[llvm] [BOLT] Gadget scanner: clarify MCPlusBuilder callbacks interface (PR #136147)

[Anatoly Trosinenko via llvm-commits]

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@@ -787,19 +794,20 @@ shouldReportSigningOracle(const BinaryContext &BC, const MCInstReference &Inst,
                           const SrcState &S) {
   static const GadgetKind SigningOracleKind("signing oracle found");
 
-  MCPhysReg SignedReg = BC.MIB->getSignedReg(Inst);
-  if (SignedReg == BC.MIB->getNoRegister())
+  std::optional<MCPhysReg> SignedReg = BC.MIB->getSignedReg(Inst);
+  if (!SignedReg)
     return std::nullopt;
 
+  assert(*SignedReg != BC.MIB->getNoRegister());
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atrosinenko wrote:

Replacing `if (mayLoad(Inst)) {...}` with `if (Desc.mayLoad()) {...}` in the single-instruction overload of `getAuthCheckedReg` callback does not work: while `AArch64MCPlusBuilder::mayLoad(Inst)` returns true only for load instructions (but not all instructions are handled), the `MCInstrDesc::mayLoad()` method, in accordance with its name, returns true if the instruction *may* read anything from memory (such as for `AArch64::HINT`).

Added a FIXME in c73d6fdd679886c2177b06e1b4b806e90f59e7e4.

https://github.com/llvm/llvm-project/pull/136147