[screen-devel] suspicious code in encoding.c

[Vincent Lefevre]

When compiling encoding.c from the screen-v4 branch:

encoding.c: In function ‘AddUtf8’:
display.h:322:18: warning: overflow in conversion from ‘int’ to ‘char’ changes 
value from ‘c >> 12 & 12288^ 248’ to ‘-8’ [-Woverflow]
  322 |     *D_obufp++ = (c);  \
      |                  ^
encoding.c:682:4: note: in expansion of macro ‘AddChar’
  682 |    AddChar((c & 0x3000000) >> 12 ^ 0xf8);
      |    ^~~~~~~
encoding.c: In function ‘ToUtf8’:
encoding.c:727:13: warning: overflow in conversion from ‘int’ to ‘char’ changes 
value from ‘c >> 12 & 12288^ 248’ to ‘-8’ [-Woverflow]
  727 |      *p++ = (c & 0x3000000) >> 12 ^ 0xf8;
      |             ^

Indeed, the code is really strange:

  (c & 0x3000000) >> 12 ^ 0xf8

will be stored in a char. So why dealing with bits more significant
than the 8 bits of a char?

The master branch has a similar code, but with int replaced by
uint32_t, so that the warning disappears just because of the unsigned
type (the behavior becomes fully specified by the C standard instead
of being implementation defined).

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)