[RISC-V] optimize Zicond conditional select cases.

When one of the two input operands is 0, ADD and IOR are functionally
equivalent.
ADD is slightly preferred over IOR because ADD has a higher likelihood
of being implemented as a compressed instruction when compared to IOR.
C.ADD uses the CR format with any of the 32 RVI registers availble,
while C.OR uses the CA format with limit to just 8 of them.

Conditional select, if zero case:
rd = (rc == 0) ? rs1 : rs2

before patch:

  czero.nez rd, rs1, rc
  czero.eqz rtmp, rs2, rc
  or rd, rd, rtmp

after patch:

  czero.eqz rd, rs1, rc
  czero.nez rtmp, rs2, rc
  add rd, rd, rtmp

Same trick applies for the conditional select, if non-zero case:
rd = (rc != 0) ? rs1 : rs2

riscv-gnu-toolchain regression tests have been passed with no new failure.
---
 gcc/config/riscv/riscv.cc                        |  2 +-
 .../gcc.target/riscv/zicond-prefer-add-to-or.c   | 16 ++++++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c

It's simple enough, so LGTM for trunk :)

Fei Gao <gaofei@eswincomputing.com> 於 2024年4月15日 週一 14:38 寫道：

> When one of the two input operands is 0, ADD and IOR are functionally
> equivalent.
> ADD is slightly preferred over IOR because ADD has a higher likelihood
> of being implemented as a compressed instruction when compared to IOR.
> C.ADD uses the CR format with any of the 32 RVI registers availble,
> while C.OR uses the CA format with limit to just 8 of them.
>
> Conditional select, if zero case:
> rd = (rc == 0) ? rs1 : rs2
>
> before patch:
>
>   czero.nez rd, rs1, rc
>   czero.eqz rtmp, rs2, rc
>   or rd, rd, rtmp
>
> after patch:
>
>   czero.eqz rd, rs1, rc
>   czero.nez rtmp, rs2, rc
>   add rd, rd, rtmp
>
> Same trick applies for the conditional select, if non-zero case:
> rd = (rc != 0) ? rs1 : rs2
>
> riscv-gnu-toolchain regression tests have been passed with no new failure.
> ---
>  gcc/config/riscv/riscv.cc                        |  2 +-
>  .../gcc.target/riscv/zicond-prefer-add-to-or.c   | 16 ++++++++++++++++
>  2 files changed, 17 insertions(+), 1 deletion(-)
>  create mode 100644
> gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c
>
> diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
> index e5f00806bb9..93c736549c9 100644
> --- a/gcc/config/riscv/riscv.cc
> +++ b/gcc/config/riscv/riscv.cc
> @@ -4709,7 +4709,7 @@ riscv_expand_conditional_move (rtx dest, rtx op, rtx
> cons, rtx alt)
>                                   gen_rtx_IF_THEN_ELSE (mode, cond1,
>                                                         CONST0_RTX (mode),
>                                                         alt)));
> -         riscv_emit_binary (IOR, dest, reg1, reg2);
> +         riscv_emit_binary (PLUS, dest, reg1, reg2);
>           return true;
>         }
>      }
> diff --git a/gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c
> b/gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c
> new file mode 100644
> index 00000000000..f3f7beb0b5e
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/riscv/zicond-prefer-add-to-or.c
> @@ -0,0 +1,16 @@
> +/* { dg-do compile } */
> +/* { dg-options "-march=rv64gc_zicond -mabi=lp64d -mbranch-cost=4" {
> target { rv64 } } } */
> +/* { dg-options "-march=rv32gc_zicond -mabi=ilp32f -mbranch-cost=4" {
> target { rv32 } } } */
> +/* { dg-skip-if "" { *-*-* } {"-O0" "-Og" "-Os" "-Oz"} } */
> +
> +long cond_select_if_zero(long a, long b, long c) {
> +  return a == 0 ? c : b;
> +}
> +
> +long cond_select_if_non_zero(long a, long b, long c) {
> +  return a != 0 ? c : b;
> +}
> +
> +/* { dg-final { scan-assembler-times {add\t}  2 } } */
> +/* { dg-final { scan-assembler-not {or\t} } } */
> +
> --
> 2.17.1
>
>

On 4/15/24 6:58 AM, Kito Cheng wrote:
> It's simple enough, so LGTM for trunk :)
We're already doing this internally.  I just hadn't submitted it due to 
being deep into stage4.

Jeff

On 2024-04-15 21:04  Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
>
>On 4/15/24 6:58 AM, Kito Cheng wrote:
>> It's simple enough, so LGTM for trunk :)
>We're already doing this internally.  I just hadn't submitted it due to
>being deep into stage4.
>
>Jeff 

Hi Jeff

Would you like me to commit it now or leave it to you with your bunch of optimizations in Zicond?

BR
Fei

On 4/15/24 7:27 PM, Fei Gao wrote:
> On 2024-04-15 21:04  Jeff Law <jeffreyalaw@gmail.com> wrote:
>>
>>
>>
>> On 4/15/24 6:58 AM, Kito Cheng wrote:
>>> It's simple enough, so LGTM for trunk :)
>> We're already doing this internally.  I just hadn't submitted it due to
>> being deep into stage4.
>>
>> Jeff
> 
> Hi Jeff
> 
> Would you like me to commit it now or leave it to you with your bunch of optimizations in Zicond?
Go ahead and commit it.  It's extremely low risk.

jeff

Committed. Thanks Kito and Jeff for the reveiw. 

BR
Fei
>
>
>On 4/15/24 7:27 PM, Fei Gao wrote:
>> On 2024-04-15 21:04  Jeff Law <jeffreyalaw@gmail.com> wrote:
>>>
>>>
>>>
>>> On 4/15/24 6:58 AM, Kito Cheng wrote:
>>>> It's simple enough, so LGTM for trunk :)
>>> We're already doing this internally.  I just hadn't submitted it due to
>>> being deep into stage4.
>>>
>>> Jeff
>>
>> Hi Jeff
>>
>> Would you like me to commit it now or leave it to you with your bunch of optimizations in Zicond?
>Go ahead and commit it.  It's extremely low risk.
>
>jeff