Prior to f747e6667ebb2 __is_constexpr() was in its only user minmax.h.
That commit moved it to const.h - but that file just defines ULL(x) and
  UL(x) so that constants can be defined for .S and .c files.
So apart from the word 'const' it wasn't really a good location.
Instead move the definition to compiler.h just before the similar
  is_signed_type() and is_unsigned_type().
This may not be a good long-term home, but the three definitions
  belong together.

Signed-off-by: David Laight <david.laight@aculab.com>
---
This makes it possible to use __is_constexpr() inside is_signed_type()
so that the result is constant integer expression for pointer types.
In particular (void *)1 isn't constant enough.

v2: Add a copy into tools/linux/compiler.h to fix perf build.

 include/linux/compiler.h       | 8 ++++++++
 include/linux/const.h          | 8 --------
 tools/include/linux/compiler.h | 8 ++++++++
 tools/include/linux/const.h    | 8 --------
 4 files changed, 16 insertions(+), 16 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index d7779a18b24f..2efec9bfcc40 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -230,6 +230,14 @@ static inline void *offset_to_ptr(const int *off)
 /* &a[0] degrades to a pointer: a different type from an array */
 #define __must_be_array(a)	BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
 
+/*
+ * This returns a constant expression while determining if an argument is
+ * a constant expression, most importantly without evaluating the argument.
+ * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
+ */
+#define __is_constexpr(x) \
+	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
+
 /*
  * Whether 'type' is a signed type or an unsigned type. Supports scalar types,
  * bool and also pointer types.
diff --git a/include/linux/const.h b/include/linux/const.h
index 435ddd72d2c4..81b8aae5a855 100644
--- a/include/linux/const.h
+++ b/include/linux/const.h
@@ -3,12 +3,4 @@
 
 #include <vdso/const.h>
 
-/*
- * This returns a constant expression while determining if an argument is
- * a constant expression, most importantly without evaluating the argument.
- * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
- */
-#define __is_constexpr(x) \
-	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
-
 #endif /* _LINUX_CONST_H */
diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
index 1684216e826a..7b65566f3e42 100644
--- a/tools/include/linux/compiler.h
+++ b/tools/include/linux/compiler.h
@@ -63,6 +63,14 @@
 # define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
 #endif
 
+/*
+ * This returns a constant expression while determining if an argument is
+ * a constant expression, most importantly without evaluating the argument.
+ * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
+ */
+#define __is_constexpr(x) \
+	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
+
 #ifdef __ANDROID__
 /*
  * FIXME: Big hammer to get rid of tons of:
diff --git a/tools/include/linux/const.h b/tools/include/linux/const.h
index 435ddd72d2c4..81b8aae5a855 100644
--- a/tools/include/linux/const.h
+++ b/tools/include/linux/const.h
@@ -3,12 +3,4 @@
 
 #include <vdso/const.h>
 
-/*
- * This returns a constant expression while determining if an argument is
- * a constant expression, most importantly without evaluating the argument.
- * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de>
- */
-#define __is_constexpr(x) \
-	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
-
 #endif /* _LINUX_CONST_H */
-- 
2.17.1

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

On Thu, Oct 05, 2023 at 11:39:54AM +0000, David Laight wrote:
> Prior to f747e6667ebb2 __is_constexpr() was in its only user minmax.h.
> That commit moved it to const.h - but that file just defines ULL(x) and
>   UL(x) so that constants can be defined for .S and .c files.
> So apart from the word 'const' it wasn't really a good location.
> Instead move the definition to compiler.h just before the similar
>   is_signed_type() and is_unsigned_type().
> This may not be a good long-term home, but the three definitions
>   belong together.
> 
> Signed-off-by: David Laight <david.laight@aculab.com>

Yeah, this is what I'd expect (a duplicate move in tools/). Thanks!

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook

It can also be defined more portably as:

#define __is_constexpr(x) \
	_Generic(0 ? (void *)((long)(x) * 0) : (int *)0, int *: 1, void *: 0)

Which doesn't rely on 'sizeof (void)' being valid.

But that is for another day.

There is also:
#define is_pointer_type(type) __is_constexpr((type)1)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

From: Kees Cook
> Sent: 05 October 2023 17:22
> 
> On Thu, Oct 05, 2023 at 11:39:54AM +0000, David Laight wrote:
> > Prior to f747e6667ebb2 __is_constexpr() was in its only user minmax.h.
> > That commit moved it to const.h - but that file just defines ULL(x) and
> >   UL(x) so that constants can be defined for .S and .c files.
> > So apart from the word 'const' it wasn't really a good location.
> > Instead move the definition to compiler.h just before the similar
> >   is_signed_type() and is_unsigned_type().
> > This may not be a good long-term home, but the three definitions
> >   belong together.
> >
> > Signed-off-by: David Laight <david.laight@aculab.com>
> 
> Yeah, this is what I'd expect (a duplicate move in tools/). Thanks!

I did try to run something that might have built 'perf'.
But it complained about a lot of stuff not being installed.

I do wonder if a better home for all these is_xxx() and __is_xxx()
would be at the top of build_bug.h.

While they can be used on their own (eg with __builtin_choose_expr())
they are typically used with the compile-time asserts
(or at least with code that is also doing asserts.)
So there might only be a few places that need the extra #include.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)