On Wed, 2 Aug 2023, Richard Sandiford wrote:
> Richard Biener <rguent...@suse.de> writes:
> > [...]
> >> >> in vect_determine_precisions_from_range. Maybe we should drop
> >> >> the shift handling from there and instead rely on
> >> >> vect_determine_precisions_from_users, extending:
> >> >>
> >> >> if (TREE_CODE (shift) != INTEGER_CST
> >> >> || !wi::ltu_p (wi::to_widest (shift), precision))
> >> >> return;
> >> >>
> >> >> to handle ranges where the max is known to be < precision.
> >> >>
> >> >> There again, if masking is enough for right shifts and right rotates,
> >> >> maybe we should keep the current handling for then (with your fix)
> >> >> and skip the types_compatible_p check for those cases.
> >> >
> >> > I think it should be enough for left-shifts as well? If we lshift
> >> > out like 0x100 << 9 so the lhs range is [0,0] the input range from
> >> > op0 will still make us use HImode. I think we only ever get overly
> >> > conservative answers for left-shifts from this function?
> >>
> >> But if we have:
> >>
> >> short x, y;
> >> int z = (int) x << (int) y;
> >>
> >> and at runtime, x == 1, y == 16, (short) z should be 0 (no UB),
> >> whereas x << y would invoke UB and x << (y & 15) would be 1.
> >
> > True, but we start with the range of the LHS which in this case
> > would be of type 'int' and thus 1 << 16 and not zero. You
> > might call that a failure of vect_determine_precisions_from_range
> > of course, since it makes it not exactly a forward propagation ...
>
> Ah, right, sorry. I should have done more checking.
>
> > [...]
> >> > Originally I completely disabled shift support but that regressed
> >> > the over-widen testcases a lot which at least have widened shifts
> >> > by constants a lot.
> >> >
> >> > x86 has vector rotates only for AMD XOP (which is dead) plus
> >> > some for V1TImode AFAICS, but I think we pattern-match rotates
> >> > to shifts, so maybe the precision stuff is interesting for the
> >> > case where we match the pattern rotate sequence for widenings?
> >> >
> >> > So for the types_compatible_p issue something along
> >> > the following? We could also exempt the shift operand from
> >> > being covered by min_precision so the consumer would have
> >> > to make sure it can be represented (I think that's never going
> >> > to be an issue in practice until we get 256bit integers vectorized).
> >> > It will have to fixup the shift operands anyway.
> >> >
> >> > diff --git a/gcc/tree-vect-patterns.cc b/gcc/tree-vect-patterns.cc
> >> > index e4ab8c2d65b..cdeeaf98a47 100644
> >> > --- a/gcc/tree-vect-patterns.cc
> >> > +++ b/gcc/tree-vect-patterns.cc
> >> > @@ -6378,16 +6378,26 @@ vect_determine_precisions_from_range
> >> > (stmt_vec_info stmt_info, gassign *stmt)
> >> > }
> >> > else if (TREE_CODE (op) == SSA_NAME)
> >> > {
> >> > - /* Ignore codes that don't take uniform arguments. */
> >> > - if (!types_compatible_p (TREE_TYPE (op), type))
> >> > + /* Ignore codes that don't take uniform arguments. For
> >> > shifts
> >> > + the shift amount is known to be in-range. */
> >>
> >> I guess it's more "we can assume that the amount is in range"?
> >
> > Yes.
> >
> >> > + if (code == LSHIFT_EXPR
> >> > + || code == RSHIFT_EXPR
> >> > + || code == LROTATE_EXPR
> >> > + || code == RROTATE_EXPR)
> >> > + {
> >> > + min_value = wi::min (min_value, 0, sign);
> >> > + max_value = wi::max (max_value, TYPE_PRECISION (type),
> >> > sign);
> >>
> >> LGTM for shifts right. Because of the above lshift thing, I think we
> >> need something like:
> >>
> >> if (code == LSHIFT_EXPR || code == LROTATE_EXPR)
> >> {
> >> wide_int op_min_value, op_max_value;
> >> if (!vect_get_range_info (op, &op_min_value, op_max_value))
> >> return;
> >>
> >> /* We can ignore left shifts by negative amounts, which are UB. */
> >> min_value = wi::min (min_value, 0, sign);
> >>
> >> /* Make sure the highest non-UB shift amount doesn't become UB. */
> >> op_max_value = wi::umin (op_max_value, TYPE_PRECISION (type));
> >> auto mask = wi::mask (TYPE_PRECISION (type), false,
> >> op_max_value.to_uhwi ());
> >> max_value = wi::max (max_value, mask, sign);
> >> }
> >>
> >> Does that look right?
