llvm dev - Mar 2013 - [LLVMdev] Convert C variable to LLVM IR Variable

Hi everyone,

  I am doing some work with LLVM IR, I need to use LLVM IR to do operation
on C variables.

Code emission is done by LLVM JIT.

That variable is C thread local , for example

__thread int* gvar;


I think some methods to convert that variable to LLVM IR, 

(1) use external function

I know LLVM IR is able to call an external function, so I can write codes
that look like:

int* load()
{
  return gvar;
}

%x = call load(); // LLVM IR

so gvar is converted to %x


This method takes a lot of work and slow down the performance.

Method 2

use LLVM InlineAsm to load that gvar 
so It may look like
%x = call InlineAsm(... );

but I am not really sure how to write LLVM InlineAsm.



The reason why I take such indirect method is that as I know, JIT is not
able to

do with LLVM GlobalVariable with ThreadLocal type.


I would be grateful to you for any idea on handling this problem


Have A Nice Day

Chia Lun Liu




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> Method 2
> 
> use LLVM InlineAsm to load that gvar 
> so It may look like
> %x = call InlineAsm(... );
> 
> but I am not really sure how to write LLVM InlineAsm.
  Are you looking for this?
  http://llvm.org/docs/LangRef.html#inline-assembler-expressions

HTH,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667
Homepage: http://people.cs.nctu.edu.tw/~chenwj

Hi Chia Lun Liu,

 > The reason why I take such indirect method is that as I know, JIT is
not
> able to
>
> do with LLVM GlobalVariable with ThreadLocal type.
did you try mcjit?  Run lli with -use-mcjit

Ciao, Duncan.

Hi Wei-Ren Chen,
      Thanks for your information, I also find a method to write LLVM
InlineAsm

First, write a c program  which contain gcc asm  ( I know how to write gcc
asm )

then use clang -S -emit-llvm $prog 

next do llc -march=cpp $prog.s 

You will see how to write LLVM InlineAsm in the output file by llc

Chia Lun Liu



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Hi Ciao, Duncan

   I had tested your method before but fail. I did it  today but fail again.


I write a simple c program

#include<stdio.h>

__thread int g[128];

int main()
{
        int i;
        for(i = 0 ; i< 128 ; i++)
        {       g[i] = 2*i;
          printf("%d\n",g[i]);
        }
        return 0;
}

-------------
I turn it to LLVM IR by
clang -S -emit-llvm $file
then use lli -use-mcjit $file.s

the following is my output



 :~/Desktop/program$ lli threadLocal.s  -use-mcjit
Cannot allocate thread local storage on this arch!
UNREACHABLE executed at
/home/bendog/Desktop/llvm-3.2.src/lib/Target/X86/X86JITInfo.cpp:585!
0  lli             0x0000000000d14acf
1  lli             0x0000000000d14fe9
2  libpthread.so.0 0x00007fae25fc9cb0
3  libc.so.6       0x00007fae25218425 gsignal + 53
4  libc.so.6       0x00007fae2521bb8b abort + 379
5  lli             0x0000000000d00fbc
6  lli             0x00000000006176ec
7  lli             0x0000000000845747
llvm::JIT::getMemoryForGV(llvm::GlobalVariable const*) + 343
8  lli             0x00000000008471ed
llvm::JIT::getOrEmitGlobalVariable(llvm::GlobalVariable const*) + 333
9  lli             0x0000000000850739
10 lli             0x000000000085222b
11 lli             0x000000000057c151
12 lli             0x0000000000cadd1f
llvm::FPPassManager::runOnFunction(llvm::Function&) + 607
13 lli             0x0000000000cb134b
llvm::FunctionPassManagerImpl::run(llvm::Function&) + 139
14 lli             0x0000000000cb14c9
llvm::FunctionPassManager::run(llvm::Function&) + 105
15 lli             0x0000000000847647
llvm::JIT::jitTheFunction(llvm::Function*, llvm::MutexGuard const&) + 39
16 lli             0x0000000000847cac
llvm::JIT::runJITOnFunctionUnlocked(llvm::Function*, llvm::MutexGuard
const&) + 28
17 lli             0x0000000000847e69
llvm::JIT::getPointerToFunction(llvm::Function*) + 265
18 lli             0x000000000050b71a main + 3210
19 libc.so.6       0x00007fae2520376d __libc_start_main + 237
20 lli             0x0000000000512d09
Stack dump:
0.	Program arguments: lli threadLocal.s -use-mcjit 
1.	Running pass 'X86 Machine Code Emitter' on function '@main'
Aborted (core dumped)


As can seen from the result, I use llvm 3.2, and my machine is x86_64 , OS :
Ubuntu 12.xx


Chia Lun Liu




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