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

[Hari Limaye via llvm-commits]

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@@ -1528,6 +1563,107 @@ static PHINode *getRecurrenceVar(Value *VarX, Instruction *DefX,
   return nullptr;
 }
 
+/// Return true if the idiom is detected in the loop.
+///
+/// Additionally:
+/// 1) \p CntInst is set to the instruction Counting Leading Zeros (CTLZ)
+///       or nullptr if there is no such.
+/// 2) \p CntPhi is set to the corresponding phi node
+///       or nullptr if there is no such.
+/// 3) \p InitX is set to the value whose CTLZ could be used.
+/// 4) \p DefX is set to the instruction calculating Loop exit condition.
+/// 5) \p Threshold is set to the constant involved in the unsigned less-than
+///       comparison.
+///
+/// The core idiom we are trying to detect is:
+/// \code
+///    if (x0 < 2)
+///      goto loop-exit // the predcondition of the loop
+///    cnt0 = init-val
+///    do {
+///      x = phi (x0, x.next);   //PhiX
+///      cnt = phi (cnt0, cnt.next)
+///
+///      cnt.next = cnt + 1;
+///       ...
+///      x.next = x >> 1;   // DefX
+///    } while (x < 4)
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hazzlim wrote:

Done.

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