep 28, 2016, at 11:22 AM, Violeta Vukobrat <violeta.vukobrat at rt-rk.com> wrote:
>>
>> Hi Jessica,
>>
>>
>> I am looking into trying out your patch and reproducing results that you got for code size reduction. Could you tell me what compiler options you used? Also, which clang commit did you work with?
>>
>>
>> Violeta
>>
>> -- 
>> Violeta Vukobrat
>> Software Engineer
>> RT-RK Computer Based Systems LLC
>> www.rt-rk.com

-- 
Violeta Vukobrat
Software Engineer
RT-RK Computer Based Systems LLC
www.rt-rk.com