[PATCH] powerpc: Use aligned stores in memset

[Joseph Myers]

On Tue, 12 Sep 2017, Zack Weinberg wrote:

> On Tue, Sep 12, 2017 at 6:30 AM, Florian Weimer <fweimer@redhat.com> wrote:
> >
> > I could not find the manual which has the requirement that the mem*
> > functions do not use unaligned accesses.  Unless they are worded in a
> > very peculiar way, right now, the GCC/glibc combination does not comply
> > with a requirement that memset & Co. can be used for device memory access.
> 
> mem* are required to behave as-if they access memory as an array of
> unsigned char.  Therefore it is valid to give them arbitrarily
> (un)aligned pointers.  The C abstract machine doesn't specifically
> contemplate the possibility of a CPU that can do unaligned word reads
> but maybe not to all memory addresses, but I would argue that if there
> is such a CPU, then mem* are obliged to cope with it.

Only if there is a way, within the standard, in which you might obtain a 
pointer to such memory.

It is explicitly undefined in ISO C to access "an object defined with a 
volatile-qualified type through use of an lvalue with 
non-volatile-qualified type" (C11 6.7.3#6).  Thus you can't use mem* 
functions on objects defined as volatile.  I think device memory with 
special access requirements should be considered to be defined as 
volatile.  (So any access from C code should use volatile-qualified 
lvalues.)

-- 
Joseph S. Myers
joseph@codesourcery.com