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SubjectRe: [thisops uV2 04/10] x86: Support for this_cpu_add,sub,dec,inc_return
On Mon, 29 Nov 2010, Mathieu Desnoyers wrote:

> > > > +#define percpu_add_return_op(var, val) \
> > > > +({ \
> > > > + typedef typeof(var) pao_T__; \
> > > > + typeof(var) pfo_ret__ = val; \
> > > > + if (0) { \
> > > > + pao_T__ pao_tmp__; \
> > > > + pao_tmp__ = (val); \
> > > > + (void)pao_tmp__; \
> > > > + } \
> > >
> > > OK, I'm dumb: why is the above needed ?
> >
> > Ensure that the compiler agrees that *var and val are compatible. Taken
> > over from percpu_add_op().
>
> Isn't that the purpose of __builtin_types_compatible_p(t1, t2) ?

We also have a __same_type() macro in linux/compiler.h... But if I use
that the kernel build fails. I guess the check is too strict.

---
arch/x86/include/asm/percpu.h | 6 +-----
1 file changed, 1 insertion(+), 5 deletions(-)

Index: linux-2.6/arch/x86/include/asm/percpu.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/percpu.h 2010-11-29 12:46:50.000000000 -0600
+++ linux-2.6/arch/x86/include/asm/percpu.h 2010-11-29 12:52:25.000000000 -0600
@@ -127,11 +127,7 @@ do { \
typedef typeof(var) pao_T__; \
const int pao_ID__ = (__builtin_constant_p(val) && \
((val) == 1 || (val) == -1)) ? (val) : 0; \
- if (0) { \
- pao_T__ pao_tmp__; \
- pao_tmp__ = (val); \
- (void)pao_tmp__; \
- } \
+ BUILD_BUG_ON(!__same_type(var, val)); \
switch (sizeof(var)) { \
case 1: \
if (pao_ID__ == 1) \



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