Re: [PATCH] slab: Remove unnecessary __builtin_constant_p()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 22 Apr 2013 16:58:21 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:

> On Mon, 2013-04-22 at 13:44 -0700, Andrew Morton wrote:
> > On Wed, 17 Apr 2013 17:03:21 -0700 (PDT) David Rientjes <rientjes@google.com> wrote:
> > 
> > > On Wed, 17 Apr 2013, Steven Rostedt wrote:
> > > 
> > > > The slab.c code has a size check macro that checks the size of the
> > > > following structs:
> > > > 
> > > > struct arraycache_init
> > > > struct kmem_list3
> > > > 
> > > > The index_of() function that takes the sizeof() of the above two structs
> > > > and does an unnecessary __builtin_constant_p() on that. As sizeof() will
> > > > always end up being a constant making this always be true. The code is
> > > > not incorrect, but it just adds added complexity, and confuses users and
> > > > wastes the time of reviewers of the code, who spends time trying to
> > > > figure out why the builtin_constant_p() was used.
> > > > 
> > > > This patch is just a clean up that makes the index_of() code a little
> > > > bit less complex.
> > > > 
> > > > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> > > 
> > > Acked-by: David Rientjes <rientjes@google.com>
> > > 
> > > Adding Pekka to the cc.
> > 
> > I ducked this patch because it seemed rather pointless - but a little
> > birdie told me that there is a secret motivation which seems pretty
> > reasonable to me.  So I shall await chirp-the-second, which hopefully
> > will have a fuller and franker changelog ;)
> 
> <little birdie voice>
> The real motivation behind this patch was it prevents LLVM (Clang) from
> compiling the kernel. There's currently a bug in Clang where it can't
> determine if a variable is constant or not, so instead, when
> __builtin_constant_p() is used, it just treats it like it isn't a
> constant (always taking the slow *safe* path).
> 
> Unfortunately, the "confusing" code of slub.c that unnecessarily uses
> the __builtin_constant_p() will fail to compile if the variable passed
> in is not constant. As Clang will say constants are not constant at this
> point, the compile fails.
> 
> When looking into this, we found the only two users of the index_of()
> static function that has this issue, passes in size_of(), which will
> always be a constant, making the check redundant.

Looking at the current callers is cheating.  What happens if someone
adds another caller which doesn't use sizeof?

> Note, this is a bug in Clang that will hopefully be fixed soon. But for
> now, this strange redundant compile time check is preventing Clang from
> even testing the Linux kernel build.
> </little birdie voice>
> 
> And I still think the original change log has rational for the change,
> as it does make it rather confusing to what is happening there.

The patch made index_of() weaker!

It's probably all a bit academic, given that linux-next does

-/*
- * This function must be completely optimized away if a constant is passed to
- * it.  Mostly the same as what is in linux/slab.h except it returns an index.
- */
-static __always_inline int index_of(const size_t size)
-{
-	extern void __bad_size(void);
-
-	if (__builtin_constant_p(size)) {
-		int i = 0;
-
-#define CACHE(x) \
-	if (size <=x) \
-		return i; \
-	else \
-		i++;
-#include <linux/kmalloc_sizes.h>
-#undef CACHE
-		__bad_size();
-	} else
-		__bad_size();
-	return 0;
-}
-

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