Yes,
#define ALPHA_OFFSET 0x3
should turn off the diagnostic. (We skip hex decimals).
Dňa 19. 8. 2019 o 20:06 užívateľ Arthur O'Dwyer <arthur.j.odw...@gmail.com>
napísal:
>> On Mon, Aug 19, 2019 at 1:53 PM Nico Weber via cfe-commits
>> <cfe-commits@lists.llvm.org> wrote:
>
>> Hi,
>>
>> this results in a false positive on webrtc, on this code:
>>
>> https://cs.chromium.org/chromium/src/third_party/libwebp/src/enc/picture_csp_enc.c?q=picture_csp_enc.c&sq=package:chromium&dr&l=1002
>>
>> The code does this:
>>
>> #ifdef WORDS_BIGENDIAN
>> #define ALPHA_OFFSET 0 // uint32_t 0xff000000 is 0xff,00,00,00 in memory
>> #else
>> #define ALPHA_OFFSET 3 // uint32_t 0xff000000 is 0x00,00,00,ff in memory
>> #endif
>>
>> // ...
>>
>> const uint8_t* const argb = (const uint8_t*)picture->argb;
>> const uint8_t* const a = argb + (0 ^ ALPHA_OFFSET);
>> const uint8_t* const r = argb + (1 ^ ALPHA_OFFSET);
>> const uint8_t* const g = argb + (2 ^ ALPHA_OFFSET);
>> const uint8_t* const b = argb + (3 ^ ALPHA_OFFSET);
>>
>> The idea is to get bytes 0, 1, 2, 3 as a, r, g, b if ALPHA_OFFSET is 0, or
>> bytes 3, 2, 1, 0 if ALPHA_OFFSET is 3.
>>
>> Maybe this shouldn't fire if the rhs is a macro?
>
> Does it show a fix-it that suggests replacing
> #define ALPHA_OFFSET 3
> with
> #define ALPHA_OFFSET 0x3
> ? That would be the quickest way to shut up the warning, if I understand
> correctly. The use of hexadecimal or octal or binary indicates unambiguously
> that you mean to do a bitwise operation (xor), not an arithmetic one (pow).
>
> If the use of a macro here prevents the hex workaround from working, then I'd
> call that a bug in the diagnostic.
>
> It would be great if Clang could do what humans do instinctively, and see
> that this "2 ^ FOO" is actually part of a larger pattern — "0 ^ FOO, 1 ^ FOO,
> 2 ^ FOO, 3 ^ FOO" — and therefore it probably isn't wrong unless the three
> surrounding lines are also wrong. However, I'm pretty sure that Clang isn't
> set up to discover "patterns" like this in general. It sees "2 ^
> LITERAL_EXPONENT" and gives a warning regardless of the surrounding lines.
>
> –Arthur
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