Fix loop unrolling initialization in the new pass manager

[echristo]

Summary

In the new pass manager use PTO.LoopUnrolling to determine when and how
we will unroll loops. Also comment a few occasions where we need to
know whether or not we're forcing the unwinder or not.

Testcase is in clang because it's controlling how the loop optimizer
is being set up and there's no other way to trigger the behavior.

[chandlerc]

Adding Alina so she is aware of the change and can comment if she spots anything I'm missing...

I think this is fine to go in as-is to fix the immediate issues. It'd be good to find a way to write an LLVM test for the unrolling functionality change and the unroll-and-jam change I comment on below. But I'm OK w/ those landing in follow-up changes, no need to hold up the simple fix here.

clang/test/Misc/loop-opt-setup.c

2

I'd use -fno-experimental-new-pass-manager here so we don't lose coverage when flipping defaults.

llvm/lib/Passes/PassBuilder.cpp

953

We don't have a test for this sadly.... Not sure the best way to add one.

[asbirlea]

Maybe elaborate in the patch description what determine when and how we will unroll loops. means?
e.g.:
"The default before and after this patch is for LoopUnroll to be enabled, and for it to use a cost model to determine whether to unroll the loop (OnlyWhenForced = false). Before this patch, disabling loop unroll would not run the LoopUnroll pass. After this patch, the LoopUnroll pass is being run, but it restricts unrolling to only the loops marked by a pragma (OnlyWhenForced = true).
In addition, this patch disables the UnrollAndJam pass when disabling unrolling."

Otherwise LGTM.

clang/test/Misc/loop-opt-setup.c

+// RUN: %clang -O1 -fexperimental-new-pass-manager -fno-unroll-loops -S -o - %s -emit-llvm | FileCheck %s
+// RUN: %clang -O1 -fno-unroll-loops -S -o - %s -emit-llvm | FileCheck %s
+extern int a[16];
+int b = 0;
+int foo(void) {
+#pragma unroll
+  for (int i = 0; i < 16; ++i)
+    a[i] = b += 2;
+  return b;
+}
+// CHECK-NOT: br i1
+

llvm/lib/Passes/PassBuilder.cpp

   if ((Phase != ThinLTOPhase::PreLink || !PGOOpt ||
        PGOOpt->Action != PGOOptions::SampleUse) &&
       PTO.LoopUnrolling)
-    LPM2.addPass(
-        LoopFullUnrollPass(Level, false, PTO.ForgetAllSCEVInLoopUnroll));
+    LPM2.addPass(LoopFullUnrollPass(Level, /*OnlyWhenForced=*/false,
+                                    PTO.ForgetAllSCEVInLoopUnroll));
 
   for (auto &C : LoopOptimizerEndEPCallbacks)
     C(LPM2, Level);

   // combiner for cleanup here so that the unrolling and LICM can be pipelined
   // across the loop nests.
   // We do UnrollAndJam in a separate LPM to ensure it happens before unroll
-  if (EnableUnrollAndJam) {
+  if (EnableUnrollAndJam && PTO.LoopUnrolling) {
     OptimizePM.addPass(
         createFunctionToLoopPassAdaptor(LoopUnrollAndJamPass(Level)));
   }
-  if (PTO.LoopUnrolling)
-    OptimizePM.addPass(LoopUnrollPass(
-        LoopUnrollOptions(Level, false, PTO.ForgetAllSCEVInLoopUnroll)));
+  OptimizePM.addPass(LoopUnrollPass(
+      LoopUnrollOptions(Level, /*OnlyWhenForced=*/!PTO.LoopUnrolling,
+                        PTO.ForgetAllSCEVInLoopUnroll)));
   OptimizePM.addPass(WarnMissedTransformationsPass());
   OptimizePM.addPass(InstCombinePass());
   OptimizePM.addPass(RequireAnalysisPass<OptimizationRemarkEmitterAnalysis, Function>());