[PATCH 1/2] aarch64: use macro trickery to automate feature array size replication

[Richard Earnshaw (lists)]

On 06/06/2025 14:08, Jan Beulich wrote:
> On 06.06.2025 14:27, Richard Earnshaw wrote:
>> --- a/include/opcode/aarch64.h
>> +++ b/include/opcode/aarch64.h
>> @@ -290,13 +290,44 @@ enum aarch64_feature_bit {
>>    AARCH64_NUM_FEATURES
>>  };
>>  
>> +typedef uint64_t aarch64_feature_word;
>> +#define AARCH64_BITS_PER_FEATURE_WORD 64
>> +
>> +/* Maximum number of features that we can currently support.  If you
>> +   need to increase this value you will need to increase the number of
>> +   replication statements in AA64_REPLICATE below.  */
>> +#define AARCH64_MAX_FEATURES (AARCH64_BITS_PER_FEATURE_WORD * 2)
> 
> Maybe you can get away without this (and hence without any need to keep
> two things in sync)?
> 
>> +static_assert (AARCH64_MAX_FEATURES > AARCH64_NUM_FEATURES,
> 
> (As an aside - isn't >= sufficient here?)
> 
>> +	       "Insufficent capacity in AARCH64_MAX_FEATURES");
>> +
>> +#define AA64_REPLICATE(SEP, BODY, ...)	\
>> +  BODY (0, __VA_ARGS__) SEP		\
>> +  BODY (1, __VA_ARGS__)
> 
> Move the static_assert() further down and use
> 
> #define XYZ_BODY(x, ...) [x] = (x)
> 
> static_assert (AARCH64_BITS_PER_FEATURE_WORD
> 	       * sizeof ((char[]){ AA64_REPLICATE (REP_COMMA, XYZ_BODY) })
> 	       >= AARCH64_NUM_FEATURES,
> 	       "Insufficient repetitions in AA64_REPLICATE()");
> 
> Jan


I don't think we need anything that complicated.

static_assert ((AA64_REPLICATE (REP_PLUS, AA64_REPVAL, AARCH64_BITS_PER_FEATURE_WORD))
	       >= AARCH64_NUM_FEATURES,
	       "Insufficient repetitions in AA64_REPLICATE()");

Is perfectly adequate (where REP_PLUS is simply '+' of course).

Thanks for the suggestion.

R.