[llvm] [BOLT][BTI] Refactor: move applyBTIFixup under MCPlusBuilder (PR #177164)

[Gergely Bálint via llvm-commits]

bgergely0 wrote:

thanks for taking a look @yavtuk!

I think that during passes, we generally have access to the BC. When we pass the BC as a param to a MIB function, it is to get the function based on the symbol:
```c++
void applyBTIFixupToSymbol(BinaryContext &BC, const MCSymbol *TargetSymbol,
                             MCInst &Call) override {
    BinaryFunction *TargetFunction = BC.getFunctionForSymbol(TargetSymbol);
    ...
```  

What you are suggesting is to move the usesBTI check inside these functions? So the usage would be e.g.:

```c++
static void relaxStubToLongJmp(BinaryBasicBlock &StubBB, const MCSymbol *Tgt) {
    const BinaryContext &BC = StubBB.getFunction()->getBinaryContext();
    InstructionListType Seq;
    BC.MIB->createLongJmp(Seq, Tgt, BC.Ctx.get());
    StubBB.clear();
    StubBB.addInstructions(Seq.begin(), Seq.end());
    BC.MIB->applyBTIFixupToTarget(StubBB);
}
```  
and inside the function, we would return early if BTI is off, like so:
```c++
void applyBTIFixupToSymbol(BinaryContext &BC, const MCSymbol *TargetSymbol,
                             MCInst &Call) override {
    if(!usesBTI)
        return;
    ...
```  

I think this is a reasonable idea, although not sure if a string-based storage of the feature is the best. I'll look a bit more into this SubtargetFeatures.


https://github.com/llvm/llvm-project/pull/177164