[BOLT] Don't apply ICP to instructions with unknown control flow

[Amir]

Summary

ICP has two modes: jump table promotion and indirect call promotion.
The selection is based on whether an instruction has a jump table or not.
An instruction with unknown control flow doesn't have a jump table and will
fall under indirect call promotion policy which might be incorrect/unsafe
(if an instruction is not a tail call, i.e. has local jump targets).

Prevent ICP for instructions with unknown control flow by adding a corresponding
check in canPromoteCallsite.

[Amir]

Narrow down to non-tailcall indirect jump with no jump table

[maksfb]

Filtering out functions with hasUnknownControlFlow() property in shouldOptimize() sounds more straightforward. But if you feel confident that some call sites could still be optimized, go for it.

[Amir]

In D128870#3677746, @maksfb wrote:

Filtering out functions with hasUnknownControlFlow() property in shouldOptimize() sounds more straightforward. But if you feel confident that some call sites could still be optimized, go for it.

Good idea to filter on a function basis.

bolt/include/bolt/Core/MCPlusBuilder.h

public:
   virtual bool isUnconditionalBranch(const MCInst &Inst) const {
     return Analysis->isUnconditionalBranch(Inst) && !isTailCall(Inst);
   }
 
   virtual bool isIndirectBranch(const MCInst &Inst) const {
     return Analysis->isIndirectBranch(Inst);
   }
 
+  virtual bool isUnknownControlFlow(const MCInst &Inst) const {
+    return !isCall(Inst) && isIndirectBranch(Inst) && !getJumpTable(Inst);
+  }
+
   /// Returns true if the instruction is memory indirect call or jump
   virtual bool isBranchOnMem(const MCInst &Inst) const {
     llvm_unreachable("not implemented");
     return false;
   }
 
   /// Returns true if the instruction is register indirect call or jump
   virtual bool isBranchOnReg(const MCInst &Inst) const {

bolt/lib/Passes/IndirectCallPromotion.cpp

 size_t IndirectCallPromotion::canPromoteCallsite(
     const BinaryBasicBlock &BB, const MCInst &Inst,
     const std::vector<Callsite> &Targets, uint64_t NumCalls) {
   BinaryFunction *BF = BB.getFunction();
   const BinaryContext &BC = BF->getBinaryContext();
 
   if (BB.getKnownExecutionCount() < opts::ExecutionCountThreshold)
     return 0;
+  if (BC.MIB->isUnknownControlFlow(Inst))
+    return 0;
 
   const bool IsJumpTable = BF->getJumpTable(Inst);
 
   auto computeStats = [&](size_t N) {
     for (size_t I = 0; I < N; ++I)
       if (IsJumpTable)
         TotalNumFrequentJmps += Targets[I].Branches;
       else