linux-kernel - Re: [PATCH 3/3] powerpc: use __builtin

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Date:   Mon, 19 Aug 2019 09:37:00 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 3/3] powerpc: use __builtin_trap() in BUG/WARN macros.

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