Re: [PATCH v4] match.pd: turn more conditional bit_iors unconditional [PR124667]

On Tue, Jul 28, 2026 at 4:30 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
>
> Following Andrea's suggestion we're also handling the variants that
> results in just 'A' if we switch the conditionals:
>
> (A & BIT) EQ 0 ? A | BIT : A , BIT = 1 << val => A | BIT
> (A & BIT) NE 0 ? A | BIT : A , BIT = 1 << val => A
>
> (A & BIT) NE 0 ? A : A | BIT , BIT = 1 << val => A | BIT
> (A & BIT) EQ 0 ? A : A | BIT , BIT = 1 << val => A
>
> 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 : A | MASK => A
>
> (A & MASK) NE MASK ? A | MASK : A => A | MASK
> (A & MASK) EQ MASK ? A | MASK : A => A
>
> 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|NE 0 ? A | BIT : A`): New
>         pattern.
>         (`(A & BIT) EQ|NE 0 ? A : A | BIT`): New pattern.
>         (`(A & MASK) EQ|NE MASK ? A : A | MASK`): New pattern.
>         (`(A & MASK) EQ|NE MASK ? A | MASK : A`): New pattern.
>         (`A | (((A >> N) & 1) << N)`): New pattern.


Ok.

>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/tree-ssa/pr124667-2.c: New test.
>         * gcc.dg/tree-ssa/pr124667.c: New test.
> ---
>
> Changes from v3, both suggested by Andrea:
> - Do a partial match instead of a full match to cover more cases when
>   dealing with the rshift + lshift pattern;
> - Rename all @A instances to @0;
> - v3 link: https://gcc.gnu.org/pipermail/gcc-patches/2026-July/723884.html
>
>  gcc/match.pd                               | 100 +++++++++++++++++++
>  gcc/testsuite/gcc.dg/tree-ssa/pr124667-2.c |  67 +++++++++++++
>  gcc/testsuite/gcc.dg/tree-ssa/pr124667.c   | 107 +++++++++++++++++++++
>  3 files changed, 274 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr124667-2.c
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr124667.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 723780b8c38..1bb9ad9c7e8 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -6193,6 +6193,106 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>    (if (wi::to_wide (@2) == ~wi::to_wide (@1))
>     @3))
>
> +/* PR124667: 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.
> +
> +   The same pattern, if using '!=' instead of '==', will
> +   result in just 'A'.
> +
> +   (A & (1 << val)) == 0 ? A |= (1 << val) : A => A |= (1 << val)
> +   (A & (1 << val)) != 0 ? A |= (1 << val) : A => A;  */
> +(for neeq (eq ne)
> + (simplify
> +  (cond
> +   (neeq (bit_and:c @0 (lshift@1 integer_onep @2)) integer_zerop)
> +        (bit_ior:c@3 @0 @1)
> +        @0)
> +    (if (neeq == EQ_EXPR)
> +     @3
> +     @0))
> +
> +/* PR124667: NE variant of the previous match with switched true/false
> +   legs:
> +
> +   (A & (1 << val)) != 0 ? A : A |= (1 << val) => A |= (1 << val)
> +   (A & (1 << val)) == 0 ? A : A |= (1 << val) => A;  */
> + (simplify
> +  (cond
> +   (neeq (bit_and:c @0 (lshift@1 integer_onep @2)) integer_zerop)
> +        @0
> +        (bit_ior:c@3 @0 @1))
> +    (if (neeq == NE_EXPR)
> +     @3
> +     @0))
> +
> +/* PR124667: a bitmask set simplification that is possible as
> +   long as the whole bitmask is being checked.  Switching the
> +   comparison from '==' to '!=' results in 'A':
> +
> +   (A & B) == B ? A : A |= B  => A |= B
> +   (A & B) != B ? A : A |= B  => A;  */
> + (simplify
> +  (cond
> +   (neeq:c (bit_and @0 @1) @1)
> +          @0
> +          (bit_ior@2 @0 @1))
> +    (if (neeq == EQ_EXPR)
> +     @2
> +     @0))
> +
> +/* PR124667: similar from above with true/false legs switched:
> +
> +   (A & B) != B ? A |= B : A => A | B.
> +   (A & B) == B ? A |= B : A => A;  */
> +(simplify
> + (cond
> +  (neeq:c (bit_and @0 @1) @1)
> +         (bit_ior@2 @0 @1)
> +         @0)
> +   (if (neeq == NE_EXPR)
> +    @2
> +    @0)))
> +
> +/* PR124667: these also reduces to just "A":
> +   ((A >> val) & 1) == 0 ?  A : (A | (1 << val))
> +   ((A >> val) & 1) != 0 ?  (A | (1 << val)) : A;
> +
> +   For both cases the zero_one patterns in lines 4860-4890
> +   will turn both into the following 'original':
> +
> +   intD.7 bitshiftD.4489 = 1 << bitD.4485;
> +   arrD.4488 = arrD.4488 >> bitD.4485 & 1) * bitshiftD.4489 | arrD.4488;
> +
> +   And it'll appear back in phiopt as:
> +
> +   _1 = arrD.4482;
> +   _2 = _1 >> bit_7;
> +   _3 = _2 & 1;
> +   _4 = _3 << bit_7;
> +   _6 = _1 | _4;
> +
> +   I.e. it'll drop the 'mult' and lshift the zero_one result from the
> +   bit check.  The 3 middle stmts can be reduced to:
> +
> +   _3 = 1 << bit_7;
> +   _4 = _1 & _3;
> +
> +   Resulting in:
> +
> +   _1 = arrD.4482;
> +   _3 = 1 << bit_7;
> +   _4 = _1 & _3
> +   _6 = _1 | _4;
> +
> +   Which will reduce to just _1.  Simplifying the middle stmts has the
> +   advantage of also benefitting other optimizations as well.  */
> +(simplify
> +  (lshift
> +    (bit_and:c (rshift @0 @1) integer_onep)
> +     @1)
> +  (bit_and @0 (lshift { build_one_cst (type); } @1)))

