llvm dev - Jul 2011 - [LLVMdev] Typo in IsLegalToCallImmediateAddr?

It seems that the || should be && here?

/// IsLegalToCallImmediateAddr - Return true if the subtarget allows calls
/// to immediate address.
bool X86Subtarget::IsLegalToCallImmediateAddr(const TargetMachine &TM) const
{
  if (Is64Bit)
    return false;
  return isTargetELF() || TM.getRelocationModel() == Reloc::Static;
}

For example, if you are doing ELF PIC (i.e. for a shared library), it
is not valid to use a call immediate.

- David

On 2011-07-01 17:13, David Meyer wrote:
> It seems that the || should be && here?
> 
> /// IsLegalToCallImmediateAddr - Return true if the subtarget allows calls
> /// to immediate address.
> bool X86Subtarget::IsLegalToCallImmediateAddr(const TargetMachine &TM)
const {
>   if (Is64Bit)
>     return false;
>   return isTargetELF() || TM.getRelocationModel() == Reloc::Static;
> }
> 
> For example, if you are doing ELF PIC (i.e. for a shared library), it
> is not valid to use a call immediate.
I think that on 32 bits it is legal, just not profitable.

I tried compiling two .c files without -fPIC:

---------------
int g(void) {
        return 42;
}
--------------
int g(void);
int f(void) {
        return g();
}
-------------

When they get combined into a .so, the code of f has:

452:   e9 fc ff ff ff          jmp    453 <f+0x7>

and readelf shows

00000453  00000802 R_386_PC32        00000458   g

so the dynamic linker will patch the code segment (making it non sharable)

So maybe the problem is that this function is being called in a place
that really wants a "IsProfitableToCallImmediateAddr"?
> - David
Cheers,
Rafael

Rafael,

I believe your example is not related to IsLegalToCallImmediateAddr.

This is an example of calling to an immediate address:

typedef int (*funcptr)(void);

int main() {
  funcptr foo = (funcptr)0x100;
  foo();
}

If IsLegalToCallImmedateAddr is true, this generates a call to
absolute address 0x100:

call 0x100

This requires a relocation of the value 0x100 - PC.
(NOTE: this is NOT the same as: "foo: call foo+0x100", which requires
no relocation)

This can't be done correctly in PIC mode. If you do this in a shared
library, the relocation is ignored and you get a jump to module_start
+ 0x100 instead of absolute 0x100.

- pdox