[all-commits] [llvm/llvm-project] 80079c: [libc] Fix new warning in DyadicFloat::as_mantissa...

[Simon Tatham via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 80079c9c2f55ddfc5b1d5235d5ed6a2981fa6a97
      https://github.com/llvm/llvm-project/commit/80079c9c2f55ddfc5b1d5235d5ed6a2981fa6a97
  Author: Simon Tatham <simon.tatham at arm.com>
  Date:   2025-03-14 (Fri, 14 Mar 2025)

  Changed paths:
    M libc/src/__support/FPUtil/dyadic_float.h

  Log Message:
  -----------
  [libc] Fix new warning in DyadicFloat::as_mantissa_type_rounded (#131148)

The affected line of code converts a float's exponent from `int` to
`size_t`, negating it in the process. Following clang commit
773e88f9d61399c, this provokes a warning, presumably because the
conversion goes wrong if `size_t` is wider than `int` and the input
value is `INT_MIN`: negating it within the `int` type is undefined
behavior, with the likely (though not guaranteed) effect of leaving it
still at `INT_MIN` and then sign-extending that on promotion to
`size_t`.

This fix adds a cast so that the promotion to `size_t` happens _before_
the negation, so that the negative input value will _always_ be
sign-extended, and then the negation will make it positive again.

(I don't believe this case will actually come up. `DyadicFloat` is a
helper system used in processing ordinary float formats, so nothing is
expected to generate an exponent with even a 16-bit absolute value, let
alone 31. But it's as easy to fix it to be robust as it is to just
suppress the warning!)



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