Hi Richard,

Please find below the new patch where I addressed your comments and 
updated the changelog.

rev16 pattern was not recognised anymore as a change in the bswap tree
pass was introducing a new GIMPLE form, not recognized by the assembly
final transformation pass.

More details in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108933

gcc/ChangeLog:

	PR target/108933
	* config/arm/arm.md (arm_rev16si2): Convert to define_insn.
	Correct generated RTL.
	(arm_rev16si2_alt1): Correctly handle conditional execution.
	(arm_rev16si2_alt2): Likewise.

gcc/testsuite/ChangeLog:

	PR target/108933
	* gcc.target/arm/rev16.c: Moved to...
	* gcc.target/arm/rev16_1.c: ...here.
	* gcc.target/arm/rev16_2.c: New test to check that rev16 is
	emitted.

On 2024-01-22 16:25, Richard Earnshaw (lists) wrote:
> On 22/01/2024 12:18, Matthieu Longo wrote:
>> rev16 pattern was not recognised anymore as a change in the bswap tree
>> pass was introducing a new GIMPLE form, not recognized by the assembly
>> final transformation pass.
>>
>> More details in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108933
>>
>> gcc/ChangeLog:
>>
>>          PR target/108933
>>          * config/arm/arm.md (*arm_rev16si2_alt3): new pattern to convert
>>            a bswap + rotate by 16 bits into rev16
> 
> ChangeLog entries need to be written as sentences, so start with a capital letter and end with a full stop; continuation lines should start in column 8 (one hard tab, don't use spaces).  But in this case, "New pattern." is sufficient.
> 
>>
>> gcc/testsuite/ChangeLog:
>>
>>          PR target/108933
>>          * gcc.target/arm/rev16.c: Moved to...
>>          * gcc.target/arm/rev16_1.c: ...here.
>>          * gcc.target/arm/rev16_2.c: New test to check that rev16 is
>>            emitted.
> 
> 
> +;; Similar pattern to match (rotate (bswap) 16)
> +(define_insn "*arm_rev16si2_alt3"
> +  [(set (match_operand:SI 0 "register_operand" "=l,r")
> +        (rotate:SI (bswap:SI (match_operand:SI 1 "register_operand" "l,r"))
> +                     (const_int 16)))]
> +  "arm_arch6"
> +  "rev16\\t%0, %1"
> +  [(set_attr "arch" "t,32")
> +   (set_attr "length" "2,4")
> +   (set_attr "type" "rev")]
> +)
> +
> 
> Unfortunately, this is insufficient.  When generating Arm or Thumb2 code (but not thumb1) we also have to handle conditional execution: we need to have '%?' in the output template at the point where a condition code might be needed.  That means we need separate output templates for all three alternatives (as we need a 16-bit variant for thumb2 that's conditional and a 16-bit for thumb1 that isn't).  See the output of arm_rev16 for a guide of what is really needed.
> 
> I note that the arm_rev16si2_alt1, and arm_rev16si2_alt2 patterns are incorrect in this regard as well; that will need fixing.
> 
> I also see that arm_rev16si2 currently expands to the alt1 variant above; given that the preferred canonical form would now appear to use bswap + rotate, we should change that as well.  In fact, we can merge your new pattern with the expand entirely and eliminate the need to call gen_arm_rev16si2_alt1.  Something like:
> 
> (define_insn "arm_rev16si2"
>    [(set (match_operand:SI 0 "s_register_operand")
>          (rotate:SI (bswap:SI (match_operand:SI 1 "s_register_operand")) (const_int 16))]
>    "arm_arch6"
>    "@
>    rev16...
>    ...
> 
> 
> R.
>