[llvm] [LoopIdiom] Support 'shift until less-than' idiom (PR #95002)

[Hari Limaye via llvm-commits]

================
@@ -1818,35 +1961,108 @@ bool LoopIdiomRecognize::recognizeAndInsertFFS() {
     ZeroCheck = true;
   }
 
-  // Check if CTLZ / CTTZ intrinsic is profitable. Assume it is always
-  // profitable if we delete the loop.
-
-  // the loop has only 6 instructions:
+  // FFS idiom loop has only 6 instructions:
   //  %n.addr.0 = phi [ %n, %entry ], [ %shr, %while.cond ]
   //  %i.0 = phi [ %i0, %entry ], [ %inc, %while.cond ]
   //  %shr = ashr %n.addr.0, 1
   //  %tobool = icmp eq %shr, 0
   //  %inc = add nsw %i.0, 1
   //  br i1 %tobool
+  size_t IdiomCanonicalSize = 6;
+  if (!isProfitableToInsertFFS(IntrinID, InitX, ZeroCheck, IdiomCanonicalSize))
+    return false;
 
-  const Value *Args[] = {InitX,
-                         ConstantInt::getBool(InitX->getContext(), ZeroCheck)};
+  transformLoopToCountable(IntrinID, PH, CntInst, CntPhi, InitX, DefX,
+                           DefX->getDebugLoc(), ZeroCheck,
+                           IsCntPhiUsedOutsideLoop);
+  return true;
+}
 
-  // @llvm.dbg doesn't count as they have no semantic effect.
-  auto InstWithoutDebugIt = CurLoop->getHeader()->instructionsWithoutDebug();
-  uint32_t HeaderSize =
-      std::distance(InstWithoutDebugIt.begin(), InstWithoutDebugIt.end());
+/// Recognize CTLZ or CTTZ idiom in a non-countable loop and convert the loop
+/// to countable (with CTLZ / CTTZ trip count). If CTLZ / CTTZ inserted as a new
+/// trip count returns true; otherwise, returns false.
+bool LoopIdiomRecognize::recognizeAndInsertFFS() {
+  // Give up if the loop has multiple blocks or multiple backedges.
+  if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
+    return false;
 
-  IntrinsicCostAttributes Attrs(IntrinID, InitX->getType(), Args);
-  InstructionCost Cost =
-    TTI->getIntrinsicInstrCost(Attrs, TargetTransformInfo::TCK_SizeAndLatency);
-  if (HeaderSize != IdiomCanonicalSize &&
-      Cost > TargetTransformInfo::TCC_Basic)
+  Intrinsic::ID IntrinID;
+  Value *InitX;
+  Instruction *DefX = nullptr;
+  PHINode *CntPhi = nullptr;
+  Instruction *CntInst = nullptr;
+
+  if (!detectShiftUntilZeroIdiom(CurLoop, *DL, IntrinID, InitX, CntInst, CntPhi,
+                                 DefX))
+    return false;
+
+  return insertFFS(IntrinID, InitX, DefX, CntPhi, CntInst);
+}
+
+bool LoopIdiomRecognize::recognizeShiftUntilLessThan() {
+  // Give up if the loop has multiple blocks or multiple backedges.
+  if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
+    return false;
+
+  Intrinsic::ID IntrinID;
+  Value *InitX;
+  Instruction *DefX = nullptr;
+  PHINode *CntPhi = nullptr;
+  Instruction *CntInst = nullptr;
+
+  unsigned LoopThreshold;
+  if (!detectShiftUntilLessThanIdiom(CurLoop, *DL, IntrinID, InitX, CntInst,
+                                     CntPhi, DefX, LoopThreshold))
+    return false;
+
+  if (LoopThreshold == 2) {
+    // Treat as regular FFS.
+    return insertFFS(IntrinID, InitX, DefX, CntPhi, CntInst);
+  }
+
+  // Look for Floor Log2 Idiom.
+  if (LoopThreshold != 4)
+    return false;
+
+  // Abort if CntPhi is used outside of the loop.
+  for (User *U : CntPhi->users())
+    if (!CurLoop->contains(cast<Instruction>(U))) {
----------------
hazzlim wrote:

Done.

https://github.com/llvm/llvm-project/pull/95002