Update the documentation and implementation of OpcodeCounter

In #35, `-O1` was added
to the instructions for building `input_for_cc.bc` (i.e. the sample
input for the `OpcodeCounter` pass). Unfortunately, `-O1` causes all
instructions to be optimized away and hence there's nothing to print for
the `OpcodeCounter` pass. This patch fixes that by restoring the
original instruction for building `input_for_cc.bc` .

In order to work around `optnone` (the original rationale behind adding
`-O1` in the previous patch), `isRequired()` [1] is added to the
implementation of `OpcodeCounter.`

Fixes #38

[1] https://llvm.org/docs/WritingAnLLVMNewPMPass.html#required-passes

README.md

@@ -297,7 +297,7 @@ need to choose which pass manager you want to use (see
 ```bash
 export LLVM_DIR=<installation/dir/of/llvm/12>
 # Generate an LLVM file to analyze
-$LLVM_DIR/bin/clang -O1 -emit-llvm -c <source_dir>/inputs/input_for_cc.c -o input_for_cc.bc
+$LLVM_DIR/bin/clang -emit-llvm -c <source_dir>/inputs/input_for_cc.c -o input_for_cc.bc
 # Run the pass through opt - Legacy PM
 $LLVM_DIR/bin/opt -load <build_dir>/lib/libOpcodeCounter.so -legacy-opcode-counter -analyze input_for_cc.bc
 # Run the pass through opt - New PM

include/OpcodeCounter.h

@@ -30,6 +30,9 @@ struct OpcodeCounter : public llvm::AnalysisInfoMixin<OpcodeCounter> {
                               llvm::FunctionAnalysisManager &);
 
   OpcodeCounter::Result generateOpcodeMap(llvm::Function &F);
+  // Part of the official API:
+  //  https://llvm.org/docs/WritingAnLLVMNewPMPass.html#required-passes
+  static bool isRequired() { return true; }
 
 private:
   // A special type used by analysis passes to provide an address that
@@ -46,6 +49,9 @@ class OpcodeCounterPrinter : public llvm::PassInfoMixin<OpcodeCounterPrinter> {
   explicit OpcodeCounterPrinter(llvm::raw_ostream &OutS) : OS(OutS) {}
   llvm::PreservedAnalyses run(llvm::Function &Func,
                               llvm::FunctionAnalysisManager &FAM);
+  // Part of the official API:
+  //  https://llvm.org/docs/WritingAnLLVMNewPMPass.html#required-passes
+  static bool isRequired() { return true; }
 
 private:
   llvm::raw_ostream &OS;