We could do slightly better here of a generic non 1 simplification but
I think 1 is the most common case.

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/pr124667-2.c b/gcc/testsuite/gcc.dg/tree-ssa/pr124667-2.c
> new file mode 100644
> index 00000000000..aa7ac22e07d
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr124667-2.c
> @@ -0,0 +1,67 @@
> +/* { dg-additional-options -O2 } */
> +/* { dg-additional-options -fdump-tree-optimized } */
> +
> +int test1 (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 test1_alt (int n, int bit)
> +{
> +  int arr[16];
> +  int bitshift = 1 << bit;
> +
> +  arr[n] = (arr[n] & bitshift) != 0 ? arr[n] | bitshift : arr[n];
> +
> +  return arr[n];
> +}
> +
> +int test2 (int n, int bit)
> +{
> +  int arr[16];
> +  int bitshift = 1 << bit;
> +
> +  arr[n] = ((arr[n] >> bit) & 1) == 0 ? arr[n] : arr[n] | bitshift;
> +
> +  return arr[n];
> +}
> +
> +int test2_alt (int n, int bit)
> +{
> +  int arr[16];
> +  int bitshift = 1 << bit;
> +
> +  arr[n] = ((arr[n] >> bit) & 1) != 0 ? arr[n] | bitshift : arr[n];
> +
> +  return arr[n];
> +}
> +
> +int test3 (int n)
> +{
> +  int arr[16];
> +  int bits = 0xF;
> +
> +  arr[n] = (arr[n] & bits) == bits ? arr[n] | bits : arr[n];
> +
> +  return arr[n];
> +}
> +
> +int test3_alt (int n)
> +{
> +  int arr[16];
> +  int bits = 0xF;
> +
> +  arr[n] = (arr[n] & bits) != bits ? arr[n] : arr[n] | bits;
> +
> +  return arr[n];
> +}
> +
> +/* { dg-final { scan-tree-dump-times " \\| " 0 optimized } } */
> +/* { dg-final { scan-tree-dump-times " \& " 0 optimized } } */
> +/* { dg-final { scan-tree-dump-times " << " 0 optimized } } */
> +/* { dg-final { scan-tree-dump-times " >> " 0 optimized } } */
> 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
>