Re: [PATCH] powerpc: Use aligned stores in memset

[Florian Weimer <fweimer@redhat.com>]

On 09/12/2017 02:18 PM, 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.

I disagree.  On most architectures, including x86-64, you can tell, with
certain hardware devices, that our mem* functions do not perform
byte-wise read or write access.  On many architectures, just a hardware
watchpoint installed using ptrace (a supported API) is sufficient.  But
this theoretical possibility does not mean that we cannot or should not
optimize the mem* functions.

If you need specific memory access patterns, you need to use inline
assembly.  In many cases, volatile loads and stores are sufficient, too.

>> ...the current glibc
>> implementation accesses locations which are outside the specified object
>> boundaries.
> 
> I think that's technically a defect.  Nothing in the C standard
> licenses it to do that;

It's permitted under the as-if rule.

Florian