[llvm] [LoopUnroll] Rotate loop to make it countable for runtime unrolling (PR #146540)

[Florian Hahn via llvm-commits]

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@@ -484,8 +485,27 @@ llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
 
   assert(ULO.Count > 0);
 
-  // All these values should be taken only after peeling because they might have
-  // changed.
+  if (ULO.Runtime && SE) {
+    BasicBlock *OrigHeader = L->getHeader();
+    BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
+    // Rotate loop if it makes it countable (for later unrolling)
+    if (BI && !BI->isUnconditional() &&
+        isa<SCEVCouldNotCompute>(SE->getExitCount(L, L->getLoopLatch())) &&
+        !isa<SCEVCouldNotCompute>(SE->getExitCount(L, OrigHeader))) {
+      LLVM_DEBUG(dbgs() << "  Rotating loop to make the loop countable.\n");
+      SimplifyQuery SQ{OrigHeader->getDataLayout()};
+      SQ.TLI = nullptr;
+      SQ.DT = DT;
+      SQ.AC = AC;
+      llvm::LoopRotation(L, LI, TTI, AC, DT, SE, nullptr /*MemorySSAUpdater*/,
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fhahn wrote:

Would it be possible to first perform the profitability checks on the unrotated form, and then only rotate if runtime unrolling is profitable?

Otherwise we may rotate w/o actually unrolling the loop. Also, if we do it here, we need to indicate the the IR has been changed even if nothing gets unrolled

https://github.com/llvm/llvm-project/pull/146540