gcc-7 has an "optimization" pass that completely screws up, and
generates the code expansion for the (impossible) case of calling
ilog2() with a zero constant, even when the code gcc compiles does not
actually have a zero constant.

And we try to generate a compile-time error for anybody doing ilog2() on
a constant where that doesn't make sense (be it zero or negative).  So
now gcc7 will fail the build due to our sanity checking, because it
created that constant-zero case that didn't actually exist in the source
code.

There's a whole long discussion on the kernel mailing about how to work
around this gcc bug.  The gcc people themselevs have discussed their
"feature" in

   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785

but it's all water under the bridge, because while it looked at one
point like it would be solved by the time gcc7 was released, that was
not to be.

So now we have to deal with this compiler braindamage.

And the only simple approach seems to be to just delete the code that
tries to warn about bad uses of ilog2().

So now "ilog2()" will just return 0 not just for the value 1, but for
any non-positive value too.

It's not like I can recall anybody having ever actually tried to use
this function on any invalid value, but maybe the sanity check just
meant that such code never made it out in public.
Reported-by: NLaura Abbott <labbott@redhat.com>
Cc: John Stultz <john.stultz@linaro.org>,
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

The function order_base_2() is defined (according to the comment block)
as returning zero on input zero, but subsequently passes the input into
roundup_pow_of_two(), which is explicitly undefined for input zero.

This has gone unnoticed until now, but optimization passes in GCC 7 may
produce constant folded function instances where a constant value of
zero is passed into order_base_2(), resulting in link errors against the
deliberately undefined '____ilog2_NaN'.

So update order_base_2() to adhere to its own documented interface.

[ See

     http://marc.info/?l=linux-kernel&m=147672952517795&w=2

  and follow-up discussion for more background. The gcc "optimization
  pass" is really just broken, but now the GCC trunk problem seems to
  have escaped out of just specially built daily images, so we need to
  work around it in mainline.    - Linus ]
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Exactly like roundup_pow_of_two(1), the rounddown version was buggy for
the case of a compile-time constant '1' argument.  Probably because it
originated from the same code, sharing history with the roundup version
from before the bugfix (for that one, see commit 1a06a52e: "Fix
roundup_pow_of_two(1)").

However, unlike the roundup version, the fix for rounddown is to just
remove the broken special case entirely.  It's simply not needed - the
generic code

    1UL << ilog2(n)

does the right thing for the constant '1' argment too.  The only reason
roundup needed that special case was because rounding up does so by
subtracting one from the argument (and then adding one to the result)
causing the obvious problems with "ilog2(0)".

But rounddown doesn't do any of that, since ilog2() naturally truncates
(ie "rounds down") to the right rounded down value.  And without the
ilog2(0) case, there's no reason for the special case that had the wrong
value.

tl;dr: rounddown_pow_of_two(1) should be 1, not 0.
Acked-by: NDmitry Torokhov <dtor@vmware.com>
Cc: stable@kernel.org
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Given a number of places in the tree that need to calculate this value
explicitly, might as well just create a macro for it.

(akpm: must be implemented as a macro for callee typeof() usage)
Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

To go along with the existing "roundup_pow_of_two" routine, add one for
rounding down since that operation appears to crop up on a regular basis in
the source tree.

[m.kozlowski@tuxland.pl: fix unbalanced parentheses]
Signed-off-by: NRobert P. J. Day <rpjday@mindspring.com>
Signed-off-by: NMariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

1 is a power of two, therefore roundup_pow_of_two(1) should return 1. It does
in case the argument is a variable but in case it's a constant it behaves
wrong and returns 0. Probably nobody ever did it so this was never noticed.
Signed-off-by: NRolf Eike Beer <eike-kernel@sf-tec.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Single typo correction in include/linux/log2.h.
Signed-off-by: NRobert P. J. Day <rpjday@mindspring.com>
Signed-Off-By: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NAdrian Bunk <bunk@stusta.de>

Add the inline function "is_power_of_2()" to log2.h, where the value
zero is *not* considered to be a power of two.
Signed-off-by: NRobert P. J. Day <rpjday@mindspring.com>
Acked-by: NKumar Gala <galak@kernel.crashing.org>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

Alter roundup_pow_of_two() so that it can make use of ilog2() on a constant to
produce a constant value, retaining the ability for an arch to override it in
the non-const case.

This permits the function to be used to initialise variables.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

This facility provides three entry points:

	ilog2()		Log base 2 of unsigned long
	ilog2_u32()	Log base 2 of u32
	ilog2_u64()	Log base 2 of u64

These facilities can either be used inside functions on dynamic data:

	int do_something(long q)
	{
		...;
		y = ilog2(x)
		...;
	}

Or can be used to statically initialise global variables with constant values:

	unsigned n = ilog2(27);

When performing static initialisation, the compiler will report "error:
initializer element is not constant" if asked to take a log of zero or of
something not reducible to a constant.  They treat negative numbers as
unsigned.

When not dealing with a constant, they fall back to using fls() which permits
them to use arch-specific log calculation instructions - such as BSR on
x86/x86_64 or SCAN on FRV - if available.

[akpm@osdl.org: MMC fix]
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: David Howells <dhowells@redhat.com>
Cc: Wojtek Kaniewski <wojtekka@toxygen.net>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>