On Wed, Apr 20, 2011 at 10:27:36AM -0700, Richard Henderson wrote:
> On 04/20/2011 09:09 AM, Jakub Jelinek wrote:
> > Hi!
> >
> > This splitter allows us to optimize (x {* {2,4,8},<< {1,2,3}}) {|,^} y
> > for constant integer y <= {1ULL,3ULL,7ULL} using lea{l,q} (| or ^ in
> > that case, when the low bits are known to be all 0, is like plus).
> >
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> >
> > 2011-04-20 Jakub Jelinek <ja...@redhat.com>
> >
> > PR target/48688
> > * config/i386/i386.md (*lea_general_4): New define_insn_and_split.
>
> Any chance you could do this in combine instead? Shift-and-add patterns
> are a fairly common architectural feature...
I've tried to do it in simplify-rtx.c, unfortunately combine.c does exactly
opposite canonicalization and thus it results in endless recursion:
/* If we are adding two things that have no bits in common, convert
the addition into an IOR. This will often be further simplified,
for example in cases like ((a & 1) + (a & 2)), which can
become a & 3. */
if (GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_WIDE_INT
&& (nonzero_bits (XEXP (x, 0), mode)
& nonzero_bits (XEXP (x, 1), mode)) == 0)
{
/* Try to simplify the expression further. */
rtx tor = simplify_gen_binary (IOR, mode, XEXP (x, 0), XEXP (x, 1));
temp = combine_simplify_rtx (tor, mode, in_dest, 0);
/* If we could, great. If not, do not go ahead with the IOR
replacement, since PLUS appears in many special purpose
address arithmetic instructions. */
if (GET_CODE (temp) != CLOBBER && temp != tor)
return temp;
}
So at least it can't be done in simplify_binary_operation_1.
Jakub
