[PATCH] D142271: [ValueTracking] Add KnownBits patterns `xor(x, x - 1)` and `and(x, -x)` for knowing upper bits to be zero

[Jay Foad via Phabricator via llvm-commits]

foad added inline comments.


================
Comment at: llvm/lib/Analysis/ValueTracking.cpp:1087
+      if (match(I, m_c_BinOp(m_Value(X), m_Neg(m_Deferred(X))))) {
+        // -(-x) == x so pick whichever we can get a better result with.
+        if (Known.countMaxTrailingZeros() <= Known2.countMaxTrailingZeros())
----------------
A less "if"fy way to implement this would be:
```
  Known = Known.blsi();
  Known2 = Known2.blsi();
  // should probably have a helper for the following two lines, similar to KnownBits::commonBits
  Known.Zero |= Known2.Zero;
  Known.One |= Known2.One;
```
(But I'm still sceptical that it gives any practical benefit.)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D142271/new/

https://reviews.llvm.org/D142271