[LoopUnroll][NFCI] Clean up remainder followup metadata handling (#165272)

Followup metadata for remainder loops is handled by two implementations,
both added by 7244852:

1. `tryToUnrollLoop` in `LoopUnrollPass.cpp`.
2. `CloneLoopBlocks` in `LoopUnrollRuntime.cpp`.

As far as I can tell, 2 is useless: I added `assert(!NewLoopID)` for the
`NewLoopID` returned by the `makeFollowupLoopID` call, and it never
fails throughout check-all for my build.

Moreover, if 2 were useful, it appears it would have a bug caused by
7cd826a. That commit skips adding loop metadata to a new remainder
loop if the remainder loop itself is to be completely unrolled because
it will then no longer be a loop. However, that commit incorrectly
assumes that `UnrollRemainder` dictates complete unrolling of a
remainder loop, and thus it skips adding loop metadata even if the
remainder loop will be only partially unrolled.

To avoid further confusion here, this patch removes 2. check-all
continues to pass for my build. If 2 actually is useful, please advise
so we can create a test that covers that usage.

Near 2, this patch retains the `UnrollRemainder` guard on the
`setLoopAlreadyUnrolled` call, which adds `llvm.loop.unroll.disable` to
the remainder loop. That behavior exists both before and after
7cd826a. The logic appears to be that remainder loop unrolling
(whether complete or partial) is opt-in. That is, unless
`UnrollRemainder` is true, `UnrollRuntimeLoopRemainder` skips running
remainder loop unrolling, and `llvm.loop.unroll.disable` suppresses any
later attempt at it.

This patch also extends testing of remainder loop followup metadata to
be sure remainder loop partial unrolling is handled correctly by 1.

Author: jdenny-ornl

llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp

@@ -460,25 +460,10 @@ CloneLoopBlocks(Loop *L, Value *NewIter, const bool UseEpilogRemainder,
 
   Loop *NewLoop = NewLoops[L];
   assert(NewLoop && "L should have been cloned");
-  MDNode *LoopID = NewLoop->getLoopID();
-
-  // Only add loop metadata if the loop is not going to be completely
-  // unrolled.
-  if (UnrollRemainder)
-    return NewLoop;
-
-  std::optional<MDNode *> NewLoopID = makeFollowupLoopID(
-      LoopID, {LLVMLoopUnrollFollowupAll, LLVMLoopUnrollFollowupRemainder});
-  if (NewLoopID) {
-    NewLoop->setLoopID(*NewLoopID);
-
-    // Do not setLoopAlreadyUnrolled if loop attributes have been defined
-    // explicitly.
-    return NewLoop;
-  }
 
   // Add unroll disable metadata to disable future unrolling for this loop.
-  NewLoop->setLoopAlreadyUnrolled();
+  if (!UnrollRemainder)
+    NewLoop->setLoopAlreadyUnrolled();
   return NewLoop;
 }
 

llvm/test/Transforms/LoopUnroll/followup.ll

@@ -1,9 +1,20 @@
-; RUN: opt < %s -S -passes=loop-unroll -unroll-count=2 | FileCheck %s -check-prefixes=COUNT,COMMON
-; RUN: opt < %s -S -passes=loop-unroll -unroll-runtime=true -unroll-runtime-epilog=true  | FileCheck %s -check-prefixes=EPILOG,COMMON
-; RUN: opt < %s -S -passes=loop-unroll -unroll-runtime=true -unroll-runtime-epilog=false | FileCheck %s -check-prefixes=PROLOG,COMMON
-;
-; Check that followup-attributes are applied after LoopUnroll.
+; Check that followup attributes are applied after LoopUnroll.
 ;
+; We choose -unroll-count=3 because it produces partial unrolling of remainder
+; loops.  Complete unrolling would leave no remainder loop to which to copy
+; followup attributes.
+
+; DEFINE: %{unroll} = opt < %s -S -passes=loop-unroll -unroll-count=3
+; DEFINE: %{epilog} = %{unroll} -unroll-runtime -unroll-runtime-epilog=true
+; DEFINE: %{prolog} = %{unroll} -unroll-runtime -unroll-runtime-epilog=false
+; DEFINE: %{fc} = FileCheck %s -check-prefixes
+
+; RUN: %{unroll} | %{fc} COMMON,COUNT
+; RUN: %{epilog} | %{fc} COMMON,EPILOG,EPILOG-NO-UNROLL
+; RUN: %{prolog} | %{fc} COMMON,PROLOG,PROLOG-NO-UNROLL
+; RUN: %{epilog} -unroll-remainder | %{fc} COMMON,EPILOG,EPILOG-UNROLL
+; RUN: %{prolog} -unroll-remainder | %{fc} COMMON,PROLOG,PROLOG-UNROLL
+
 target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
 
 define i32 @test(ptr nocapture %a, i32 %n) nounwind uwtable readonly {
@@ -36,15 +47,17 @@ for.end:                                          ; preds = %for.body, %entry
 ; COMMON-LABEL: @test(
 
