On Mon, Aug 19, 2019 at 04:08:43PM +0200, Christophe Leroy wrote:
> Le 19/08/2019 à 15:23, Segher Boessenkool a écrit :
> >On Mon, Aug 19, 2019 at 01:06:31PM +0000, Christophe Leroy wrote:
> >>Note that we keep using an assembly text using "twi 31, 0, 0" for
> >>inconditional traps because GCC drops all code after
> >>__builtin_trap() when the condition is always true at build time.
> >
> >As I said, it can also do this for conditional traps, if it can prove
> >the condition is always true.
>
> But we have another branch for 'always true' and 'always false' using
> __builtin_constant_p(), which don't use __builtin_trap(). Is there
> anything wrong with that ?:
The compiler might not realise it is constant when it evaluates the
__builtin_constant_p, but only realises it later. As the documentation
for the builtin says:
A return of 0 does not indicate that the
value is _not_ a constant, but merely that GCC cannot prove it is a
constant with the specified value of the '-O' option.
(and there should be many more and more serious warnings here).
> #define BUG_ON(x) do { \
> if (__builtin_constant_p(x)) { \
> if (x) \
> BUG(); \
> } else { \
> if (x) \
> __builtin_trap(); \
> BUG_ENTRY("", 0); \
> } \
> } while (0)
I think it may work if you do
#define BUG_ON(x) do { \
if (__builtin_constant_p(x)) { \
if (x) \
BUG(); \
} else { \
BUG_ENTRY("", 0); \
if (x) \
__builtin_trap(); \
} \
} while (0)
or even just
#define BUG_ON(x) do { \
BUG_ENTRY("", 0); \
if (x) \
__builtin_trap(); \
} \
} while (0)
if BUG_ENTRY can work for the trap insn *after* it.
> >Can you put the bug table asm *before* the __builtin_trap maybe? That
> >should make it all work fine... If you somehow can tell what machine
> >instruction is that trap, anyway.
>
> And how can I tell that ?
I don't know how BUG_ENTRY works exactly.
Segher
