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
