nathanchance added a comment.
> In D139114#4053166 <https://reviews.llvm.org/D139114#4053166>, @nathanchance
> wrote:
>
>> For what it’s worth, this triggers a LOT in the Linux kernel for the pattern
>> that @MaskRay pointed out:
>
> Out of curiosity, has it found any true positives that you can tell?
I have not been able to look too closely since I am on mobile until Wednesday
but of the instances I examined, they all appear to be false positives (or
maybe true positives that we really just don’t care about) .
The Linux kernel has a macro `BIT(x)`, which is just `1UL << x`, so any
negation is going to be a really large number but expected to truncate when
assigning to a narrower width variable because the bit being cleared might only
be in the first few positions. That is at least the case with the really noisy
instance of the warning that I mentioned above
#define FWNODE_FLAG_LINKS_ADDED BIT(0)
#define FWNODE_FLAG_NOT_DEVICE BIT(1)
#define FWNODE_FLAG_INITIALIZED BIT(2)
#define FWNODE_FLAG_NEEDS_CHILD_BOUND_ON_ADD BIT(3)
#define FWNODE_FLAG_BEST_EFFORT BIT(4)
struct fwnode_handle {
struct fwnode_handle *secondary;
const struct fwnode_operations *ops;
struct device *dev;
struct list_head suppliers;
struct list_head consumers;
u8 flags;
};
…
if (initialized)
fwnode->flags |= FWNODE_FLAG_INITIALIZED;
else
fwnode->flags &= ~FWNODE_FLAG_INITIALIZED;
>> It would be nice if this was split into a separate flag that could be
>> disabled, as -Wconstant-conversion has not been very noisy up until this
>> point.
>
> We can definitely split it into a separate flag. Alternatively, we could
> investigate disabling the warning for that code pattern (or moving that bit
> under its own flag).
Right, I would suspect that emitting this for `&=` will rarely show anything
interesting. I’ll have to filter the warnings to see if there are any other
instances with other operators that appear problematic. I’ll see how easy that
is or if there is a simple way to omit those instances that can be added to the
patch itself for testing.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D139114/new/
https://reviews.llvm.org/D139114
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