On Mon, May 22, 2023 at 4:56 AM Richard Biener via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Fri, May 19, 2023 at 4:15 AM Andrew Pinski via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > While working something else, I noticed we could improve
> > the following function code generation:
> > ```
> > unsigned f(unsigned t)
> > {
> > if (t & ~(1<<30)) __builtin_unreachable();
> > return t != 0;
> > }
> > ```
> > Right know we just emit a comparison against 0 instead
> > of just a shift right by 30.
> > There is code in do_store_flag which already optimizes
> > `(t & 1<<30) != 0` to `(t >> 30) & 1`. This patch
> > extends it to handle the case where we know t has a
> > nonzero of just one bit set.
> >
> > OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions.
> >
> > gcc/ChangeLog:
> >
> > * expr.cc (do_store_flag): Extend the one bit checking case
> > to handle the case where we don't have an and but rather still
> > one bit is known to be non-zero.
> > ---
> > gcc/expr.cc | 27 +++++++++++++++++++++------
> > 1 file changed, 21 insertions(+), 6 deletions(-)
> >
> > diff --git a/gcc/expr.cc b/gcc/expr.cc
> > index 5ede094e705..91528e734e7 100644
> > --- a/gcc/expr.cc
> > +++ b/gcc/expr.cc
> > @@ -13083,15 +13083,30 @@ do_store_flag (sepops ops, rtx target,
> > machine_mode mode)
> > && integer_zerop (arg1)
> > && (TYPE_PRECISION (ops->type) != 1 || TYPE_UNSIGNED (ops->type)))
> > {
> > - gimple *srcstmt = get_def_for_expr (arg0, BIT_AND_EXPR);
> > - if (srcstmt
> > - && integer_pow2p (gimple_assign_rhs2 (srcstmt)))
> > + wide_int nz = tree_nonzero_bits (arg0);
> > +
> > + if (wi::popcount (nz) == 1)
> > {
> > + tree op0;
> > + tree op1;
> > + gimple *srcstmt = get_def_for_expr (arg0, BIT_AND_EXPR);
> > + /* If the defining statement was (x & POW2), then remove the and
> > + as we are going to add it back. */
> > + if (srcstmt
> > + && integer_pow2p (gimple_assign_rhs2 (srcstmt)))
> > + {
> > + op0 = gimple_assign_rhs1 (srcstmt);
> > + op1 = gimple_assign_rhs2 (srcstmt);
> > + }
> > + else
> > + {
> > + op0 = arg0;
> > + op1 = wide_int_to_tree (TREE_TYPE (op0), nz);
> > + }
> > enum tree_code tcode = code == NE ? NE_EXPR : EQ_EXPR;
> > type = lang_hooks.types.type_for_mode (mode, unsignedp);
> > - tree temp = fold_build2_loc (loc, BIT_AND_EXPR, TREE_TYPE (arg1),
> > - gimple_assign_rhs1 (srcstmt),
> > - gimple_assign_rhs2 (srcstmt));
> > + tree temp = fold_build2_loc (loc, BIT_AND_EXPR, TREE_TYPE (op0),
> > + op0, op1);
> > temp = fold_single_bit_test (loc, tcode, temp, arg1, type);
> > if (temp)
> > return expand_expr (temp, target, VOIDmode, EXPAND_NORMAL);
>
> I wonder if, instead of expanding expand with these kind of tricks we
> want to instead
> add to ISEL and use direct optab IFNs for things we matched? In
> particular I think
> we do want to get rid of TER but the above adds another use of
> get_def_for_expr.
The above does not add another at all. It was there before, it just
moves it around slightly. Instead we depend on the non-zero bits to be
correct before even trying get_def_for_expr .
The get_def_for_expr is there to remove the & if it can be ter'ed.
>
> As Jeff says the above doesn't look like it includes costing so that would be
> an
> argument to make it a generic match.pd transform (it appears to be "simpler")?
For the TER case, it would be same number of gimple instructions so
that can happen if we want
t = a & CST
result = t != 0
vs:
t1 = BIT_FIELD_REF <a, 1, N>
result = (bool)t1
For the non-TER case (which is what this patch is trying to solve).
we just have `t != 0` (where t has a non-zero value of CST) so it might increase
the number of gimple instructions by 1.
Is that ok? Or should that still happen in expand only.
The cost issue between a != 0 vs bit_extraction (for the non-ter case)
is something which I will be solving next weekend.
>
> Richard.
>
> > --
> > 2.31.1
> >