 
-; COUNT: br i1 %exitcond.1, label %for.end.loopexit, label %for.body, !llvm.loop ![[LOOP:[0-9]+]]
+; COUNT: br i1 %exitcond.2, label %for.end.loopexit, label %for.body, !llvm.loop ![[LOOP:[0-9]+]]
 
 ; COUNT: ![[FOLLOWUP_ALL:[0-9]+]] = !{!"FollowupAll"}
 ; COUNT: ![[FOLLOWUP_UNROLLED:[0-9]+]] = !{!"FollowupUnrolled"}
 ; COUNT: ![[LOOP]] = distinct !{![[LOOP]], ![[FOLLOWUP_ALL]], ![[FOLLOWUP_UNROLLED]]}
 
 
-; EPILOG: br i1 %niter.ncmp.7, label %for.end.loopexit.unr-lcssa, label %for.body, !llvm.loop ![[LOOP_0:[0-9]+]]
-; EPILOG: br i1 %epil.iter.cmp, label %for.body.epil, label %for.end.loopexit.epilog-lcssa, !llvm.loop ![[LOOP_2:[0-9]+]]
+; EPILOG: br i1 %niter.ncmp.2, label %for.end.loopexit.unr-lcssa, label %for.body, !llvm.loop ![[LOOP_0:[0-9]+]]
+; EPILOG-NO-UNROLL: br i1 %epil.iter.cmp, label %for.body.epil, label %for.end.loopexit.epilog-lcssa, !llvm.loop ![[LOOP_2:[0-9]+]]
+; EPILOG-UNROLL: br i1 %epil.iter.cmp, label %for.body.epil.1, label %for.end.loopexit.epilog-lcssa
+; EPILOG-UNROLL: br i1 %epil.iter.cmp.1, label %for.body.epil, label %for.end.loopexit.epilog-lcssa, !llvm.loop ![[LOOP_2:[0-9]+]]
 
 ; EPILOG: ![[LOOP_0]] = distinct !{![[LOOP_0]], ![[FOLLOWUP_ALL:[0-9]+]], ![[FOLLOWUP_UNROLLED:[0-9]+]]}
 ; EPILOG: ![[FOLLOWUP_ALL]] = !{!"FollowupAll"}
@@ -53,8 +66,10 @@ for.end:                                          ; preds = %for.body, %entry
 ; EPILOG: ![[FOLLOWUP_REMAINDER]] = !{!"FollowupRemainder"}
 
 
-; PROLOG:  br i1 %prol.iter.cmp, label %for.body.prol, label %for.body.prol.loopexit.unr-lcssa, !llvm.loop ![[LOOP_0:[0-9]+]]
-; PROLOG:  br i1 %exitcond.7, label %for.end.loopexit.unr-lcssa, label %for.body, !llvm.loop ![[LOOP_2:[0-9]+]]
+; PROLOG-UNROLL:  br i1 %prol.iter.cmp, label %for.body.prol.1, label %for.body.prol.loopexit.unr-lcssa
+; PROLOG-UNROLL:  br i1 %prol.iter.cmp.1, label %for.body.prol, label %for.body.prol.loopexit.unr-lcssa, !llvm.loop ![[LOOP_0:[0-9]+]]
+; PROLOG-NO-UNROLL:  br i1 %prol.iter.cmp, label %for.body.prol, label %for.body.prol.loopexit.unr-lcssa, !llvm.loop ![[LOOP_0:[0-9]+]]
+; PROLOG:  br i1 %exitcond.2, label %for.end.loopexit.unr-lcssa, label %for.body, !llvm.loop ![[LOOP_2:[0-9]+]]
 
 ; PROLOG: ![[LOOP_0]] = distinct !{![[LOOP_0]], ![[FOLLOWUP_ALL:[0-9]+]], ![[FOLLOWUP_REMAINDER:[0-9]+]]}
 ; PROLOG: ![[FOLLOWUP_ALL]] = !{!"FollowupAll"}