[llvm] [SCEV] Add predicate in SolveLinEq to ensure B is a multiple of A. (PR #108777)

[David Sherwood via llvm-commits]

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@@ -10137,8 +10140,17 @@ static const SCEV *SolveLinEquationWithOverflow(const APInt &A, const SCEV *B,
   //
   // B is divisible by D if and only if the multiplicity of prime factor 2 for B
   // is not less than multiplicity of this prime factor for D.
-  if (SE.getMinTrailingZeros(B) < Mult2)
-    return SE.getCouldNotCompute();
+  if (SE.getMinTrailingZeros(B) < Mult2) {
+    if (!Predicates)
+      return SE.getCouldNotCompute();
+    // Try to add a predicate ensuring B is a multiple of A.
+    const SCEV *URem = SE.getURemExpr(B, SE.getConstant(A));
+    const SCEV *Zero = SE.getZero(B->getType());
+    // Avoid adding a predicate that is known to be false.
+    if (SE.isKnownPredicate(CmpInst::ICMP_NE, URem, Zero))
+      return SE.getCouldNotCompute();
+    Predicates->insert(SE.getEqualPredicate(URem, Zero));
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david-arm wrote:

Just thinking in terms of compile-time here - can you get the equivalent result if you first compute `SE.getEqualPredicate(URem, Zero)` and then test if the answer is constant-false? I don't know if `isKnownPredicate` is expensive or not and whether or not we care.

https://github.com/llvm/llvm-project/pull/108777