QEmu ARC port - decoder implementation feedback

[Cupertino Miranda]

Hi Richard,

Last few weeks I have been working on cleaning up ARC port for further
reviewing.

In the context of the decoder, if I recall well, your observation was
that a linear search was not Ok.
Your suggestion was not to reinvent the wheel and generate decodetree to
create the decoder.

We started to do that and in the process we realize that the approach
would bring us yet another encoding language description to maintain.
Also that decodetree alone would not allow us to properly disassembly
code, still requiring to keep the initial structure.

Taking in consideration that we do all the other toolchain tools for ARC
and that the decoder structure is already upstreamed in binutils we
would definitely prefer to keep that structure and optimize the
surroundings for proper decoding speeds.

So far, we did the following:
   - converted opcodes.def to macros instead of table entries.
   - created a script that reads those entries and outputs macros that
directly translate to a switch/case decision tree (example below), just
like the ones produced by decodetree. The difference is that the switch
will return the enum entry for the proper decoder structure instead of
calling a translation function.
   - the script can either be contributed in C or python language as it
is based on a simple recursive algorithm.

As you have been the one giving attention to our target, I seek for your
early feedback and to make sure that the solution is still inline with
your expectations.

Looking forward for your feedback.

Regards,
Cupertino

PS: Would it be asking too much if we could have a call, just to
get to meet each other. Unfortunately, no one now is able to do it any
other way. I miss the traveling and conferences.

Example1 (first few lines of the file):

   MATCH_PATTERN(0xf8000000) /* 11111000000000000000000000000000 */
    MATCH_VALUE(0x0) /* 0 */
     MATCH_PATTERN(0x10000) /* 10000000000000000 */
      MATCH_VALUE(0x0) /* 0 */
RETURN_MATCH(OPCODE_ARCv2HS_b_0x00000000_0xF8010000_BRANCH_OP_SIMM21_A16_5)
/* 00000ssssssssss0SSSSSSSSSSNQQQQQ */
      END_MATCH_VALUE(0x0) /* 0 */
      MATCH_VALUE(0x10000) /* 10000000000000000 */
RETURN_MATCH(OPCODE_ARCv2HS_b_0x00010000_0xF8010000_BRANCH_OP_SIMM25_A16_5)
/* 00000ssssssssss1SSSSSSSSSSNRtttt */
      END_MATCH_VALUE(0x10000) /* 10000000000000000 */
     END_MATCH_PATTERN(0x10000) /* 10000000000000000 */
    END_MATCH_VALUE(0x0) /* 0 */
    MATCH_VALUE(0x8000000) /* 1000000000000000000000000000 */
     MATCH_PATTERN(0x10000) /* 10000000000000000 */
      MATCH_VALUE(0x0) /* 0 */
       MATCH_PATTERN(0x20000) /* 100000000000000000 */
        MATCH_VALUE(0x0) /* 0 */
RETURN_MATCH(OPCODE_ARCv2HS_bl_0x08000000_0xF8030000_BRANCH_OP_SIMM21_A32_5)
/* 00001sssssssss00SSSSSSSSSSNQQQQQ */
        END_MATCH_VALUE(0x0) /* 0 */
        MATCH_VALUE(0x20000) /* 100000000000000000 */
RETURN_MATCH(OPCODE_ARCv2HS_bl_0x08020000_0xF8030000_BRANCH_OP_SIMM25_A32_5)
/* 00001sssssssss10SSSSSSSSSSNRtttt */
        END_MATCH_VALUE(0x20000) /* 100000000000000000 */
       END_MATCH_PATTERN(0x20000) /* 100000000000000000 */
      END_MATCH_VALUE(0x0) /* 0 */
      MATCH_VALUE(0x10000) /* 10000000000000000 */
       MATCH_PATTERN(0x17) /* 10111 */
        MATCH_VALUE(0x0) /* 0 */
MULTI_MATCH(OPCODE_ARCv2HS_breq_0x08010000_0xF8010017_BRCC_OP_RB_RC_SIMM9_A16_8)
/* 00001bbbsssssss1SBBBCCCCCCN0Y000 */
MULTI_MATCH(OPCODE_ARCv2HS_breq_0x08010F80_0xF8010FF7_BRCC_OP_RB_LIMM_SIMM9_A16_8)
/* 00001bbbsssssss1SBBB11111000Y000 */
MULTI_MATCH(OPCODE_ARCv2HS_breq_0x0E017000_0xFF017037_BRCC_OP_LIMM_RC_SIMM9_A16_8)
/* 00001110sssssss1S111CCCCCC00Y000 */
MULTI_MATCH(OPCODE_ARCv2HS_breq_0x0E017F80_0xFF017FF7_BRCC_OP_LIMM_LIMMdup_SIMM9_A16_8)
/* 00001110sssssss1S11111111000Y000 */
        END_MATCH_VALUE(0x0) /* 0 */