[llvm] Fix exact backedge count algorithm in Scalar-Evolution (PR #92560)

[via llvm-commits]

https://github.com/mrdaybird created https://github.com/llvm/llvm-project/pull/92560

Fixes #92554 (std::reverse will auto-vectorize now)

When calculating number of times a exit condition containing a comparison is executed, we mostly assume that RHS of comparison should be loop invariant, but it may be another add-recurrence.

In that case, we can try the computation with `LHS = LHS - RHS` and `RHS = 0`.

For now, I have only changed `howManyLessThans`, so it only affects `X < Y` type of conditions.
Other conditions will also need to be modified. 
@fhahn 

>From 250adcbc8456d31a8601528b43f586b618a90aa2 Mon Sep 17 00:00:00 2001
From: Vaibhav Pathak <pathakvaibhav at protonmail.com>
Date: Fri, 17 May 2024 19:29:36 +0530
Subject: [PATCH] Update howManyLessThans

---
 llvm/lib/Analysis/ScalarEvolution.cpp | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 515b9d0744f6e..9bf5bf80b4570 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -12941,12 +12941,16 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
       return RHS;
   }
 
-  // When the RHS is not invariant, we do not know the end bound of the loop and
-  // cannot calculate the ExactBECount needed by ExitLimit. However, we can
-  // calculate the MaxBECount, given the start, stride and max value for the end
-  // bound of the loop (RHS), and the fact that IV does not overflow (which is
-  // checked above).
   if (!isLoopInvariant(RHS, L)) {
+    // If RHS is an add recurrence, try again with lhs=lhs-rhs and rhs=0
+    if(auto RHSAddRec = dyn_cast<SCEVAddRecExpr>(RHS)){
+       return howManyLessThans(getMinusSCEV(IV, RHSAddRec), 
+        getZero(IV->getType()), L, true, ControlsOnlyExit, AllowPredicates);
+    }
+    // If we cannot calculate ExactBECount, we can calculate the MaxBECount, 
+    // given the start, stride and max value for the end bound of the 
+    // loop (RHS), and the fact that IV does not overflow (which is
+    // checked above).
     const SCEV *MaxBECount = computeMaxBECountForLT(
         Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
     return ExitLimit(getCouldNotCompute() /* ExactNotTaken */, MaxBECount,