On Thu, 19 Apr 2018, Richard Biener wrote:
On Thu, 19 Apr 2018, Marc Glisse wrote:
On Thu, 19 Apr 2018, Jakub Jelinek wrote:
As mentioned in the PR, this optimization can't work if @0's precision
is higher than @1's precision, because originally it compares just some set
of lower bits, but in the new comparison compares all bits.
If @0's precision is smaller than @1's precision (in this case @0 can't be
a pointer, as we disallow such direct casts), then in theory it can be
handled, but will not match what the comment says and we'd need to verify
that the @1 constant can be represented in the @0's precision.
This patch just verifies the precision is the same and does small formatting
cleanup. Bootstrapped/regtested on x86_64-linux and i686-linux, ok for
trunk?
That certainly seems safe, but I am surprised to see a direct cast from 64-bit
pointer to 32-bit integer. I've always seen gcc represent those with an
intermediate cast to a 64-bit integer, even if verify_gimple_assign_unary
allows the direct cast. Does it depend on the platform? It might be nice to
canonicalize this a bit, either by forbidding narrowing pointer-to-integer
casts, or by simplifying cast chains to direct casts.
We are only (well, that was the intention until I broke the verifier...)
disallowing widening casts from pointers because whether there is
zero- or sign-extension involved isn't specified (in fact TYPE_SIGN
of the pointer isn't what matters here but POINTERS_EXTEND_UNSIGNED,
and that's even not well-defined for random address-spaces I think).
Not sure if it's really required to restrict things further.
Then we should probably go with option 2 "simplifying cast chains to
direct casts". Currently,
unsigned f(char*p){return p;}
is turned into
p.0_1 = (long int) p_2(D);
_3 = (unsigned int) p.0_1;
instead of the simpler (more canonical?)
_3 = (unsigned int) p_2(D);
(ideally to me, the type should be part of the operations more than the
objects, so "p.0_1 = (long int) p_2(D)" would just be a copy and not a
(nop) conversion, but that would be way too big a change)
--
Marc Glisse
