[LLVMdev] load bytecode from string for jiting problem

[Will Dietz]

On Thu, Mar 13, 2014 at 9:02 AM, Willy WOLFF <willy.wolff at etu.unistra.fr> wrote:
> Hello,
>
> I having a weird problem while writing a bytecode module to a string,
> and after read/parse it for unsing on a jit.
>
> I write a pass to export function to module, and put this module inside
> a global variable.
> I use WriteBitcodeToFile for this.
> For debuging, after this write, I try to load the exported module with
> parseBitcodeFile.
> This two step works.
>
>
>
> After, while the compiled program is running, I try to read and parse
> this global variable for jiting the function.
>
> 1) I read the global variable with
>   StringRef sr (gv, gv_length);
>
> 2) I manually test this bytecode by
> (inspired by  inline bool isRawBitcode(const unsigned char *BufPtr,
> const unsigned char *BufEnd) at
> http://llvm.org/docs/doxygen/html/ReaderWriter_8h_source.html#l00067)
>   if (sr.str()[0] == 'B')
>     std::cout << "B ok\n";
>   if (sr.str()[1] == 'C')
>     std::cout << "C ok\n";
>   if (sr.str()[2] == (char) 0xc0)
>     std::cout << "0xc0 ok\n";
>   if (sr.str()[3] == (char) 0xde)
>     std::cout << "0xde ok\n";
>
> 3) I try to parse the gv by
>   MemoryBuffer* mbjit = MemoryBuffer::getMemBuffer (sr.str());

Not sure if this is your issue, but should be fixed anyway:

The std::string created by "sr.str()" ends its lifetime in this
statement, and MemoryBuffer for efficiency reasons
avoids copying data it doesn't have to (like StringRef) so will be
referencing the freed memory.

To resolve this:
* Pass MemoryBuffer your StringRef directly
* Use getMemBufferCopy()
* Preserve the result of sr.str() into a stack variable and pass that
to getMemoryBuffer() instead.

As a final note, check if your bitcode buffer "string" is
null-terminated or not.  If not, be sure to be careful and
do things like informing MemoryBuffer that this is the case.

Hope this helps,
~Will

>   LLVMContext& context = getGlobalContext();
>   ErrorOr<Module*> ModuleOrErr = parseBitcodeFile (mbjit, context);
>   if (error_code EC = ModuleOrErr.getError())
>   {
>     std::cout << ModuleOrErr.getError().message() << "\n";
>     assert(false);
>   }
>
>
>
>
> This is the execution result:
> B ok
> C ok
> 0xc0 ok
> 0xde ok
> Invalid bitcode signature
>
>
>
> Ok is not working :/
> But why ???
>
>
>
> For debuging, between 2) and 3), I export the readed module and write to
> a file on my hard drive,
> and try llvm-dis, and the dissasembly of the module works.
>
> Wath's wrong? Any idea for solve this problem?
>
> Thanks you very much.
>
> Regards,
> Willy
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