Re: ARM memcpy on unaligned pointers and -O2

[Shaun Jackman <sjackman at pathwayconnect dot com>]

On December 13, 2001 03:24, you wrote:
> > I believe I've found a bug in GCC 3.0.2 wrt the ARM target and -O2
> > optimisation. I found it when I wrote the following functions to load and
> > store unaligned pointers. This is the objdump -S output.
[ clip ]
> > As you can see this did not have the intended effect. GCC recognised the
> > length of the memcpy as being constant and optimised it away to a simple
> > ldr or str instruction. What it doesn't seem to have taken into account
> > though, is the fact that the pointer it's passed may not be aligned. I
> > believe it's allowed to pass unaligned pointers to memcpy and the
> > compiler should handle it correctly.
>
> No, your pointer is aligned.  You declared it to be so in the declarations
> of your functions.  Having done this, the compiler can and does use this
> knowledge to optimise the code.
>
> R.

I recognized that afterwords and cast the arguments to memcpy()

uint32
ld32( const uint32* p)
{
	uint32 ret;
	memcpy( &ret, (uint8*)p, sizeof( ret));
	return ret;
}

void
str32( uint32* p, uint32 val)
{
	memcpy( (uint8*)p, &val, sizeof( *p));
}

It seems the behaviour persists though. In fact, it has the exact same 
behaviour if I change the parameters from uint32* to void*.

Please cc me as I'm not a member of this list,
Thanks,
Shaun