Extend basic block address map with IR indexes.

Since LLVM reorders machine basic blocks during compilation, the order
in the basic block address map doesn't match the order of LLVM IR basic
blocks. This commit extends the basic block address map
(SHT_LLVM_BB_ADDR_MAP) with an additional field which exposes the LLVM
IR basic block index for each machine basic block. This allows us to map
virtual addresses back to BasicBlocks in the IR.

Author: ptersilie

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -1151,6 +1151,7 @@ void AsmPrinter::emitBBAddrMapSection(const MachineFunction &MF) {
   OutStreamer->emitSymbolValue(FunctionSymbol, getPointerSize());
   // Emit the total number of basic blocks in this function.
   OutStreamer->emitULEB128IntValue(MF.size());
+  const Function &F = MF.getFunction();
   // Emit BB Information for each basic block in the funciton.
   for (const MachineBasicBlock &MBB : MF) {
     const MCSymbol *MBBSymbol =
@@ -1161,6 +1162,16 @@ void AsmPrinter::emitBBAddrMapSection(const MachineFunction &MF) {
     // always be computed from their offsets.
     emitLabelDifferenceAsULEB128(MBB.getEndSymbol(), MBBSymbol);
     OutStreamer->emitULEB128IntValue(getBBAddrMapMetadata(MBB));
+    // Emit the index of the corresponding LLVMIR basic block.
+    size_t i = 0;
+    for (auto It = F.begin(); It != F.end(); It++) {
+      const BasicBlock *BB = &*It;
+      if (BB == MBB.getBasicBlock()) {
+          break;
+      }
+      i++;
+    }
+    OutStreamer->emitULEB128IntValue(i);
   }
   OutStreamer->PopSection();
 }