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

Hello, Chris


> 2010/3/20 Chris Lattner <clattner at apple.com>
>
>
>> On Mar 19, 2010, at 2:40 PM, Louis Gerbarg wrote:
>>
>>
>>> What do you mean by "variable sized pointers"?  What
does:
>>>
>>> struct  S {void *X; };
>>>
>>> return for sizeof(struct S); ?
>>>
>>
>> It doesn't, at least not for Intel's EBC compiler. They error
out on any
>> sizeof that include a pointer.  A piece of EBC code can run in either a
32
>> bit or 64 bit environment, and everything in the compiler either needs
to
>> cope with it (by conditionally choosing the size of offsets into
structs,
>> for instance) or give up on it and abort. That also means that you
cannot
>> compile code that depends on knowing pointer sizes in the preprocessor,
etc.
>>
>>
>> Ok, that makes sense.  It could be done by generalizing the notions of
>> variably modified types (which are VLAs in C99) to include pointers.
>>
>
I have read the sizeof and VLA in C99
I found a example:
EXAMPLE 3 In this example, the size of a variable-length array is computed
and returned from a function:
#include <stddef.h>
size_t fsize3(int n)
{
  char b[n+3]; // variable length array
  return sizeof b; // execution time sizeof
}
int main()
{
  size_t size;
  size = fsize3(10); // fsize3 returns 13
  return 0;
}
And I found some information with clang about VLA
(http://clang.llvm.org/cxx_compatibility.html#vla)
Does llvm/clang doesn't support sizeof is evaluated at run time??

thanks

ching
>
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Hello Chris,

I have survey the efi specification and ask some question to efi engineer.
Difference between EFI C and ANSI C is as following:
1. void*
    In EFI C, the void* is 4-byte for 32-bit processor and 8-byte for 64-bit
processor.
    And it can appears in any where like ANSI C.
    So the main problem is that struct layout like
    struct S{
        void* X;
    };
    is not static.
2. no floating support in EFI C
3. no C++ support in EFI C
4. no assembly support in EFI C, all assembly must convert to C

I am wondering that does LLVM support model which structure layout is
determined at run time??
If not, do I need to modify the parser in clang to support this feature??

thanks

ching
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