[LoopUnroll] Add RuntimeUnrollMultiExit to loop unroll options (NFC) (#124462)

Add an extra knob to RuntimeUnrollMultiExit to let backends control
whether to allow multi-exit unrolling on a per-loop basis.

This gives backends more fine-grained control on deciding if multi-exit
unrolling is profitable for a given loop and uarch. Similar to
4226e0a0c75.

PR: https://github.com/llvm/llvm-project/pull/124462

1 files changed, 6 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
index b0bc55c..524b268 100644
--- a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -461,7 +461,7 @@ CloneLoopBlocks(Loop *L, Value *NewIter, const bool UseEpilogRemainder,
 
 /// Returns true if we can profitably unroll the multi-exit loop L. Currently,
 /// we return true only if UnrollRuntimeMultiExit is set to true.
-static bool canProfitablyUnrollMultiExitLoop(
+static bool canProfitablyRuntimeUnrollMultiExitLoop(
     Loop *L, SmallVectorImpl<BasicBlock *> &OtherExits, BasicBlock *LatchExit,
     bool UseEpilogRemainder) {
 
@@ -583,7 +583,8 @@ bool llvm::UnrollRuntimeLoopRemainder(
     bool UseEpilogRemainder, bool UnrollRemainder, bool ForgetAllSCEV,
     LoopInfo *LI, ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC,
     const TargetTransformInfo *TTI, bool PreserveLCSSA,
-    unsigned SCEVExpansionBudget, Loop **ResultLoop) {
+    unsigned SCEVExpansionBudget, bool RuntimeUnrollMultiExit,
+    Loop **ResultLoop) {
   LLVM_DEBUG(dbgs() << "Trying runtime unrolling on Loop: \n");
   LLVM_DEBUG(L->dump());
   LLVM_DEBUG(UseEpilogRemainder ? dbgs() << "Using epilog remainder.\n"
@@ -632,8 +633,9 @@ bool llvm::UnrollRuntimeLoopRemainder(
     if (!PreserveLCSSA)
       return false;
 
-    if (!canProfitablyUnrollMultiExitLoop(L, OtherExits, LatchExit,
-                                          UseEpilogRemainder)) {
+    if (!RuntimeUnrollMultiExit &&
+        !canProfitablyRuntimeUnrollMultiExitLoop(L, OtherExits, LatchExit,
+                                                 UseEpilogRemainder)) {
       LLVM_DEBUG(
           dbgs()
           << "Multiple exit/exiting blocks in loop and multi-exit unrolling not "