On Mon, May 18, 2020 at 8:38 PM Brian Gerst <brge...@gmail.com> wrote:
>
> On Mon, May 18, 2020 at 5:15 PM Nick Desaulniers
> <ndesaulni...@google.com> wrote:
> >
> > On Sun, May 17, 2020 at 8:29 AM Brian Gerst <brge...@gmail.com> wrote:
> > >
> > > The core percpu macros already have a switch on the data size, so the
> > > switch
> > > in the x86 code is redundant and produces more dead code.
> > >
> > > Also use appropriate types for the width of the instructions. This avoids
> > > errors when compiling with Clang.
> > >
> > > Signed-off-by: Brian Gerst <brge...@gmail.com>
> > > ---
> > > arch/x86/include/asm/percpu.h | 90 ++++++++++++++---------------------
> > > 1 file changed, 35 insertions(+), 55 deletions(-)
> > >
> > > diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
> > > index 89f918a3e99b..233c7a78d1a6 100644
> > > --- a/arch/x86/include/asm/percpu.h
> > > +++ b/arch/x86/include/asm/percpu.h
> > > @@ -117,37 +117,17 @@ extern void __bad_percpu_size(void);
> > > #define __pcpu_reg_imm_4(x) "ri" (x)
> > > #define __pcpu_reg_imm_8(x) "re" (x)
> > >
> > > -#define percpu_to_op(qual, op, var, val) \
> > > -do { \
> > > - typedef typeof(var) pto_T__; \
> > > - if (0) { \
> > > - pto_T__ pto_tmp__; \
> > > - pto_tmp__ = (val); \
> > > - (void)pto_tmp__; \
> > > - } \
> > > - switch (sizeof(var)) { \
> > > - case 1: \
> > > - asm qual (op "b %1,"__percpu_arg(0) \
> > > - : "+m" (var) \
> > > - : "qi" ((pto_T__)(val))); \
> > > - break; \
> > > - case 2: \
> > > - asm qual (op "w %1,"__percpu_arg(0) \
> > > - : "+m" (var) \
> > > - : "ri" ((pto_T__)(val))); \
> > > - break; \
> > > - case 4: \
> > > - asm qual (op "l %1,"__percpu_arg(0) \
> > > - : "+m" (var) \
> > > - : "ri" ((pto_T__)(val))); \
> > > - break; \
> > > - case 8: \
> > > - asm qual (op "q %1,"__percpu_arg(0) \
> > > - : "+m" (var) \
> > > - : "re" ((pto_T__)(val))); \
> > > - break; \
> > > - default: __bad_percpu_size(); \
> > > - } \
> > > +#define percpu_to_op(size, qual, op, _var, _val) \
> > > +do { \
> > > + __pcpu_type_##size pto_val__ = __pcpu_cast_##size(_val); \
> > > + if (0) { \
> > > + typeof(_var) pto_tmp__; \
> > > + pto_tmp__ = (_val); \
> > > + (void)pto_tmp__; \
> > > + } \
> >
> > Please replace the whole `if (0)` block with:
> > ```c
> > __same_type(_var, _val);
> > ```
> > from include/linux/compiler.h.
>
> The problem with __builtin_types_compatible_p() is that it considers
> unsigned long and u64 (aka unsigned long long) as different types even
> though they are the same width on x86-64. While this may be a good
> cleanup to look at in the future, it's not a simple drop-in
> replacement.
Does it trigger errors in this case?
It's interesting to know how this trick differs from
__builtin_types_compatible_p(). Might even be helpful to wrap this
pattern in a macro with a comment with the pros/cons of this approach
vs __same_type.
On the other hand, the use of `long` seems tricky in x86 code as x86
(32b) is ILP32 but x86_64 (64b) is LP64. So the use of `long` is
ambiguous in the sense that it's a different size depending on the
target ABI. Wouldn't it potentially be a bug for x86 kernel code to
use `long` percpu variables (or rather mix `long` and `long long` in
the same operation) in that case, since the sizes of the two would be
different for i386?
--
Thanks,
~Nick Desaulniers
