llvm dev - Nov 2006 - [LLVMdev] Byte code portability (was Re: libstdc++ as bytecode, and compiling C++ to C)

On Wed, 22 Nov 2006, [ISO-8859-1] Pertti Kellom�ki
wrote:
> Aside from stuff that depends on system headers, are there any other
> dependencies on the host system? In other words, will llvm-gcc produce
> exactly the same byte code for a given set of source files regardless of
> where compilation takes place? This has obvious implications on
Yes.  Many aspects of the target compiler can leak through.  One trivial 
example is:

int X = sizeof(long);

gcc evaluates 'sizeof' before LLVM gets control, therefore the var will 
get initialized with 8 or 4 depending on the target.

Another example is code like:

#ifdef __POWERPC__
int X = 1;
#else
int X = 0;
#endif

There are many other examples.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

Chris Lattner wrote:
> Many aspects of the target compiler can leak through.
So if one wants to use the LLVM system as a cross compiler, one
has to configure llvm-gcc as a cross compiler? Fair enough, I guess.
> One trivial example is:
> 
> int X = sizeof(long);
So I assume this also means that while getelementptr insulates
llvm byte code from the details of target specific address calculations,
the target back end has to agree with gcc on how much space values
of each data type consume.
-- 
Pertti

On Thu, 2006-11-23 at 16:51 +0200, Pertti Kellomäki
wrote:
> Chris Lattner wrote:
> > Many aspects of the target compiler can leak through.
> 
> So if one wants to use the LLVM system as a cross compiler, one
> has to configure llvm-gcc as a cross compiler? Fair enough, I guess.
Yes.
> 
> > One trivial example is:
> > 
> > int X = sizeof(long);
> 
> So I assume this also means that while getelementptr insulates
> llvm byte code from the details of target specific address calculations,
> the target back end has to agree with gcc on how much space values
> of each data type consume.
They agree through the llvm IR. LLVM integer and floating point types
are fixed in size. The front end compiler must use the appropriate type
to get the size that it wants. The only difference might be in structure
layout as LLVM doesn't support alignment attributes in structures. 

Reid.

Pertti Kellomäki schrieb:
> Chris Lattner wrote:
>> Many aspects of the target compiler can leak through.
> 
> So if one wants to use the LLVM system as a cross compiler, one
> has to configure llvm-gcc as a cross compiler? Fair enough, I guess.
I hope the C backend is still meant to generate portable code though.

Philipp