[llvm] [SCEV] Avoid overflow in howManyGreaterThans. (PR #217744)

[Mikael Holmen via llvm-commits]

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@@ -13860,12 +13863,21 @@ ScalarEvolution::ExitLimit ScalarEvolution::howManyGreaterThans(
       return End;
   }
 
-  // Compute ((Start - End) + (Stride - 1)) / Stride.
-  // FIXME: This can overflow. Holding off on fixing this for now;
-  // howManyGreaterThans will hopefully be gone soon.
-  const SCEV *One = getOne(Stride->getType());
-  const SCEV *BECount = getUDivExpr(
-      getAddExpr(getMinusSCEV(Start, End), getMinusSCEV(Stride, One)), Stride);
+  const SCEV *Delta = getMinusSCEV(Start, End);
+  const SCEV *BECount;
+  if (MayAddOverflow) {
+    // The ceiling division instead needs Start >= End, so that (Start - End) is
+    // the exact unsigned distance between them.
+    if (!isLoopEntryGuardedByCond(
+            L, IsSigned ? ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE, Start, End))
+      return getCouldNotCompute();
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mikaelholmen wrote:

I saw that with b96b66160 the regressions I mentioned went away again. Nice and thanks!

https://github.com/llvm/llvm-project/pull/217744