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05 April 2024, 19:05:47 UTC
Revision 29905b52fad0854351f57bab867647e4982285bf authored by Ard Biesheuvel on 02 February 2017, 18:05:26 UTC, committed by Linus Torvalds on 03 February 2017, 17:56:43 UTC 
log2: make order_base_2() behave correctly on const input value zero
 
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 <ard.biesheuvel@linaro.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent 34e00ac
History
Tip revision: 29905b52fad0854351f57bab867647e4982285bf authored by Ard Biesheuvel on 02 February 2017, 18:05:26 UTC
log2: make order_base_2() behave correctly on const input value zero
Tip revision: 29905b5
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