On Wed, Jul 8, 2026 at 4:35 AM Daniel Barboza
<[email protected]> wrote:
>
> Add patterns to handle the case where we're checking if a bit is set and
> setting it in case it's not. Similar to the work done in PR64567 but in
> this case the bit is a result of a 1 << val lshift:
>
> (A & BIT) EQ 0 ? A | BIT : A , BIT = 1 << val => A | BIT
> (A & BIT) NE 0 ? A : A | BIT , BIT = 1 << val => A | BIT
>
> We're also adding cases where a full bitmask is tested, i.e.:
>
> (A & MASK) EQ MASK ? A : A | MASK => A | MASK
> (A & MASK) NE MASK ? A | MASK : A => A | MASK
>
> Note that for these simplifications we're not limited to a bit/pow2
> value like the zero comparisons, which don't work with multiple bits
> because there's no guarantees to preserve the 'non-zero' cases. E.g.:
> "(A & 0xF) == 0 ? A | 0xF : A" can't be simplified to just "A | 0xF"
> because there's a whole range of A lower bits (1,2...E) that would be
> turned to 0xF in the simplification - bits that would be preserved in
> the original pattern. This is the same scenario discussed before in
> PR64567.
>
> Bootstrapped and regression tested in x86_64, aarch64 and riscv64.
>
> PR tree-optimization/124667
>
> gcc/ChangeLog:
>
> * match.pd(`(A & BIT) EQ 0 ? A | BIT : A`): New
> pattern.
> (`(A & BIT) NE 0 ? A : A | BIT`): New pattern.
> (`(A & MASK) EQ MASK ? A : A | MASK`): New pattern.
> (`(A & MASK) NE MASK ? A | MASK : A`): New pattern.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/tree-ssa/pr124667.c: New test.
> ---
>
> Hi,
>
> This is a follow-up of:
>
> [PATCH v3] tree-ssa-phiopt.cc: add cond_removal_mispredict_memop [PR124667]
>
> But now we're using match.pd instead of using cselim. There's a
> bitclear case that we're not covering here since match.pd isn't
> able (yet) to deal with more than one stmt in the middle_bb from
> phiopt. As soon as we get to that point I'll either re-send this
> patch or, in case this is already merged, send a follow-up.
>
>
>
> gcc/match.pd | 31 +++++++
> gcc/testsuite/gcc.dg/tree-ssa/pr124667.c | 107 +++++++++++++++++++++++
> 2 files changed, 138 insertions(+)
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr124667.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index a7cec25dbad..a6580926ee3 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -6102,6 +6102,37 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> (if (wi::to_wide (@2) == ~wi::to_wide (@1))
> @3))
>
> +/* PR124667: (A & (1 << val)) == 0 ? A |= (1 << val) : A
> + Similar to PR64567 above but with the bit given
> + by an (1 << val) lshift. Note that this shift pattern
> + guarantees that this is a single bit operation. */
> +(simplify
> + (cond
> + (eq (bit_and:c @0 (lshift@1 integer_onep @2)) integer_zerop)
> + (bit_ior:c@3 @0 @1) @0)
> + @3)
> +
> +/* PR124667: (A & (1 << val)) != 0 ? A : A |= (1 << val)
> + NE variant of the previous match. */
> +(simplify
> + (cond
> + (ne (bit_and:c @0 (lshift@1 integer_onep @2)) integer_zerop)
> + @0 (bit_ior:c@3 @0 @1))
> + @3)
So it looks like you might need to recognize:
((a >> val) & 1) == 0 ? (A | (1 << val)) : A
Which at least will be caught at the generic level.
Can you also handle:
(A & (1 << val)) == 0 ? A: (A| (1 << val) )
(A & (1 << val)) != 0 ?(A| (1 << val) ) : A
These are both just A.
And the `((a >> val) & 1) == 0`/`((a >> val) & 1) != 0` versions where
the result is just A.
