Re: How do disable generation of LDRD in ARM

[Andrew Haley <aph at redhat dot com>]

On 12/24/2012 07:53 PM, Roger Cruz wrote:
> 
> 
> I am compiling this piece of code from an open source project that I
> wish not to have to change.  The problem is that when compiled for
> ARM, it generates an LDRD instruction, which when executed, causes a
> bus error since the address in ptr is not doubleword aligned.

Well, that's going to be tough.  Your code isn't legal C, and a
compiler is not obligated to generate anything sensible for it.  You
can't just convert an arbitrary byte pointer to a UINT64* and expect
working code.  If you lie to the compiler it will bite you.

> I know I can change the code but I prefer to leave it intact as
> there may be many more instances of this and I don't wish to
> maintain the code.

There's no point talking about "maintenance" because the code is
already broken.

> 248    static DWORD64 dwarf2_get_u8(const unsigned char* ptr)
> 249    {
> 250        return *(const UINT64*)ptr;
> 251    }

This is the right way to do it:

static DWORD64 dwarf2_get_u8(const unsigned char* ptr)
{
  UINT64 poo;
  memcpy(&poo, ptr, sizeof poo);
  return poo;
}

If GCC can detect that ptr is correctly aligned, it will generate an
LDRD or LDM instruction, as appropriate.

> Happy Holidays,

You too!  :-)

Andrew.