[PATCH] D87034: [KnownBits] Implement accurate unsigned and signed max and min

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/lib/Support/KnownBits.cpp:113
+  LHSgeRHS.setLowBits(BitWidth - LHSgtRHS.countLeadingZeros());
+  if (LHSgeRHS.isAllOnesValue()) {
+    Res.Zero |= LHS.Zero;
----------------
nikic wrote:
> How does this differ from `LHS.getMinValue().uge(RHS.getMaxValue())`?
Generally I think this code should be structured something like this:

```
  if (LHS.getMinValue().uge(RHS.getMaxValue()))
    return LHS;
  if (RHS.getMinValue().uge(LHS.getMaxValue()))
    return RHS;

  // If the result of the umax is LHS then it must be greater than or equal to
  // the minimum possible value of RHS. Likewise for RHS. Any known bits that
  // are common to these two values are also known in the result.
  KnownBits L = LHS.makeGE(RHS.getMinValue());
  KnownBits R = RHS.makeGE(LHS.getMinValue());
  return KnownBits(L.Zero & R.Zero, L.One & R.One);
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D87034/new/

https://reviews.llvm.org/D87034