llvm dev - Mar 2012 - [LLVMdev] MC JIT on ARM can't generate valid code for external functions call

Hello.

We found the following problem with MC JIT, on ARM   it can't generate
valid code for  instruction "bl <external_function>" like:

bl printf

Because the ELF file in memory generated by MC JIT does not have the
.plt section, but we need to have the following code to be emitted in it:

.plt:00008290                 STR             LR, [SP,#-4]!
.plt:00008294                 LDR             LR, =_GLOBAL_OFFSET_TABLE_
; PIC mode
.plt:00008298                 NOP
.plt:0000829C                 LDR             PC, [LR,#8]!

Also GOT section doesn't exists.

To fix this we need to generate the valid entries in GOT and PLT sections

We propose adding these sections and generating a thunk, same as in the
usual compilation pipeline.

 What is the best way to fix these issues?


---
Kurmangaleev Shamil,

On Tue, Mar 13, 2012 at 05:27:58PM +0400, Shamil Kurmangaleev
wrote:
> Because the ELF file in memory generated by MC JIT does not have the
> .plt section, but we need to have the following code to be emitted in it:
Why do you need it to emit PIC? You know the offsets of all functions it
is calling.

Joerg

Instruction BL in the ARM mode can jump by offset +/- 32 MB only. 


If using absolute offset, we need generate a few additional
instructions same as: 

load offset into register and jump to address
from register, or save LR, and load into PC offset. 

But in both cases
change size of code in the function will lead to possibility invalidate
offsets  

computed in the "ARM constant island placement and branch
shortening pass" and we still need 

in the thunk.  

--- 

Kurmangaleev
Shamil 

 13.03.2012 17:47, Joerg Sonnenberger пишет:  

On Tue, Mar 13,
2012 at 05:27:58PM +0400, Shamil Kurmangaleev wrote:

Because the ELF
file in memory generated by MC JIT does not have the
.plt section, but
we need to have the following code to be emitted in it:

Why do you need
it to emit PIC? You know the offsets of all functions it
is
calling.

Joerg
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There is a patch in progress
(http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120220/137666.html
-- I think it's still just in progress) which refactors the MCJIT dynamic
loading and adds some new features.  I believe that support for external
functions on ARM is one of the features that was added.

-Andy

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Shamil Kurmangaleev
Sent: Tuesday, March 13, 2012 6:28 AM
To: llvmdev at cs.uiuc.edu
Subject: [LLVMdev] MC JIT on ARM can't generate valid code for external
functions call

Hello.

We found the following problem with MC JIT, on ARM   it can't generate
valid code for  instruction "bl <external_function>" like:

bl printf

Because the ELF file in memory generated by MC JIT does not have the .plt
section, but we need to have the following code to be emitted in it:

.plt:00008290                 STR             LR, [SP,#-4]!
.plt:00008294                 LDR             LR, =_GLOBAL_OFFSET_TABLE_
; PIC mode
.plt:00008298                 NOP
.plt:0000829C                 LDR             PC, [LR,#8]!

Also GOT section doesn't exists.

To fix this we need to generate the valid entries in GOT and PLT sections

We propose adding these sections and generating a thunk, same as in the usual
compilation pipeline.

 What is the best way to fix these issues?


---
Kurmangaleev Shamil,

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