[Mesa-dev] [PATCH] util: Unbreak usage of assert()/debug_assert() inside expressions.

[Jose Fonseca]

On 12/12/14 16:30, Patrick Baggett wrote:
>
>
> On Fri, Dec 12, 2014 at 10:17 AM, Roland Scheidegger <sroland at vmware.com
> <mailto:sroland at vmware.com>> wrote:
>
>     Am 12.12.2014 um 15:09 schrieb Jose Fonseca:
>      > From: José Fonseca <jfonseca at vmware.com <mailto:jfonseca at vmware.com>>
>      >
>      > f0ba7d897d1c22202531acb70f134f2edc30557d made debug_assert()/assert()
>      > unsafe for expressions, but only now with u_atomic.h started to
>     rely on
>      > them for Windows this became an issue.
>      >
>      > This fixes non-debug builds with MSVC.
>      > ---
>      >  src/gallium/auxiliary/util/u_debug.h | 2 +-
>      >  1 file changed, 1 insertion(+), 1 deletion(-)
>      >
>      > diff --git a/src/gallium/auxiliary/util/u_debug.h
>     b/src/gallium/auxiliary/util/u_debug.h
>      > index badd5e2..4c22fdf 100644
>      > --- a/src/gallium/auxiliary/util/u_debug.h
>      > +++ b/src/gallium/auxiliary/util/u_debug.h
>      > @@ -185,7 +185,7 @@ void _debug_assert_fail(const char *expr,
>      >  #ifdef DEBUG
>      >  #define debug_assert(expr) ((expr) ? (void)0 :
>     _debug_assert_fail(#expr, __FILE__, __LINE__, __FUNCTION__))
>      >  #else
>      > -#define debug_assert(expr) do { } while (0 && (expr))
>      > +#define debug_assert(expr) (void)(0 && (expr))
>      >  #endif
>      >
>      >
>      >
>
>
> Just for my own education, can someone explain what the need for
> `debug_assert()` to have any expansion of `expr` at all? Rather, what
> breaks with something like:
>
>    #define debug_assert(expr) ((void)0)
>
>

Did you read the above mentioned 
f0ba7d897d1c22202531acb70f134f2edc30557d commit message?  It's explained 
there:

$ git show f0ba7d897d1c22202531acb70f134f2edc30557d
commit f0ba7d897d1c22202531acb70f134f2edc30557d
Author: Keith Whitwell <keithw at vmware.com>
Date:   Mon Dec 28 11:05:06 2009 +0000

     util: better fix for unused variable warnings with asserts

     Modify the non-debug (ie disabled) version of assert to expose the
     value in the expression to the compiler (avoiding the unused variable
     messages) while still expanding to a noop.

diff --git a/src/gallium/auxiliary/util/u_debug.h 
b/src/gallium/auxiliary/util/u_debug.h
index abd834c..facc30a 100644
--- a/src/gallium/auxiliary/util/u_debug.h
+++ b/src/gallium/auxiliary/util/u_debug.h
@@ -188,7 +188,7 @@ void _debug_assert_fail(const char *expr,
  #ifdef DEBUG
  #define debug_assert(expr) ((expr) ? (void)0 : 
_debug_assert_fail(#expr, __FILE__, __LINE__, __FUNCTION__))
  #else
-#define debug_assert(expr) ((void)0)
+#define debug_assert(expr) do { } while (0 && (expr))
  #endif



Jose