On Mon, Aug 19, 2019 at 05:05:46PM +0200, Christophe Leroy wrote:
> Le 19/08/2019 à 16:37, Segher Boessenkool a écrit :
> >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.
>
> So you mean GCC would not be able to prove that
> __builtin_constant_p(cond) is always true but it would be able to prove
> that if (cond) is always true ?
Not sure what you mean, sorry.
> And isn't there a away to tell GCC that '__builtin_trap()' is
> recoverable in our case ?
No, GCC knows that a trap will never fall through.
> >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)
>
> It doesn't work:
You need to make a BUG_ENTRY so that it refers to the *following* trap
instruction, if you go this way.
> >I don't know how BUG_ENTRY works exactly.
>
> It's basic, maybe too basic: it adds an inline asm with a label, and
> adds a .long in the __bug_table section with the address of that label.
>
> When putting it after the __builtin_trap(), I changed it to using the
> address before the one of the label which is always the twxx instruction
> as far as I can see.
>
> #define BUG_ENTRY(insn, flags, ...) \
> __asm__ __volatile__( \
> "1: " insn "\n" \
> ".section __bug_table,\"aw\"\n" \
> "2:\t" PPC_LONG "1b, %0\n" \
> "\t.short %1, %2\n" \
> ".org 2b+%3\n" \
> ".previous\n" \
> : : "i" (__FILE__), "i" (__LINE__), \
> "i" (flags), \
> "i" (sizeof(struct bug_entry)), \
> ##__VA_ARGS__)
#define MY_BUG_ENTRY(lab, flags) \
__asm__ __volatile__( \
".section __bug_table,\"aw\"\n" \
"2:\t" PPC_LONG "%4, %0\n" \
"\t.short %1, %2\n" \
".org 2b+%3\n" \
".previous\n" \
: : "i" (__FILE__), "i" (__LINE__), \
"i" (flags), \
"i" (sizeof(struct bug_entry)), \
"i" (lab))
called as
#define BUG_ON(x) do { \
MY_BUG_ENTRY(&&lab, 0); \
lab: if (x) \
__builtin_trap(); \
} while (0)
not sure how reliable that works -- *if* it works, I just typed that in
without testing or anything -- but hopefully you get the idea.
Segher
