Simplify finding decoder logic

[brchiu]

Taking 'FileCreationTimeStamp' of sample TAP3 as example, the corresponding ASN.1 expression of tap3.asn1 are :

FileCreationTimeStamp ::= [APPLICATION 108] DateTimeLong

DateTimeLong ::= SEQUENCE 
{
    localTimeStamp     LocalTimeStamp OPTIONAL, -- *m.m.
    utcTimeOffset      UtcTimeOffset  OPTIONAL
}

Part of generated FileCreationTimeStamp.c is :

...
asn_dec_rval_t
FileCreationTimeStamp_decode_ber(asn_codec_ctx_t *opt_codec_ctx, asn_TYPE_descriptor_t *td,
        void **structure, const void *bufptr, size_t size, int tag_mode) {
    FileCreationTimeStamp_1_inherit_TYPE_descriptor(td);
    return td->ber_decoder(opt_codec_ctx, td, structure, bufptr, size, tag_mode);
}

static void
FileCreationTimeStamp_1_inherit_TYPE_descriptor(asn_TYPE_descriptor_t *td) {
    td->free_struct    = asn_DEF_DateTimeLong.free_struct;
    td->print_struct   = asn_DEF_DateTimeLong.print_struct;
    td->check_constraints = asn_DEF_DateTimeLong.check_constraints;
    td->ber_decoder    = asn_DEF_DateTimeLong.ber_decoder;
    td->der_encoder    = asn_DEF_DateTimeLong.der_encoder;
    td->xer_decoder    = asn_DEF_DateTimeLong.xer_decoder;
    td->xer_encoder    = asn_DEF_DateTimeLong.xer_encoder;
    td->uper_decoder   = asn_DEF_DateTimeLong.uper_decoder;
    td->uper_encoder   = asn_DEF_DateTimeLong.uper_encoder;
    if(!td->per_constraints)
        td->per_constraints = asn_DEF_DateTimeLong.per_constraints;
    td->elements       = asn_DEF_DateTimeLong.elements;
    td->elements_count = asn_DEF_DateTimeLong.elements_count;
    td->specifics      = asn_DEF_DateTimeLong.specifics;
}

asn_TYPE_descriptor_t asn_DEF_FileCreationTimeStamp = {
    "FileCreationTimeStamp",
    "FileCreationTimeStamp",
    FileCreationTimeStamp_free,
    FileCreationTimeStamp_print,
    FileCreationTimeStamp_constraint,
    FileCreationTimeStamp_decode_ber,
    FileCreationTimeStamp_encode_der,
    FileCreationTimeStamp_decode_xer,
    FileCreationTimeStamp_encode_xer,
    0, 0,   /* No PER support, use "-gen-PER" to enable */
    0,  /* Use generic outmost tag fetcher */
    asn_DEF_FileCreationTimeStamp_tags_1,
    sizeof(asn_DEF_FileCreationTimeStamp_tags_1)
        /sizeof(asn_DEF_FileCreationTimeStamp_tags_1[0]) - 1, /* 1 */
    asn_DEF_FileCreationTimeStamp_tags_1,   /* Same as above */
    sizeof(asn_DEF_FileCreationTimeStamp_tags_1)
        /sizeof(asn_DEF_FileCreationTimeStamp_tags_1[0]), /* 2 */
    0,  /* No PER visible constraints */
    0, 0,   /* Defined elsewhere */
    0   /* No specifics */
};

And part of generated DateTimeLong.c

asn_TYPE_descriptor_t asn_DEF_DateTimeLong = {
    "DateTimeLong",
    "DateTimeLong",
    SEQUENCE_free,
    SEQUENCE_print,
    SEQUENCE_constraint,
    SEQUENCE_decode_ber,
    SEQUENCE_encode_der,
    SEQUENCE_decode_xer,
    SEQUENCE_encode_xer,
    0, 0,   /* No PER support, use "-gen-PER" to enable */
    0,  /* Use generic outmost tag fetcher */
    asn_DEF_DateTimeLong_tags_1,
    sizeof(asn_DEF_DateTimeLong_tags_1)
        /sizeof(asn_DEF_DateTimeLong_tags_1[0]), /* 1 */
    asn_DEF_DateTimeLong_tags_1,    /* Same as above */
    sizeof(asn_DEF_DateTimeLong_tags_1)
        /sizeof(asn_DEF_DateTimeLong_tags_1[0]), /* 1 */
    0,  /* No PER visible constraints */
    asn_MBR_DateTimeLong_1,
    2,  /* Elements count */
    &asn_SPC_DateTimeLong_specs_1   /* Additional specs */
};

With eye-inspection, we can see when invoking ber_decoder of asn_DEF_FileCreationTimeStamp,
function SEQUENCE_decode_ber is called eventually. And the address of asn_SPC_DateTimeLong_specs_1 is assigned to asn_DEF_FileCreationTimeStamp's member variable 'specifics'.

