[llvm] [SCEV] Use getConstantMultiple in to get divisibility info from guards. (PR #162617)

[Nikita Popov via llvm-commits]

================
@@ -15749,51 +15749,11 @@ void ScalarEvolution::LoopGuards::collectFromBlock(
       return RewriteMap.lookup_or(S, S);
     };
 
-    // Check for the SCEV expression (A /u B) * B while B is a constant, inside
-    // \p Expr. The check is done recuresively on \p Expr, which is assumed to
-    // be a composition of Min/Max SCEVs. Return whether the SCEV expression (A
-    // /u B) * B was found, and return the divisor B in \p DividesBy. For
-    // example, if Expr = umin (umax ((A /u 8) * 8, 16), 64), return true since
-    // (A /u 8) * 8 matched the pattern, and return the constant SCEV 8 in \p
-    // DividesBy.
-    std::function<bool(const SCEV *, const SCEV *&)> HasDivisibiltyInfo =
-        [&](const SCEV *Expr, const SCEV *&DividesBy) {
-          if (auto *Mul = dyn_cast<SCEVMulExpr>(Expr)) {
-            if (Mul->getNumOperands() != 2)
-              return false;
-            auto *MulLHS = Mul->getOperand(0);
-            auto *MulRHS = Mul->getOperand(1);
-            if (isa<SCEVConstant>(MulLHS))
-              std::swap(MulLHS, MulRHS);
-            if (auto *Div = dyn_cast<SCEVUDivExpr>(MulLHS))
-              if (Div->getOperand(1) == MulRHS) {
-                DividesBy = MulRHS;
-                return true;
-              }
-          }
-          if (auto *MinMax = dyn_cast<SCEVMinMaxExpr>(Expr))
-            return HasDivisibiltyInfo(MinMax->getOperand(0), DividesBy) ||
-                   HasDivisibiltyInfo(MinMax->getOperand(1), DividesBy);
-          return false;
-        };
-
-    // Return true if Expr known to divide by \p DividesBy.
-    std::function<bool(const SCEV *, const SCEV *&)> IsKnownToDivideBy =
-        [&](const SCEV *Expr, const SCEV *DividesBy) {
-          if (SE.getURemExpr(Expr, DividesBy)->isZero())
-            return true;
-          if (auto *MinMax = dyn_cast<SCEVMinMaxExpr>(Expr))
-            return IsKnownToDivideBy(MinMax->getOperand(0), DividesBy) &&
-                   IsKnownToDivideBy(MinMax->getOperand(1), DividesBy);
-          return false;
-        };
-
     const SCEV *RewrittenLHS = GetMaybeRewritten(LHS);
     const SCEV *DividesBy = nullptr;
-    if (HasDivisibiltyInfo(RewrittenLHS, DividesBy))
-      // Check that the whole expression is divided by DividesBy
-      DividesBy =
-          IsKnownToDivideBy(RewrittenLHS, DividesBy) ? DividesBy : nullptr;
+    const APInt &Multiple = SE.getConstantMultiple(RewrittenLHS);
+    if (!Multiple.isOne())
+      DividesBy = SE.getConstant(Multiple);
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nikic wrote:

As a possible followup, it would probably make sense to eliminate the DividesBy null case to avoid the `DividesBy ?` checks below. The existing code should already handle the 1 case. It doesn't look like the conversion from APInt back to SCEV is really needed either.

https://github.com/llvm/llvm-project/pull/162617