[bugpoint] Try to reduce passes after reducing everything.

[fhahn]

Summary

In some cases, we fail to reduce the pass list earlier because of
complex pass dependencies, but we can reduce it after we simplified the
reproducer.

An example of that is PR43474, which can limit the crash to
-loop-interchange. Adding a test case would require at least 2
interacting Loop passes I think.

[reames]

LGTM.

On a general note, I think we should be iterating to a fixed point between the various reductions. There's no reason we should ever be returning something another run of bugpoint could reduce. We'd need to provide a "fast" mode of course, but that seems worthwhile anyways.

[fhahn]

In D69236#1723865, @reames wrote:

LGTM.

On a general note, I think we should be iterating to a fixed point between the various reductions. There's no reason we should ever be returning something another run of bugpoint could reduce. We'd need to provide a "fast" mode of course, but that seems worthwhile anyways.

That's an interesting suggestion!

My main motivation for the initial set of recent bugpoint patches was to see how easy it would be to fix a few pain points I've seen in the past (and to kill some time on a plane). The initial patches were quite small, because I do not have any experience with the bug point code, the test coverage is really limited and I wanted to avoid breaking bugpoint. But I think once the first set of patches are committed and baked a bit, I would like to explore the direction suggested a bit more.

llvm/tools/bugpoint/CrashDebugger.cpp

Error BugDriver::debugOptimizerCrash(const std::string &ID) {
   }
 
   outs() << "\n*** Found crashing pass"
          << (PassesToRun.size() == 1 ? ": " : "es: ")
          << getPassesString(PassesToRun) << '\n';
 
   EmitProgressBitcode(*Program, ID);
 
-  return DebugACrash(*this, TestForOptimizerCrash);
+  auto Res = DebugACrash(*this, TestForOptimizerCrash);
+  if (Res || DontReducePassList)
+    return Res;
+  // Try to reduce the pass list again. This covers additional cases
+  // we failed to reduce earlier, because of more complex pass dependencies
+  // triggering the crash.
+  auto SecondRes = ReducePassList(*this).reduceList(PassesToRun);
+  if (Error E = SecondRes.takeError())
+    return E;
+  outs() << "\n*** Found crashing pass"
+         << (PassesToRun.size() == 1 ? ": " : "es: ")
+         << getPassesString(PassesToRun) << '\n';
+
+  EmitProgressBitcode(getProgram(), "reduced-simplified");
+  return Res;
 }
 
 static bool TestForCodeGenCrash(const BugDriver &BD, Module *M) {
   if (Error E = BD.compileProgram(*M)) {
     if (VerboseErrors)
       errs() << toString(std::move(E)) << "\n";
     else {
       consumeError(std::move(E));