Messages in this thread | | | Date | Thu, 23 Aug 2018 16:40:22 +0100 | From | Brian Starkey <> | Subject | Re: [PATCH] drm/fourcc: Add DOC: overview comment |
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Hi Matthew,
On Thu, Aug 23, 2018 at 07:34:45AM -0700, Matthew Wilcox wrote: >On Wed, Aug 22, 2018 at 04:57:33PM +0100, Brian Starkey wrote: >> On Wed, Aug 22, 2018 at 05:11:55PM +0200, Daniel Vetter wrote: >> > On Wed, Aug 22, 2018 at 4:59 PM, Eric Engestrom >> > <eric.engestrom@intel.com> wrote: >> > > On Tuesday, 2018-08-21 17:44:17 +0100, Brian Starkey wrote: >> > > > On Tue, Aug 21, 2018 at 09:26:39AM -0700, Matthew Wilcox wrote: >> > > > > Can you turn them into enums? This seems to work ok: >> >> I'm not sure that swapping out explicit 32-bit unsigned integers for >> enums (unspecified width, signed integers) is necessarily a good idea, >> it seems like Bad Things could happen. >> >> The C spec says: >> >> "the value of an enumeration constant shall be an integer constant >> expression that has a value representable as an int" >> >> Which likely gives us 4 bytes to play with on all machines >> that run Linux, but if drm_fourcc.h is ever going to be some kind of >> standard reference, making it non-portable seems like a fail. >> >> And even if you do have 4 bytes in an enum, signed integers act >> differently from unsigned ones, and compilers do love to invoke the UB >> clause... > >I think you're exaggerating how much latitude C compilers have here. >Further down in 6.7.2.2, it says: > > Each enumerated type shall be compatible with char, a signed > integer type, or an unsigned integer type. The choice of type is > implementation-defined, but shall be capable of representing the values > of all the members of the enumeration. > >So if we include an integer which isn't representable in a plain int, >then the compiler _must_ choose a larger type.
I don't think so... the sentence I pasted says that including a value which isn't representable in a plain int would be illegal, and so the compiler doesn't _have_ to do anything (nasal demons, right?).
>It could choose a >signed-64-bit type rather than an unsigned-32-bit type, but I can't >imagine any compiler being quite so insane.
The paragraph about the implementation choosing a representation is separate from the valid range of values - the compiler can pick whatever storage it likes (smaller or even larger than an int), so long as that storage can fit all the defined values. However, providing a value in an enum definition which is not representable as an int would still be invalid (irrespective of how large the storage is) - it's a separate restriction.
Anyhow, I'm not dying to replace all the current definitions with enums, so if someone else wants to pick that up, be my guest.
Cheers, -Brian
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