llvm dev - Mar 2010 - [LLVMdev] Idea for Google Summer Code : C Compiler for EFI Byte Code implement in LLVM

On Mon, Mar 15, 2010 at 10:50 AM, Renato Golin <rengolin at
systemcall.org> wrote:
> On 10 March 2010 16:57, 琬菁楊 <ching1119.cs96 at g2.nctu.edu.tw> wrote:
>> I think the main issue is that EFI C dialect is not ANSI-C compliant:
the
>> size of pointer is determined at the run-time and therefore the layout
of
>> the structure is not static. Does LLVM support this model?
>
> Hi Ching,
>
> The LLVM IR doesn't care about the size of your pointers, and this is
> why you have the 'datalayout' explicit on the object file. I
don't
> know, however, if you can omit the layout definition and leave it for
> run time.
The layout for non-packed structures is, in this sense, left for
runtime.  The definition, if lacking any alignment attributes, will be
layed out in the backend according to the alignment rules then.  The
IR layout definition can be happily cross-platform if you always
access the structures in a type-safe way (using GEP, never castes,
etc).
> If LLVM doesn't allow omitting the layout, it should, as we now have
> an use case that needs it. If it does, it should be just a matter of
> converting the current IR into EFI bytecode and creating intrinsics to
> deal with the run-time variables. You could even benefit from having
> different languages (LLVM supports) into EFI bytecode...
How does EFI describe structures if the pointer size can change?  This
shouldn't be a harder problem than C struct -> llvm struct.  I assume
the EFI bytecode has some way to describe them.  What is it?
(Question is for Ching)

Andrew
>> And I am wondering whether this kind of idea is valuable to the LLVM
>> community? or are there any other related ideas is more valuable?
>
> I think that an open source compiler to EFI byte code is not only
> desirable, but necessary.
>
>
> cheers,
> --renato
>
> http://systemcall.org/
>
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> How does EFI describe structures if the pointer size can change?  This
> shouldn't be a harder problem than C struct -> llvm struct.  I
assume
> the EFI bytecode has some way to describe them.  What is it?
EFI describe structures almost like C.

There are EBC instructions that have two immediates: one for 32bits pointers and
one for 64bits pointers.

Hello Tristan and all,

I have already know that if I want to do this feature(c -> EFI Byte code)
for GCC
I should further modify the GCC front end(parser) to solve the problem (the
size
of pointer is determined at run time).

I have read a powerpoint about LLVM (
http://llvm.org/pubs/2008-10-04-ACAT-LLVM-Intro.pdf)
It is the LLVM-GCC design graph (
http://www.im.ntu.edu.tw/~b95030/llvm_gcc.png<http://www.im.ntu.edu.tw/%7Eb95030/llvm_gcc.png>
).
According to the above discussion , LLVM IR doesn't care about the size of
pointers.
I am wondering how could LLVM support dynamic pointer size model without
modifying
GCC front end??

thanks

ching
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On Mar 19, 2010, at 11:08 AM, 琬菁楊 wrote:
> 
> Hello Tristan and all,
> 
> I have already know that if I want to do this feature(c -> EFI Byte
code) for GCC
> I should further modify the GCC front end(parser) to solve the problem (the
size
> of pointer is determined at run time).
> 
> I have read a powerpoint about LLVM
(http://llvm.org/pubs/2008-10-04-ACAT-LLVM-Intro.pdf)
> It is the LLVM-GCC design graph
(http://www.im.ntu.edu.tw/~b95030/llvm_gcc.png).
> According to the above discussion , LLVM IR doesn't care about the size
of pointers.
> I am wondering how could LLVM support dynamic pointer size model without
modifying
> GCC front end??
What do you mean by "variable sized pointers"?  What does:

struct  S {void *X; };

return for sizeof(struct S); ?

-Chris
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Hello Tristan,
> How does EFI describe structures if the pointer size can change?  This
> > shouldn't be a harder problem than C struct -> llvm struct.  I
assume
> > the EFI bytecode has some way to describe them.  What is it?
>
> EFI describe structures almost like C.
>
> There are EBC instructions that have two immediates: one for 32bits
> pointers and one for 64bits pointers.
>
I have read the uefi specification 2.3 and survey the code of EBC VM
It seems that there is no relationship between the EBC byte code and the
processor.
like CMPI[32|64][w|d]eq {@}R1 {Index16}, Immed16|Immed32,
choosing which form is not depending on the processor but compiler(means
32-bit processor could even support 64-bit operand)
Does it more feasible to implement it??

thanks

ching
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