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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: Laura Abbott <[email protected]>
Cc: John Stultz <[email protected]>,
Cc: Thomas Gleixner <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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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: Ard Biesheuvel <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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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 1a06a52ee1b0: "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: Dmitry Torokhov <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>
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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: Robert P. J. Day <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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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.
[[email protected]: fix unbalanced parentheses]
Signed-off-by: Robert P. J. Day <[email protected]>
Signed-off-by: Mariusz Kozlowski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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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: Rolf Eike Beer <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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Single typo correction in include/linux/log2.h.
Signed-off-by: Robert P. J. Day <[email protected]>
Signed-Off-By: David Howells <[email protected]>
Signed-off-by: Adrian Bunk <[email protected]>
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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: Robert P. J. Day <[email protected]>
Acked-by: Kumar Gala <[email protected]>
Signed-off-by: Paul Mackerras <[email protected]>
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constant
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: David Howells <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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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.
[[email protected]: MMC fix]
Signed-off-by: David Howells <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: David Howells <[email protected]>
Cc: Wojtek Kaniewski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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