[llvm] [InstCombine] Add additional known bits info for self multiply (PR #151202)

[Yingwei Zheng via llvm-commits]

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@@ -888,11 +888,27 @@ KnownBits KnownBits::mul(const KnownBits &LHS, const KnownBits &RHS,
   Res.Zero |= (~BottomKnown).getLoBits(ResultBitsKnown);
   Res.One = BottomKnown.getLoBits(ResultBitsKnown);
 
-  // If we're self-multiplying then bit[1] is guaranteed to be zero.
-  if (NoUndefSelfMultiply && BitWidth > 1) {
-    assert(Res.One[1] == 0 &&
-           "Self-multiplication failed Quadratic Reciprocity!");
-    Res.Zero.setBit(1);
+  // Self multiplying
+  if (NoUndefSelfMultiply) {
+    // bit[1] is guaranteed to be zero.
+    if (BitWidth > 1) {
+      assert(Res.One[1] == 0 &&
+             "Self-multiplication failed Quadratic Reciprocity!");
+      Res.Zero.setBit(1);
+    }
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dtcxzyw wrote:

```suggestion
```
It is covered by the logic below.


https://github.com/llvm/llvm-project/pull/151202