Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness

On 2 February 2017 at 15:43, Laura Abbott [off-list ref] wrote:

On 02/02/2017 01:17 AM, Ard Biesheuvel wrote:

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On 1 February 2017 at 21:50, Laura Abbott [off-list ref] wrote:

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On 02/01/2017 09:36 AM, Ard Biesheuvel wrote:

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On 1 February 2017 at 16:58, Laura Abbott [off-list ref] wrote:

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On 10/19/2016 09:22 AM, Will Deacon wrote:

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On Wed, Oct 19, 2016 at 09:01:33AM -0700, Linus Torvalds wrote:

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On Wed, Oct 19, 2016 at 8:56 AM, Markus Trippelsdorf
[off-list ref] wrote:

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On 2016.10.19 at 08:55 -0700, Linus Torvalds wrote:

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Well, in the meantime we apparently have to live with it. Unless Will
is using some unreleased gcc version that nobody else is using and we
can just ignore it?

Yes, he is using gcc-7 that is unreleased. (It will be released April
next year.)

Ahh, self-built? So it's not part of some experimental ARM distro
setup and this will be annoying lots of people?

Our friendly compiler guys built it, but it's just a snapshot of trunk,
so it's all heading towards GCC 7.0. AFAIU, the problematic optimisation
is also a mid-end pass, so it would affect other architectures too.

quoted

If so, still think that we could just get rid of the ____ilog2_NaN()
thing as it's not _that_ important, but it's certainly not very
high-priority. Will can do it in his tree too for testing, and it can
remind people to get the gcc problem fixed.

I'm carrying the diff below, which fixes arm64 defconfig, but I'm worried
that we might be relying on this trick elsewhere. The arm __bad_cmpxchg
function, for example.

Will

--->8

diff --git a/include/linux/log2.h b/include/linux/log2.h
index fd7ff3d91e6a..9cf5ad69065d 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h

@@ -16,12 +16,6 @@
 #include <linux/bitops.h>

 /*
- * deal with unrepresentable constant logarithms
- */
-extern __attribute__((const, noreturn))
-int ____ilog2_NaN(void);
-
-/*
  * non-constant log of base 2 calculators
  * - the arch may override these in asm/bitops.h if they can be implemented
  *   more efficiently than using fls() and fls64()

@@ -85,7 +79,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
 #define ilog2(n)                             \
 (                                            \
      __builtin_constant_p(n) ? (             \
-             (n) < 1 ? ____ilog2_NaN() :     \
+             (n) < 1 ? 0 :                   \
              (n) & (1ULL << 63) ? 63 :       \
              (n) & (1ULL << 62) ? 62 :       \
              (n) & (1ULL << 61) ? 61 :       \

@@ -149,9 +143,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
              (n) & (1ULL <<  3) ?  3 :       \
              (n) & (1ULL <<  2) ?  2 :       \
              (n) & (1ULL <<  1) ?  1 :       \
-             (n) & (1ULL <<  0) ?  0 :       \
-             ____ilog2_NaN()                 \
-                                ) :          \
+             0) :                            \
      (sizeof(n) <= 4) ?                      \
      __ilog2_u32(n) :                        \
      __ilog2_u64(n)                          \

@@ -194,7 +186,6 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
  * @n: parameter
  *
  * The first few values calculated by this routine:
- *  ob2(0) = 0
  *  ob2(1) = 0
  *  ob2(2) = 1
  *  ob2(3) = 2

Reviving this thread as gcc 7 has now hit Fedora rawhide and has this
same issue. I pulled in the above patch from Will as a temporary work
around for building. It didn't look like there was consensus on a
permanent solution though from the thread.

I still think order_base_2() is broken, since it may invoke
roundup_pow_of_two() with an input value that is documented as
producing undefined output. I would argue that the below is the
correct fix.

diff --git a/include/linux/log2.h b/include/linux/log2.h
index fd7ff3d91e6a..46523731bec0 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h

@@ -203,6 +203,18 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
  *  ... and so on.
  */

-#define order_base_2(n) ilog2(roundup_pow_of_two(n))
+static inline __attribute__((__const__))
+unsigned long __order_base_2(unsigned long n)
+{
+       return n ? 1UL << fls_long(n - 1) : 1;
+}
+
+#define order_base_2(n)                                \
+(                                              \
+       __builtin_constant_p(n) ? (             \
+               ((n) < 2) ? (n) :               \
+               ilog2((n) - 1) + 1) :           \
+       ilog2(__order_base_2(n))                \
+ )

 #endif /* _LINUX_LOG2_H */

This fixes the problem although the comments should be updated
as well.

This brings order_base_2() in line with the comments, so I am not sure
what you'd want to update here?

ob2(1) = 1  for the __builtin_constant_p case which doesn't
match the comment of ob2(1) = 0. So my statement should actually
be is this correct?

Thanks,
Laura

You are right. But the final one I proposed is correct:

#define order_base_2(n)                        \
(                                              \
       __builtin_constant_p(n) ? (             \
               ((n) == 0 || (n) == 1) ? 0 :    \
               ilog2((n) - 1) + 1) :           \
       __order_base_2(n)                       \
)

and solves Joe's issue as well.

I will submit this as a proper patch.

Thanks,
Ard.