aaron.ballman added inline comments.
================
Comment at: clang/test/Sema/constant-conversion.c:30
+ s.b = 1; // one-bit-warning {{implicit truncation from 'int' to a
one-bit wide bit-field changes value from 1 to -1}}
+ s.b = true; // no-warning (we suppress it manually to reduce false
positives)
+ s.b = false; // no-warning
----------------
bjope wrote:
> (Sorry for being late to the party, with post commit comments. I'm just
> trying to understand the reasoning about always suppressing the warning based
> on "true".)
>
> Isn't the code a bit suspicious when using true/false from stdbool.h, while
> using a signed bitfield?
>
> When doing things like
> ```
> (s.b == true) ? 1 : 0
> ```
> the result would always be zero if s.b is a signed 1-bit bitfield.
>
> So wouldn't it make more sense to actually use an unsigned bitfield (such as
> the bool type from stdbool.h) when the intent is to store a boolean value and
> using defines from stdbool.h?
>
> Is perhaps the idea that we will get warnings about `(s.b == true)` being
> redundant in situations like this, and then we do not need a warning on the
> bitfield assignment? Such a reasoning would actually make some sense, since
> `(s.b == true)` never would be true even when the bitfield is assigned a
> non-constant value, so we can't rely on catching the problem by only looking
> at bitfield assignments involving true/false.
> Isn't the code a bit suspicious when using true/false from stdbool.h, while
> using a signed bitfield?
Yes and no...
> When doing things like
> ```
> (s.b == true) ? 1 : 0
> ```
> the result would always be zero if s.b is a signed 1-bit bitfield.
You're correct, but that's a bit of a code smell because of `== true`.
> So wouldn't it make more sense to actually use an unsigned bitfield (such as
> the bool type from stdbool.h) when the intent is to store a boolean value and
> using defines from stdbool.h?
Yes, ideally.
> Is perhaps the idea that we will get warnings about (s.b == true) being
> redundant in situations like this, and then we do not need a warning on the
> bitfield assignment? Such a reasoning would actually make some sense, since
> (s.b == true) never would be true even when the bitfield is assigned a
> non-constant value, so we can't rely on catching the problem by only looking
> at bitfield assignments involving true/false.
The idea is more that someone using `true` is more likely to be treating the
field as a boolean and so they won't be comparing the bit-field against a
specific value, but instead testing it for its boolean value. This also unifies
the behavior between C and C++: https://godbolt.org/z/WKP4xcPzT
We don't currently issue a diagnostic on `== true`, but that sure seems like
something `-Wtautological-compare` should pick up on (for bit-fields in
general, not just for one-bit bit-fields): https://godbolt.org/z/Tj4711Ysc
(Note, godbolt seems to have a Clang trunk that's ~8 days old, so diagnostic
behavior doesn't match actual trunk yet. That caught me by surprise.)
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D132851/new/
https://reviews.llvm.org/D132851
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