Hi Richard,
I've recently experimented with TARGET_SHIFT_TRUNCATION_MASK
macro and have posted a patch defining it for S/390.
On S/390 only the least significant six bits of a shift count
operand are used and I therefore expected the modulo operation
in the following example to be optimized away:
long
f (long t, unsigned int shift_count)
{
return t << (shift_count % 64);
}
But I understand that this optimization can't simply be added
to simplify_rtx.c.
The macro only applies to rtxes coming from named patterns
and not to shifts emitted anywhere else in the optimization process because
combine and co are currently not aware that macro exists. Because
nobody is able to determine the origin of a shift rtx it would
lead to wrong code performing that kind of optimization in simplify-rtx
for all incoming shifts.
In:
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg00456.html
you proposed to take care of this in the 4.1 (formerly 3.6) timeframe fixing
all places where shift rtxes are generated besides optabs.
Is this still on your todo list?
Bye,
-Andreas-
