[llvm-exegesis] [AArch64] Reland Resolving "not all operands are initialized by snippet generator"

[lakshayk-nv]

Reland #142529 (Resolving "not all operands are initialized by snippet generator")

Introduced changes in implementation of randomizeTargetMCOperand() for AArch64 that omitting OPERAND_SHIFT_MSL, OPERAND_PCREL to an immediate value of 264 and 8 respectively.
PS: Omitting MCOI::OPERAND_FIRST_TARGET/llvm:AArch64:OPERAND_IMPLICIT_IMM_0 similarly, to value 0. It was low hanging change thus added in this PR only.

For any future operand type of AArch64 if not initialised will exit with error "Unimplemented operand type: MCOI::OperandType:<#Number>".

[Reland Updates]

Updated tools/llvm-exegesis/AArch64/error-resolution.s which caused problem.
Test case was failing when there is uninitialised operands error coming from secondary/consumer instruction used by exegesis in latency mode required to chain up the assembly to ensure serial execution.

i.e. We get error message like UMOVvi16_idx0: Not all operands were initialized by the snippet generator for <<<any opcode other than UMOVvi16_idx0>>> opcode. but test case want to check like
# UMOVvi16_idx0_latency: ---. Thus the testcase fails.

/llvm-project/llvm/test/tools/llvm-exegesis/AArch64/error-resolution.s:65:26: error: UMOVvi16_idx0_latency: expected string not found in input
# UMOVvi16_idx0_latency: ---
                         ^
<stdin>:1:1: note: scanning from here
UMOVvi16_idx0: Not all operands were initialized by the snippet generator for LD1W_D_IMM opcode.
^

Input file: <stdin>
Check file: /llvm-project/llvm/test/tools/llvm-exegesis/AArch64/error-resolution.s

-dump-input=help explains the following input dump.

Input was:
<<<<<<
          1: UMOVvi16_idx0: Not all operands were initialized by the snippet generator for LD1W_D_IMM opcode. 
check:65     X~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ error: no match found
>>>>>>

--

********************
********************
Failed Tests (1):
  LLVM :: tools/llvm-exegesis/AArch64/error-resolution.s

[Why it fails (only sometimes)]

Exegesis in latency mode require the generated assembly to be chained to ensure serial execution,
For this exegesis add an additional consumer instruction for some instruction, which is chosen via a random seed.
Thus, it randomly fails whenever there is secondary consumer instruction (which is unsupported/throws error) added in generated assembly.

Please review: @sjoerdmeijer @davemgreen @boomanaiden154 @abhilash1910
Thanks!

[boomanaiden154]

LGTM, assuming you've rerun the test quite a few times locally to ensure there aren't any flakes.

[abhilash1910]

nit:
but this introduces illegal instruction error for the below instructions.
System instructions ...

[lakshayk-nv]

Updated the comment. Thanks!

[abhilash1910]

Should we also count for 272 ? We may not want to enforce 264 always?
Edit: This can be extended for other imm operands which are defaulted to some value, probably in a separate patch- essentially "taking value as an arg" . @boomanaiden154 @sjoerdmeijer @davemgreen what do you think?

[sjoerdmeijer]

Agree, this area needs some work in follow ups, but let's start somewhere with this, so is okay as a start.

[abhilash1910]

LGTM otherwise!