[llvm] [BOLT] Compare and Jump (CmpJE and CmpJNE) generation in MCPlusBuilder for both X86 and AArch64. (PR #131949)

[Rodrigo Rocha via llvm-commits]

rcorcs wrote:

Hi @maksfb , thank you for the your comments.

1. `createInstrumentedIndCallHandlerEntryBB()` was not broken before because by chance it was comparing to zero which is exactly the code that represents AArch64CC::EQ.

Below is a code snippet from `AArch64MCPlusBuilder.cpp` with the call to `createCmpJE()`.
<img width="639" alt="createInstrumentedIndCallHandlerEntryBB_AArch64" src="https://github.com/user-attachments/assets/4b7480a3-6eb6-496a-865a-91ad7d55be1a" />

Below is a code snippet from `llvm/lib/Target/AArch64/Utils/AArch64BaseInfo.h` where AArch64CC::EQ is defined.
<img width="596" alt="AArch64CC" src="https://github.com/user-attachments/assets/d025de9d-b8ab-42ab-b862-386ef4e243a4" />

However, note in the example below, that the `AArch64::Bcc` instruction expects a CondCode as its immediate operand. Below we can see an example extracted from `AArch64InstrInfo.cpp`.
<img width="934" alt="BccUseExample" src="https://github.com/user-attachments/assets/434c6c25-a9fc-4aec-a60d-cc829bc691d3" />

While the immediate value passed to `createCmpJE()` must be used only in the comparison instruction, such as in `cmp RegNo, #Imm`.

2. The reason for introducing `createCmpJNE()` is because if I want to implement a multiversioning strategy with some checks, such as in the example below, I need to be able to jump if not equal as well.
<img width="663" alt="UsecaseExample" src="https://github.com/user-attachments/assets/910cd8d0-7951-42ce-ac85-d00c6e36bcb8" />

The use case for this is a work in progress.


https://github.com/llvm/llvm-project/pull/131949