Re: [PATCH 3/3] powerpc: use __builtin_trap() in BUG/WARN macros.

[Christophe Leroy]

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 ?

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