[llvm] [SCEV] Init BE vars to CouldNotCompute in howManyLT (NFC) (PR #222892)

[Ramkumar Ramachandra via llvm-commits]

https://github.com/artagnon updated https://github.com/llvm/llvm-project/pull/222892

>From cc2f6f89aaf4eb567f903c663708175242e57c0c Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <artagnon at tenstorrent.com>
Date: Fri, 11 Sep 2026 10:06:15 +0100
Subject: [PATCH 1/2] [SCEV] Init BE vars to CouldNotCompute in howManyLT (NFC)

This avoids the risk of initializing ExitLimit with nullptr BE
variables, and cast-asserts on nullptr.
---
 llvm/lib/Analysis/ScalarEvolution.cpp | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 06f350fd2179b..f7063661403ef 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -13538,8 +13538,8 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
       return RHS;
   }
 
-  const SCEV *End = nullptr, *BECount = nullptr,
-             *BECountIfBackedgeTaken = nullptr;
+  const SCEV *End = nullptr, *BECount = getCouldNotCompute(),
+             *BECountIfBackedgeTaken = getCouldNotCompute();
   if (!isLoopInvariant(RHS, L)) {
     const auto *RHSAddRec = dyn_cast<SCEVAddRecExpr>(RHS);
     if (PositiveStride && RHSAddRec != nullptr && RHSAddRec->getLoop() == L &&
@@ -13717,7 +13717,7 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
       BECount = getUDivExpr(Numerator, Stride);
     }
 
-    if (!BECount) {
+    if (isa<SCEVCouldNotCompute>(BECount)) {
       auto canProveRHSGreaterThanEqualStart = [&]() {
         auto CondGE = IsSigned ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE;
         const SCEV *GuardedRHS = applyLoopGuards(OrigRHS, L);
@@ -13780,8 +13780,7 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
   bool MaxOrZero = false;
   if (isa<SCEVConstant>(BECount)) {
     ConstantMaxBECount = BECount;
-  } else if (BECountIfBackedgeTaken &&
-             isa<SCEVConstant>(BECountIfBackedgeTaken)) {
+  } else if (isa<SCEVConstant>(BECountIfBackedgeTaken)) {
     // If we know exactly how many times the backedge will be taken if it's
     // taken at least once, then the backedge count will either be that or
     // zero.

>From 1c83b6a834bc5e3c4b075f0b38979caa2f9bc375 Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <artagnon at tenstorrent.com>
Date: Fri, 11 Sep 2026 10:32:23 +0100
Subject: [PATCH 2/2] [SCEV] Fall thru in null-BE loop-variant case in
 howManyLT (NFC)

The check for null BE is unnecessary since we initialize BE variables to
CouldNotCompute, and can be handled by the common code that follows.
---
 llvm/lib/Analysis/ScalarEvolution.cpp | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index f7063661403ef..55c84af598acb 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -13583,16 +13583,6 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
         }
       }
     }
-    if (BECount == nullptr) {
-      // 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,
-                       MaxBECount, false /*MaxOrZero*/, Predicates);
-    }
   } else {
     // Let End = max(RHS,Start).  We use the expression (End-Start)/Stride to
     // describe the backedge count: if the backedge is taken at least once then