Re: [PATCH] RISC-V: Handle "(a & twobits) == singlebit" in branches using Zbs

[Andrew Waterman <andrew@sifive.com>]

Am I wrong to worry that this will increase dynamic instruction count
when used in a loop?  The obvious code is more efficient when the
constant loads can be hoisted out of a loop.  Or does the cost model
account for this somehow?


On Sun, Nov 13, 2022 at 12:50 PM Philipp Tomsich
<philipp.tomsich@vrull.eu> wrote:
>
> Use Zbs when generating a sequence for "if ((a & twobits) == singlebit) ..."
> that can be expressed as bexti + bexti + andn.
>
> gcc/ChangeLog:
>
>         * config/riscv/bitmanip.md (*branch<X:mode>_mask_twobits_equals_singlebit):
>         Handle "if ((a & T) == C)" using Zbs, when T has 2 bits set and C has one
>         of these tow bits set.
>         * config/riscv/predicates.md (const_twobits_operand): New predicate.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/riscv/zbs-if_then_else-01.c: New test.
>
> Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
> ---
>
>  gcc/config/riscv/bitmanip.md                  | 42 +++++++++++++++++++
>  gcc/config/riscv/predicates.md                |  5 +++
>  .../gcc.target/riscv/zbs-if_then_else-01.c    | 20 +++++++++
>  3 files changed, 67 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.target/riscv/zbs-if_then_else-01.c
>
> diff --git a/gcc/config/riscv/bitmanip.md b/gcc/config/riscv/bitmanip.md
> index 7a8f4e35880..2cea394671f 100644
> --- a/gcc/config/riscv/bitmanip.md
> +++ b/gcc/config/riscv/bitmanip.md
> @@ -690,3 +690,45 @@
>    "TARGET_ZBS"
>    [(set (match_dup 0) (zero_extract:X (match_dup 1) (const_int 1) (match_dup 2)))
>     (set (match_dup 0) (xor:X (match_dup 0) (const_int 1)))])
> +
> +;; IF_THEN_ELSE: test for 2 bits of opposite polarity
> +(define_insn_and_split "*branch<X:mode>_mask_twobits_equals_singlebit"
> +  [(set (pc)
> +       (if_then_else (match_operator 1 "equality_operator"
> +                      [(and:X (match_operand:X 2 "register_operand" "r")
> +                              (match_operand:X 3 "const_twobits_operand" "i"))
> +                       (match_operand:X 4 "single_bit_mask_operand" "i")])
> +        (label_ref (match_operand 0 "" ""))
> +        (pc)))
> +   (clobber (match_scratch:X 5 "=&r"))
> +   (clobber (match_scratch:X 6 "=&r"))]
> +  "TARGET_ZBS && TARGET_ZBB && !SMALL_OPERAND (INTVAL (operands[3]))"
> +  "#"
> +  "&& reload_completed"
> +  [(set (match_dup 5) (zero_extract:X (match_dup 2)
> +                                     (const_int 1)
> +                                     (match_dup 8)))
> +   (set (match_dup 6) (zero_extract:X (match_dup 2)
> +                                     (const_int 1)
> +                                     (match_dup 9)))
> +   (set (match_dup 6) (and:X (not:X (match_dup 6)) (match_dup 5)))
> +   (set (pc) (if_then_else (match_op_dup 1 [(match_dup 6) (const_int 0)])
> +                          (label_ref (match_dup 0))
> +                          (pc)))]
> +{
> +   unsigned HOST_WIDE_INT twobits_mask = UINTVAL (operands[3]);
> +   unsigned HOST_WIDE_INT singlebit_mask = UINTVAL (operands[4]);
> +
> +   /* Make sure that the reference value has one of the bits of the mask set */
> +   if ((twobits_mask & singlebit_mask) == 0)
> +      FAIL;
> +
> +   int setbit = ctz_hwi (singlebit_mask);
> +   int clearbit = ctz_hwi (twobits_mask & ~singlebit_mask);
> +
> +   operands[1] = gen_rtx_fmt_ee (GET_CODE (operands[1]) == NE ? EQ : NE,
> +                                <X:MODE>mode, operands[6], GEN_INT(0));
> +
> +   operands[8] = GEN_INT (setbit);
> +   operands[9] = GEN_INT (clearbit);
> +})
> diff --git a/gcc/config/riscv/predicates.md b/gcc/config/riscv/predicates.md
> index 490bff688a7..6e34829a59b 100644
> --- a/gcc/config/riscv/predicates.md
> +++ b/gcc/config/riscv/predicates.md
> @@ -321,6 +321,11 @@
>    (and (match_code "const_int")
>         (match_test "popcount_hwi (~UINTVAL (op)) == 2")))
>
> +;; A CONST_INT operand that has exactly two bits set.
> +(define_predicate "const_twobits_operand"
> +  (and (match_code "const_int")
> +       (match_test "popcount_hwi (UINTVAL (op)) == 2")))
> +
>  ;; A CONST_INT operand that fits into the unsigned half of a
>  ;; signed-immediate after the top bit has been cleared.
>  (define_predicate "uimm_extra_bit_operand"
> diff --git a/gcc/testsuite/gcc.target/riscv/zbs-if_then_else-01.c b/gcc/testsuite/gcc.target/riscv/zbs-if_then_else-01.c
> new file mode 100644
> index 00000000000..d249a841ff9
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/riscv/zbs-if_then_else-01.c
> @@ -0,0 +1,20 @@
> +/* { dg-do compile } */
> +/* { dg-options "-march=rv64gc_zbb_zbs -mabi=lp64" } */
> +/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" "-O1" } } */
> +
> +void g();
> +
> +void f1 (long a)
> +{
> +  if ((a & ((1ul << 33) | (1 << 4))) == (1ul << 33))
> +    g();
> +}
> +
> +void f2 (long a)
> +{
> +  if ((a & 0x12) == 0x10)
> +    g();
> +}
> +
> +/* { dg-final { scan-assembler-times "bexti\t" 2 } } */
> +/* { dg-final { scan-assembler-times "andn\t" 1 } } */
> --
> 2.34.1
>