[llvm] [InstSimplify] Fold `u/sdiv exact (mul nsw/nuw X, C), C --> X` when C is not a power of 2 (PR #76445)

[Yingwei Zheng via llvm-commits]

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@@ -1189,14 +1189,26 @@ static Value *simplifyDiv(Instruction::BinaryOps Opcode, Value *Op0, Value *Op1,
   if (Value *V = simplifyDivRem(Opcode, Op0, Op1, Q, MaxRecurse))
     return V;
 
-  // If this is an exact divide by a constant, then the dividend (Op0) must have
-  // at least as many trailing zeros as the divisor to divide evenly. If it has
-  // less trailing zeros, then the result must be poison.
   const APInt *DivC;
-  if (IsExact && match(Op1, m_APInt(DivC)) && DivC->countr_zero()) {
-    KnownBits KnownOp0 = computeKnownBits(Op0, /* Depth */ 0, Q);
-    if (KnownOp0.countMaxTrailingZeros() < DivC->countr_zero())
-      return PoisonValue::get(Op0->getType());
+  if (IsExact && match(Op1, m_APInt(DivC))) {
+    // If this is an exact divide by a constant, then the dividend (Op0) must
+    // have at least as many trailing zeros as the divisor to divide evenly. If
+    // it has less trailing zeros, then the result must be poison.
+    if (DivC->countr_zero()) {
+      KnownBits KnownOp0 = computeKnownBits(Op0, /* Depth */ 0, Q);
+      if (KnownOp0.countMaxTrailingZeros() < DivC->countr_zero())
+        return PoisonValue::get(Op0->getType());
+    }
+
+    // udiv exact (mul nsw X, C), C --> X
+    // sdiv exact (mul nuw X, C), C --> X
+    // where C is not a power of 2.
+    Value *X;
+    if (!DivC->isPowerOf2() &&
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dtcxzyw wrote:

I have double checked that before pushing. Wouldn't it be guarded by `Q.IIQ.isExact`?
https://github.com/llvm/llvm-project/blob/fe94ae83ecf356fd2d63b7cd15f37aaafdbc14ef/llvm/lib/Analysis/InstructionSimplify.cpp#L6988-L6995

https://github.com/llvm/llvm-project/pull/76445