[cfe-users] -Wconversion does not produce warnings when it should

[Sven Köhler via cfe-users]

Am 16.11.20 um 21:49 schrieb David Blaikie via cfe-users:
> On Mon, Nov 16, 2020 at 12:36 PM Sven Köhler via cfe-users
> <cfe-users at lists.llvm.org> wrote:
>>
>> Can you elaborate what is happening here?
> 
> I believe/would guess the goal is to reduce false positives (where a
> false positive is defined as "warning on code where the code does what
> the author intended").
> 
> In the taste of testb4 this code is "as good as" the unsigned int
> equivalent, say (unigned x(unsigned a, unsigned b) { return a + b; })
> - wraparound occurs in both cases, so the code perhaps isn't too
> surprising/the semantics of widening/narrowing conversions don't
> really impact the behavior.

I do appreciate your explanation. But I do not appreciate the compiler
guessing that wrap around is OK.

The C standard could also be interpreted as "wrap around only occurs
with types of rank equal to (unsigned) int and above" as all other types
are promoted to int prior to any arithmetic.
(My guess for the origin of this promotion would be that int is the
natural word size of the underlying CPU.)

With most modern architectures, int is 32 bit which is a reasonable size
for wraparounds. Unfortunately, int doesn't always have a fixed size.

> The uint16 version of the code and the
> unsigned int version of the code would appear to the user to work in
> the same way.

I would appreciate that only if my understanding of the C language was
that arithmetic on two uint16_t operands is performed with uint16_t
precision. But that is simply false. It is performed with 32 bit
precision (if int is 32 bit).

> But, yeah, once the expressions are sufficiently complicated, as in
> testa4, clang's attempt to silence benign cases of the warning gives
> up and you get warnings. It's best-effort sort of stuff.

I would vote for -Wconversion=strict or -Wimplicit-int-conversion=strict
where we get a warning for every int conversion that loses precision.

I would be very happy if there was a clang option to generate warnings
in such cases.


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