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Subject: clang - odd macro / conditional expansion behaviour?
From: Adrian Chadd <adrian@freebsd.org>
To: Dimitry Andric <dim@freebsd.org>
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I've hit this rather amusing clang behaviour:


In file included from
/usr/home/adrian/work/freebsd/ath/head/src/sys/modules/ath/../../contrib/sys/dev/ath/ath_hal/ar9300/ar9300_eeprom.c:21:
/usr/home/adrian/work/freebsd/ath/head/src/sys/modules/ath/../../contrib/sys/dev/ath/ath_hal/ar9300/ar9300template_generic.h:107:3:
error: implicit conversion from 'int' to
      'u_int8_t' (aka 'unsigned char') changes value from -477 to 35
[-Werror,-Wconstant-conversion]
                FREQ2FBIN(2412, 1),
                ^~~~~~~~~~~~~~~~~~
/usr/home/adrian/work/freebsd/ath/head/src/sys/modules/ath/../../contrib/sys/dev/ath/ath_hal/ar9300/ar9300eep.h:136:65:
note: expanded from macro 'FREQ2FBIN'
    (((y) == HAL_FREQ_BAND_2GHZ) ? ((x) - 2300) : (((x) - 4800) / 5))
                                                   ~~~~~~~~~~~~~^~~

.. now, HAL_FREQ_BAND_2GHZ is defined here as:

typedef enum {
        HAL_FREQ_BAND_5GHZ      = 0,
        HAL_FREQ_BAND_2GHZ      = 1,
} HAL_FREQ_BAND;

And:

#define FREQ2FBIN(x,y) \
    (((y) == HAL_FREQ_BAND_2GHZ) ? ((x) - 2300) : (((x) - 4800) / 5))

And all of those macros here have '1' in the second field, yet when
they're expanded they evaluate as if it were false.

So, what's going on? :-) gcc is fine with this. :)

Thanks!



Adrian