On Mon, Aug 19, 2019 at 05:05:46PM +0200, Christophe Leroy wrote:
Le 19/08/2019 Ã 16:37, Segher Boessenkool a ÃcritÂ:
On Mon, Aug 19, 2019 at 04:08:43PM +0200, Christophe Leroy wrote:
Le 19/08/2019 Ã 15:23, Segher Boessenkool a ÃcritÂ:
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.