[LoopDist] Add metadata for checking post process state of distribute… (#153902)

…d loops

Add a count of the number of partitions LoopDist made when distributing
a loop in meta data, then check for loops which are already distributed
to prevent reprocessing.

We see this happen on some spec apps, LD is on by default at SiFive.

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopDistribute.cpp b/llvm/lib/Transforms/Scalar/LoopDistribute.cpp
index 27d3004..1099aa3 100644
--- a/llvm/lib/Transforms/Scalar/LoopDistribute.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopDistribute.cpp
@@ -113,6 +113,8 @@ static cl::opt<bool> EnableLoopDistribute(
     cl::desc("Enable the new, experimental LoopDistribution Pass"),
     cl::init(false));
 
+static const char *DistributedMetaData = "llvm.loop.isdistributed";
+
 STATISTIC(NumLoopsDistributed, "Number of loops distributed");
 
 namespace {
@@ -826,6 +828,8 @@ public:
           {LLVMLoopDistributeFollowupAll, LLVMLoopDistributeFollowupFallback},
           "llvm.loop.distribute.", true);
       LVer.getNonVersionedLoop()->setLoopID(UnversionedLoopID);
+      addStringMetadataToLoop(LVer.getNonVersionedLoop(), DistributedMetaData,
+                              true);
     }
 
     // Create identical copies of the original loop for each partition and hook
@@ -986,6 +990,13 @@ static bool runImpl(Function &F, LoopInfo *LI, DominatorTree *DT,
   for (Loop *L : Worklist) {
     LoopDistributeForLoop LDL(L, &F, LI, DT, SE, LAIs, ORE);
 
+    // Do not reprocess loops we already distributed
+    if (getOptionalBoolLoopAttribute(L, DistributedMetaData).value_or(false)) {
+      LLVM_DEBUG(
+          dbgs() << "LDist: Distributed loop guarded for reprocessing\n");
+      continue;
+    }
+
     // If distribution was forced for the specific loop to be
     // enabled/disabled, follow that.  Otherwise use the global flag.
     if (LDL.isForced().value_or(EnableLoopDistribute))