[PATCH] D66856: [Sema] Suppress -Wformat diagnostics for bool types when printed using %hhd

Thu Aug 29 06:02:36 PDT 2019

aaron.ballman added a comment.

In D66856#1649721 <https://reviews.llvm.org/D66856#1649721>, @erik.pilkington wrote:

> In D66856#1648809 <https://reviews.llvm.org/D66856#1648809>, @aaron.ballman wrote:
>
> > I'm wondering whether this should even warn pedantically. There are no format specifiers that apply directly to a `_Bool` datatype, so the user is left making a choice as to what specifier fits best. I think `hhd` and `hhu` are both equally reasonable types, as are the `d` and `u` groups directly -- I don't think we should warn on either of those specifiers with a `_Bool` argument. I could maybe see a case for pedantically diagnosing `h`, `l`, and `ll` prefixes with `_Bool` because that seems a bit type-confused to me. `c` as a specifier seems weird (given that false values will potentially terminate the string suddenly depending on terminal behavior, IIRC), but `lc` seems like type confusion again.
>
>
> Hmm... on second though I think the `l` and `ll` prefixes are worth -Wformat proper, since that can result in printing a half-initialized integer when the argument ends up on the stack (we generate `movl $0,(%rsp)` for the first stack argument).

Agreed.

> I think `%c` is probably also worth a -Wformat, it seems like it would almost certainly be a mistake.

That's my feeling as well. At the very least, -Wformat-pedantic.

> I suppose we could pedantically warn on `%hd`, but I'm not sure.

I think it makes sense to dos o.

> Who is the audience for -Wformat-pedantic?

To me, the audience are people who want to catch semantically-benign-but-logically-inconsistent format strings. e.g., it's not wrong to pass 'c' to something formatted as `%hd`, but it smells fishy to do so.

> Users that expect it to diagnose any pedantically-UB printf calls will already be disappointed, since the C standard seems to be quite a lot more restrictive then us (7.21.6.9p9 says: "If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined." (I'm assuming "correct type" means "same type")). So I think we should just concern ourselves with calls that would lead to actual problems at runtime, or are likely programmer errors, and `printf("%hd", (_Bool)0)` doesn't seem to meet either of those bars.

To me, actual problems at runtime belong in -Wformat and logical inconsistencies that don't cause runtime problems belong in -Wformat-pedantic. However, I think it's a defect that the C standard has no clearly UB-free way to print a _Bool value. I will bring this up on the reflectors to ask if we can clarify the standard here.

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