Re: [Xen-devel] [PATCH] xen: make sure that likely and unlikely convert the expression to a boolean

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-04-07 at 13:54 +0100, Jan Beulich wrote:
> >>> On 07.04.14 at 14:42, <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Mon, 2014-04-07 at 13:36 +0100, Jan Beulich wrote:
> >> >>> On 07.04.14 at 13:07, <ian.campbell@xxxxxxxxxx> wrote:
> >> > According to http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html 
> >> > __builtin_expect has the prototype:
> >> >     long __builtin_expect (long exp, long c)
> >> > 
> >> > If sizeof(exp) > sizeof(long) then this will effectively mask off the 
> >> > top 
> > bits
> >> > of exp, meaning that the if in "if (unlikey(x))" will see the masked 
> > version,
> >> > which might be false when true was expected, likely has the same issue.
> >> > 
> >> > With the x86_32 hypervisor no longer existing this is mostly likely to 
> > affect
> >> > arm32 builds. A quick grep however shows that all the existing arm32 
> >> > uses 
> > of
> >> > both likely and unlikely already pass a boolean. I noticed this with an 
> >> > as 
> > yet
> >> > unposted patch which did not have this property.
> >> 
> >> Good catch,
> > 
> > I tore out some hair before I got there of course ;-)
> > 
> >>  except that at least in the 4.2 tree we still care for the
> >> x86_32 hypervisor, and I already spotted a case having the same
> >> issue (in mtrr_var_range_msr_set()). I.e. for the purposes of
> >> backporting it might be better to make the statement above a little
> >> less tailored to arm32.
> > 
> > That's fine by me. Do you mean for me to extend it now or did you want
> > to change it as part of the backport process? Either is fine by me.
> > 
> > How about:
> >         This is mostly likely to affect x86_32 and arm32 builds.  x86_32
> >         is not present on 4.3 onwards and a quick grep of current
> >         staging shows that all the existing arm32... <etc as before>
> > ?
> 
> Yes, that reads fine for both -unstable and the backport.

Thanks. Here's what I ended up applying:

From e5545fb6d0dc5e2c48b2e450d18246d9bc1ae35b Mon Sep 17 00:00:00 2001
From: Ian Campbell <ian.campbell@xxxxxxxxxx>
Date: Mon, 7 Apr 2014 12:07:04 +0100
Subject: [PATCH] xen: make sure that likely and unlikely convert the
 expression to a boolean

According to http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
__builtin_expect has the prototype:
    long __builtin_expect (long exp, long c)

If sizeof(exp) > sizeof(long) then this will effectively mask off the top bits
of exp, meaning that the if in "if (unlikey(x))" will see the masked version,
which might be false when true was expected, likely has the same issue.

This is mostly likely to affect x86_32 and arm32 builds.  x86_32 is not
present on 4.3 onwards and a quick grep of current staging shows that all the
existing arm32  uses of both likely and unlikely already pass a boolean. I
noticed this with an as yet unposted patch which did not have this property.

Also the defintion of likely might not have had the expected affect for cases
where a true value > 1 might be passed.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Keir Fraser <keir@xxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
---
 xen/include/xen/compiler.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
index 6e07990..4b3472d 100644
--- a/xen/include/xen/compiler.h
+++ b/xen/include/xen/compiler.h
@@ -7,8 +7,8 @@
 
 #define barrier()     __asm__ __volatile__("": : :"memory")
 
-#define likely(x)     __builtin_expect((x),1)
-#define unlikely(x)   __builtin_expect((x),0)
+#define likely(x)     __builtin_expect(!!(x),1)
+#define unlikely(x)   __builtin_expect(!!(x),0)
 
 #define inline        __inline__
 #define always_inline __inline__ __attribute__ ((always_inline))
-- 
1.7.10.4




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