> +
> +/* PR124667: a bitmask set simplification that is possible as
> + long as the whole bitmask is being checked:
> + (A & B) == B ? A : A |= B => A | B
> + (A & B) != B ? A |= B : A => A | B. */
> +(simplify
> + (cond (eq (bit_and:c @0 @1) @1)
> + @0 (bit_ior:c@2 @0 @1))
> + @2)
> +(simplify
> + (cond (ne (bit_and:c @0 @1) @1)
> + (bit_ior:c@2 @0 @1) @0)
> + @2)
These 2 patterns don't need `:c` on the bit_and/bit_ior as the order
there is stable. But should have them on the eq/ne.
Can you handle:
(A & B) == B ? (A|B) : A
(A & B) != B ? A : (A|B)
too? They are both just A.
Thanks,
Andrea
> +
> #if GIMPLE
> (match (nop_atomic_bit_test_and_p @0 @1 @4)
> (bit_and (convert?@4 (ATOMIC_FETCH_OR_XOR_N @2 INTEGER_CST@0 @3))
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr124667.c
> b/gcc/testsuite/gcc.dg/tree-ssa/pr124667.c
> new file mode 100644
> index 00000000000..9bdbe05a00d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr124667.c
> @@ -0,0 +1,107 @@
> +/* { dg-additional-options -O2 } */
> +/* { dg-additional-options -fdump-tree-optimized } */
> +
> +int bitset1 (int n, int bit)
> +{
> + int arr[16];
> + int bitshift = 1 << bit;
> +
> + if ((arr[n] & bitshift) == 0)
> + arr[n] |= bitshift;
> +
> + return arr[n];
> +}
> +
> +int bitset1_alt (int n, int bit)
> +{
> + int arr[16];
> + int bitshift = 1 << bit;
> +
> + arr[n] = (arr[n] & bitshift) != 0 ? arr[n] : arr[n] | bitshift;
> +
> + return arr[n];
> +}
> +
> +int bitset1b (int n, int bit)
> +{
> + int arr[16];
> + int bitshift = 1 << bit;
> +
> + if ((bitshift & arr[n]) == 0)
> + arr[n] |= bitshift;
> +
> + return arr[n];
> +}
> +
> +int bitset1b_alt (int n, int bit)
> +{
> + int arr[16];
> + int bitshift = 1 << bit;
> +
> + arr[n] = (bitshift & arr[n]) != 0 ? arr[n] : bitshift | arr[n];
> +
> + return arr[n];
> +}
> +
> +int bitset2 (int n)
> +{
> + int arr[16];
> + int bits = 0xF;
> +
> + if ((arr[n] & bits) != bits)
> + arr[n] |= bits;
> +
> + return arr[n];
> +}
> +
> +int bitset2_alt (int n)
> +{
> + int arr[16];
> + int bits = 0xF;
> +
> + arr[n] = (arr[n] & bits) == bits ? arr[n]: arr[n] | bits;
> +
> + return arr[n];
> +}
> +
> +int bitset2b (int n)
> +{
> + int arr[16];
> + int bits = 0xF;
> +
> + if ((bits & arr[n]) != bits)
> + arr[n] |= bits;
> +
> + return arr[n];
> +}
> +
> +int bitset2b_alt (int n)
> +{
> + int arr[16];
> + int bits = 0xF;
> +
> + arr[n] = (bits & arr[n]) == bits ? arr[n]: bits | arr[n];
> +
> + return arr[n];
> +}
> +
> +/* A negative test to ensure we're not optimizing something
> + we shouldn't. */
> +int bitset_nosimplify (int n)
> +{
> + int arr[16];
> +
> + int bits = 0xF;
> +
> + /* We can't make the bit_ior unconditional here because the
> + cond matches for multiple values aside from '0xF' in
> + which the value wouldn't be touched, e.g. for arr[n] = 0x1
> + "arr[n] & bits" is also non-zero and the value wouldn't be
> + changed. */
> + if ((arr[n] & bits) == 0)
> + arr[n] |= bits;
> +
> + return arr[n];
> +}
> +
> +/* { dg-final { scan-tree-dump-times "goto" 2 optimized } } */
> --
> 2.43.0
>
