[SCEV] Strengthen inference of RHS > Start in howManyLessThans

Split off from D105216 to simplify review.  Rewritten with a lambda to be easier to follow.  Comments clarified.

Sorry for no test case, this is tricky to exercise with the current structure of the code.  It's about to be hit more frequently in a follow up patch, and the change itself is simple.

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 5cb96a2..c6a1328 100644 (file)
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -11742,10 +11742,29 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
   if (isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(OrigStart, Stride), OrigRHS))
     BECount = BECountIfBackedgeTaken;
   else {
+    auto canProveRHSGreaterThanEqualStart = [&]() {
+      auto CondGE = IsSigned ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE;
+      if (isLoopEntryGuardedByCond(L, CondGE, OrigRHS, OrigStart))
+        return true;
+
+      // (RHS > Start - 1) implies RHS >= Start.
+      // * "RHS >= Start" is trivially equivalent to "RHS > Start - 1" if
+      //   "Start - 1" doesn't overflow.
+      // * For signed comparison, if Start - 1 does overflow, it's equal
+      //   to INT_MAX, and "RHS >s INT_MAX" is trivially false.
+      // * For unsigned comparison, if Start - 1 does overflow, it's equal
+      //   to UINT_MAX, and "RHS >u UINT_MAX" is trivially false.
+      //
+      // FIXME: Should isLoopEntryGuardedByCond do this for us?
+      auto CondGT = IsSigned ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT;
+      auto *StartMinusOne = getAddExpr(OrigStart,
+                                       getMinusOne(OrigStart->getType()));
+      return isLoopEntryGuardedByCond(L, CondGT, OrigRHS, StartMinusOne);
+    };
+
     // If we know that RHS >= Start in the context of loop, then we know that
     // max(RHS, Start) = RHS at this point.
-    if (isLoopEntryGuardedByCond(
-            L, IsSigned ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE, OrigRHS, OrigStart))
+    if (canProveRHSGreaterThanEqualStart())
       End = RHS;
     else
       End = IsSigned ? getSMaxExpr(RHS, Start) : getUMaxExpr(RHS, Start);