Pranav Bhandarkar wrote:
GCC 4.3 does fine here except when the operator is "logical and" (see
attached. test.c uses logical and and test1.c uses plus)
Logical and generates control-flow instructions, i.e. compares,
branches, and labels. This makes optimizing it a very different problem
than optimizing for plus.
Try compiling with -fdump-tree-all and notice that the "gimple" dump
file already contains all of the control-flow expressed in the IL, which
means optimizing this is going to be very difficult.
We could perhaps add a new high level gimple that contains the C
language && as an operator, run a CSE pass, and then later lower it to
expose the control flow, but that will be a lot of work, and probably
won't give enough benefit to justify it.
It is simpler to rewrite the code. For instance if you change this
a[0] = ione && itwo && ithree && ifour && ifive;
to
a[0] = !!ione & !!itwo & !!ithree & !!ifour & !!ifive;
then you get the same effect (assuming none of the subexpressions have
side-effects), and gcc is able to perform the optimization. You also
get code without branches which is likely to be faster on modern
workstation cpus.
Jim
