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 ?
And isn't there a away to tell GCC that '__builtin_trap()' is
recoverable in our case ?
(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)
It doesn't work:
void test_bug1(unsigned long long a)
{
BUG_ON(a);
}
00000090 <test_bug1>:
90: 7c 63 23 78 or r3,r3,r4
94: 0f 03 00 00 twnei r3,0
98: 4e 80 00 20 blr
RELOCATION RECORDS FOR [__bug_table]:
OFFSET TYPE VALUE
00000084 R_PPC_ADDR32 .text+0x00000090
As you see, the relocation in __bug_table points to the 'or' and not to
the 'twnei'.
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.
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__)
Christophe
