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c25c172959
The following testcase shows wrong-code caused by incorrect use
of with_possible_nonzero_bits2.
That matcher is defined as
/* Slightly extended version, do not make it recursive to keep it cheap. */
(match (with_possible_nonzero_bits2 @0)
with_possible_nonzero_bits@0)
(match (with_possible_nonzero_bits2 @0)
(bit_and:c with_possible_nonzero_bits@0 @2))
and because with_possible_nonzero_bits includes the SSA_NAME case with
integral/pointer argument, both forms can actually match when a SSA_NAME
with integral/pointer type has a def stmt which is BIT_AND_EXPR
assignment with say SSA_NAME with integral/pointer type as one of its
operands (or INTEGER_CST, another with_possible_nonzero_bits case).
And in match.pd the latter actually wins if both match and so when using
(with_possible_nonzero_bits2 @0) the @0 will actually be one of the
BIT_AND_EXPR operands if that form is matched.
Now, with_possible_nonzero_bits2 and with_certain_nonzero_bits2 were added
for the
/* X == C (or X & Z == Y | C) is impossible if ~nonzero(X) & C != 0. */
(for cmp (eq ne)
(simplify
(cmp:c (with_possible_nonzero_bits2 @0) (with_certain_nonzero_bits2 @1))
(if (wi::bit_and_not (wi::to_wide (@1), get_nonzero_bits (@0)) != 0)
{ constant_boolean_node (cmp == NE_EXPR, type); })))
simplifier, but even for that one I think they do not do a good job, they
might actually pessimize stuff rather than optimize, but at least does not
result in wrong-code, because the operands are solely tested with
wi::to_wide or get_nonzero_bits, but not actually used in the
simplification. The reason why it can pessimize stuff is say if we have
# RANGE [irange] int ... MASK 0xb VALUE 0x0
x_1 = ...;
# RANGE [irange] int ... MASK 0x8 VALUE 0x0
_2 = x_1 & 0xc;
_3 = _2 == 2;
then if it used just with_possible_nonzero_bits@0, @0 would have
get_nonzero_bits (@0) 0x8 and (2 & ~8) != 0, so we can fold it into
_3 = 0;
But as it uses (with_possible_nonzero_bits2 @0), @0 is x_1 rather
than _2 and get_nonzero_bits (@0) is unnecessarily conservative,
0xb rather than 0x8 and (2 & ~0xb) == 0, so we don't optimize.
Now, with_possible_nonzero_bits2 can actually improve stuff as well in that
pattern, if say value ranges aren't fully computed yet or the BIT_AND_EXPR
assignment has been added later and the lhs doesn't have range computed yet,
get_nonzero_range on the BIT_AND_EXPR lhs will be all bits set, while
on the BIT_AND_EXPR operand might actually succeed.
I believe better would be to either modify get_nonzero_bits so that it
special cases the SSA_NAME with BIT_AND_EXPR def_stmt (but one level
deep only like with_possible_nonzero_bits2, no recursion), in that case
return bitwise and of get_nonzero_bits (non-recursive) for the lhs and
both operands, and possibly BIT_AND_EXPR itself e.g. for GENERIC
matching during by returning bitwise and of both operands.
Then with_possible_nonzero_bits2 could be needed for the GENERIC case,
perhaps have the second match #if GENERIC, but changed so that the @N
operand always is the whole thing rather than its operand which is
error-prone. Or add get_nonzero_bits wrapper with a different name
which would do that.
with_certain_nonzero_bits2 could be changed similarly, these days
we can test known non-zero bits rather than possible non-zero bits on
SSA_NAMEs too, we record both mask and value, so possible nonzero bits
(aka. get_nonzero_bits) is mask () | value (), while known nonzero bits
is value () & ~mask (), with a new function (get_known_nonzero_bits
or get_certain_nonzero_bits etc.) which handles that.
Anyway, the following patch doesn't do what I wrote above just yet,
for that single pattern it is just a missed optimization.
But the with_possible_nonzero_bits2 uses in the 3 new simplifiers are
just completely incorrect, because they don't just use the @0 operand
in get_nonzero_bits (pessimizing stuff if value ranges are fully computed),
but also use it in the replacement, then they act as if the BIT_AND_EXPR
wasn't there at all.
While we could use (with_possible_nonzero_bits2@3 @0) and use
get_nonzero_bits (@0) and use @3 in the replacement, that would still
often be a pessimization, so I've just used with_possible_nonzero_bits@0.
2024-11-22 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/117420
* match.pd ((X >> C1) << (C1 + C2) -> X << C2,
(X >> C1) * (C2 << C1) -> X * C2, X / (1 << C) -> X /[ex] (1 << C)):
Use with_possible_nonzero_bits@0 rather than
(with_possible_nonzero_bits2 @0).
* gcc.dg/torture/pr117420.c: New test.
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