> >
> > As said it looks overly conservative to me? For example with my patch
> > for
> >
> > void foo (signed char *v, int s)
> > {
> > if (s < 1 || s > 7)
> > return;
> > for (int i = 0; i < 1024; ++i)
> > v[i] = v[i] << s;
> > }
> >
> > I get
> >
> > t.c:5:21: note: _7 has range [0xffffc000, 0x3f80]
> > t.c:5:21: note: can narrow to signed:15 without loss of precision: _7 =
> > _6 << s_12(D);
> > t.c:5:21: note: only the low 15 bits of _6 are significant
> > t.c:5:21: note: _6 has range [0xffffff80, 0x7f]
> > ...
> > t.c:5:21: note: vect_recog_over_widening_pattern: detected: _7 = _6 <<
> > s_12(D);
> > t.c:5:21: note: demoting int to signed short
> > t.c:5:21: note: Splitting statement: _6 = (int) _5;
> > t.c:5:21: note: into pattern statements: patt_24 = (signed short) _5;
> > t.c:5:21: note: and: patt_23 = (int) patt_24;
> > t.c:5:21: note: created pattern stmt: patt_21 = patt_24 << patt_22;
> > t.c:5:21: note: over_widening pattern recognized: patt_18 = (int)
> > patt_21;
> >
> > and eventually
> >
> > patt_22 = (signed short) s_12(D);
> > ...
> > vect__5.7_28 = MEM <vector(16) signed char> [(signed char
> > *)vectp_v.5_10];
> > vect_patt_24.8_29 = [vec_unpack_lo_expr] vect__5.7_28;
> > vect_patt_24.8_30 = [vec_unpack_hi_expr] vect__5.7_28;
> > vect_patt_21.9_31 = vect_patt_24.8_29 << patt_22;
> > vect_patt_21.9_32 = vect_patt_24.8_30 << patt_22;
> > vect_patt_17.10_33 = VEC_PACK_TRUNC_EXPR <vect_patt_21.9_31,
> > vect_patt_21.9_32>;
> > MEM <vector(16) signed char> [(signed char *)vectp_v.11_34] =
> > vect_patt_17.10_33;
> >
> > which I think is correct.
>
> Agreed.
>
> > If I'd up the range of the shift
> > operand then the range for _7 would be larger and we wouldn't
> > be able to shrink the range. I suppose the _from_users
> > might back-propagate the truncation to signed char and enable
> > that though?
>
> Yeah, that's the mechanism I was getting confused with, but like
> you say, it isn't a factor for vect_determine_precisions_from_range.
>
> >> Thanks, and sorry for the scope creep.
> >
> > Well, if we can agree on that the patch as originally is
> > correct (but incomplete) we can maybe still install it.
>
> Yeah, works for me.
>
> > As seen above I wonder if there's really an existing problem
> > for LSHIFT_EXPR.
>
> No, maybe not. For:
>
> void foo (int *v, int s)
> {
> for (int i = 0; i < 1024; ++i)
> v[i] = 2 << ((v[i] & 3) * 8 + 7);
> }
>
> we get:
>
> note: _8 has range [0x0, 0x1010100]
> note: _7 has range [0x7, 0x1f]
> note: can narrow to unsigned:25 without loss of precision: _8 = 2 << _7;
>
> which isn't true, but I don't think anything will come of it.
>
> With a bit of work, we could in principle realise that:
>
> void foo (short *v, int s)
> {
> for (int i = 0; i < 1024; ++i)
> v[i] = 2 << (((v[i] & 3) - 1) & 31);
> }
>
> gives a shift range of [0,2] \union {31}, and so the shift result is
> either 0, 2, 4 or 8 (range [0, 8]), but we don't compute that result
> range at present.
>
> So yeah, I still can't trigger an actual problem.
>
> > I'm sure we mishandle rotates, but again at the consumer side - in
> > priciple we could analyze them as lshift + rshift which means the
> > existing behavior is probably also correct.
>
> Yeah, please go ahead with the original patch. And sorry for the
> distraction. It's been a useful exercise for me at least...
Installed. Indeed it was useful for me as well.
Richard.