Moreover, asn_MBR_DateTimeLong_1 is used for accessing child elements when proceeding the process of decoding each child element of DateTimeLong.

So after some tweak of asn1c_C.c and other files, we can eliminate the indirect access of codec function and generate the final data structure for asn_DEF_FileCreationTimeStamp directly. Thus we can remove the excessive helper functions which occupy a lot of code space.

New type descriptor in FileCreationTimeStamp.c

asn_TYPE_descriptor_t asn_DEF_FileCreationTimeStamp = {
    "FileCreationTimeStamp",
    "FileCreationTimeStamp",
    SEQUENCE_free,
    SEQUENCE_print,
    FileCreationTimeStamp_constraint,
    SEQUENCE_decode_ber,
    SEQUENCE_encode_der,
    SEQUENCE_decode_xer,
    SEQUENCE_encode_xer,
    0, 0,   /* No PER support, use "-gen-PER" to enable */
    0,  /* Use generic outmost tag fetcher */
    asn_DEF_FileCreationTimeStamp_tags_1,
    sizeof(asn_DEF_FileCreationTimeStamp_tags_1)
        /sizeof(asn_DEF_FileCreationTimeStamp_tags_1[0]) - 1, /* 1 */
    asn_DEF_FileCreationTimeStamp_tags_1,   /* Same as above */
    sizeof(asn_DEF_FileCreationTimeStamp_tags_1)
        /sizeof(asn_DEF_FileCreationTimeStamp_tags_1[0]), /* 2 */
    0,  /* No PER visible constraints */
    asn_MBR_DateTimeLong_1,
    2,  /* Elements count */
    &asn_SPC_DateTimeLong_specs_1   /* Additional specs */
};

There are some more possible improvement to this pull request :

1. check_constraints function has not been manipulated as above due to, for some types, constraint checks are really needed. For those check_constraints functions have similar content, it can be removed as well. For those check_constraints functions really do check, either leave it as current or change to a data driven function.
   We might need to modify asn1c_C.c and perhaps asn1c_constraint.c to achieve these.
2. After codec functions are all mapped to primitive types, then we can extract them into a codec function array and use a type variable to access them. Then we can reduce excessive pointers to codec function to a single numeric (or enumerated) variable. This can further reduce code size.
3. After codec functions are extracted, it become easier for us to conditionally remove codec functions that we don't need. For example, if we only want PER coder and decoder, we can conditionally remove XER/BER/DER codec. This is especially useful for RRC whose message are PER oriented.

Notice :

1. I have tested the pull request by 'make check' using examples under RRC, TAP3, LDAP. As for PKIX1 and MHEG5 and 'make distcheck', they failed the same place as 0.9.28. And ULP and MEGACO has not been tested yet due to no complete asn1 file and no test data available.
2. I am not quite acquainted with the internal of asn1c nor with ASN.1 specification. All modification are done by comparison and simple logic inference. The modified code can work with minimum level but not in good quality in terms of code structure or flow. Any revise advice is welcomed.

[brchiu]

Item 1 is implemented and add to this pull request.

[brchiu]

Fix 'make check' failures.

[coveralls]

Coverage decreased (-0.3%) to 77.538% when pulling 4ab5d5e on brchiu:simplify_finding_decoder_logic into 0eca8c3 on vlm:master.

[coveralls]

Coverage increased (+0.03%) to 77.884% when pulling 0372693 on brchiu:simplify_finding_decoder_logic into 0eca8c3 on vlm:master.

[coveralls]

Coverage decreased (-0.01%) to 77.894% when pulling 65e0e8a on brchiu:simplify_finding_decoder_logic into 4021e4b on vlm:master.

[vlm]

Intentional?

[vlm]

Merged, thank you!

(Though see my comment).