[PATCH] D144218: [Clang] [AVR] Fix USHRT_MAX for 16-bit int.

Thu Feb 23 05:32:35 PST 2023

aaron.ballman added inline comments.

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Comment at: clang/lib/Headers/limits.h:55
 #define UCHAR_MAX (__SCHAR_MAX__*2  +1)
-#define USHRT_MAX (__SHRT_MAX__ *2  +1)
+#define USHRT_MAX (__SHRT_MAX__ * 2U + 1U)
 #define UINT_MAX  (__INT_MAX__  *2U +1U)
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mysterymath wrote:
> aaron.ballman wrote:
> > It's worth double-checking that this still gives the correct type for the macro:
> > 
> > C2x 5.2.4.2.1p2: For all unsigned integer types for which <limits.h> or <stdint.h> define a macro with suffix _WIDTH holding its width N, there is a macro with suffix _MAX holding the maximal value 2N − 1 that is representable by the type and that has the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. ...
> Ah, thanks; it hadn't occurred to me that the type of the expression would be specified in the standard. It could be either `unsigned int` or `int`, depending on the target.
> 
> The most straightforward approach I could think of to produce the right type is:
> 1) Perform the arithmetic in `unsigned int` to produce the right value
> 2) Cast to `unsigned short` to produce the right type
> 3) Directly perform integer promotion using unary plus
> 
> The use of unary plus is a bit odd here, but it seems like the most direct way to express the logic, and the overall expression seems less fragile than the `#if` alternative. I've added a comment to explain this as well.
Now the trouble is with the cast operation, because that runs afoul of 5.2.4.2.1p1: The values given below shall be replaced by constant expressions suitable for use in conditional expression inclusion preprocessing directives. ...

https://godbolt.org/z/K9cs66sdK

I'm almost wondering if the most direct solution is for `__SHRT_MAX__` to be generated with or without the `U` suffix depending on target.

We should probably use this as an opportunity to add more exhaustive testing for all the _MIN and _MAX macros to ensure the type properties hold. I was playing around with something like this: https://godbolt.org/z/o7KjY3asW

Repository:
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https://reviews.llvm.org/D144218