Re: memset zero bytes at NULL - isolate-erroneous-paths

[Jeff Law <law at redhat dot com>]

On 11/18/13 04:08, Richard Biener wrote:
> > I'd say that turning memset (0, '\0', 0) into a trap is bad from a QOI
> perspective.  Jeff, is there an easy way to avoid this?  Testcase:
>
> void fn (void *addr, int a)
> {
>    if (a == 0)
>      addr = (void *)0;
>    __builtin_memset (addr, '\0', a);
> }
>
> I wonder where in isolate-paths you check for builtins at all?  ah,
> it's probably from the nonnull attribute on memset.  Which also
> means that trying to catch this case reliably isn't going to work
> (you cannot prove the program has len == 0 in this case and
> conditionally not trapping would somewhat defeat the purpose
> of isolating this path)
It's the nonnull attribute on memset.  One thought would split the 
optimization into two parts.  One which transforms *0 and the other 
which transforms calls/returns.  Have the former enabled by -O2 the 
latter off for now. For the next release, both enabled by default at -O2.

Add distinct warnings for both cases, possibly enabled by -Wall (depends 
on the noise).

That gets most of the benefit now and gives a way for users to identify 
brokenness in their code.

Sadly, this feels a lot like -fstrict-aliasing did eons ago.  Aggressive 
TBAA exposed all kinds problems and it took a lot of user (re)education 
to get them fixed.

jeff