[PATCH v1 0/5] Silence some clang -Wshorten-64-to-32 warnings

[PATCH v1 0/5] Silence some clang -Wshorten-64-to-32 warnings

[Ian Rogers]

Clang's shorten-64-to-32 can be useful to spot certain kinds of bugs
that can be more prevalent in C code due to implicit 64 to 32-bit
casting. Add some explicit casts to header files so as to avoid the
warning when these headers are used.

This patch started out as a single patch in a series for the perf tool
where a bug could have been identified were -Wshorten-64-to-32
enabled:
https://lore.kernel.org/lkml/20250401182347.3422199-3-irogers@google.com/

Ian Rogers (5):
  bitfield: Silence a clang -Wshorten-64-to-32 warning
  bitmap: Silence a clang -Wshorten-64-to-32 warning
  bitops: Silence a clang -Wshorten-64-to-32 warning
  math64: Silence a clang -Wshorten-64-to-32 warning
  hash.h: Silence a clang -Wshorten-64-to-32 warning

 include/asm-generic/bitops/fls64.h | 2 +-
 include/linux/bitfield.h           | 2 +-
 include/linux/bitmap.h             | 2 +-
 include/linux/hash.h               | 2 +-
 include/linux/math64.h             | 2 +-
 5 files changed, 5 insertions(+), 5 deletions(-)

-- 
2.49.0.504.g3bcea36a83-goog

[PATCH v1 1/5] bitfield: Silence a clang -Wshorten-64-to-32 warning

[Ian Rogers]

The clang warning -Wshorten-64-to-32 can be useful to catch
inadvertent truncation. In some instances this truncation can lead to
changing the sign of a result, for example, truncation to return an
int to fit a sort routine. Silence the warning by making the implicit
truncation explicit.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 include/linux/bitfield.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
index 63928f173223..cc5cfed041bb 100644
--- a/include/linux/bitfield.h
+++ b/include/linux/bitfield.h
@@ -176,7 +176,7 @@ static __always_inline __##type type##_encode_bits(base v, base field)	\
 {									\
 	if (__builtin_constant_p(v) && (v & ~field_mask(field)))	\
 		__field_overflow();					\
-	return to((v & field_mask(field)) * field_multiplier(field));	\
+	return to((__##type)((v & field_mask(field)) * field_multiplier(field))); \
 }									\
 static __always_inline __##type type##_replace_bits(__##type old,	\
 					base val, base field)		\
-- 
2.49.0.504.g3bcea36a83-goog

[PATCH v1 2/5] bitmap: Silence a clang -Wshorten-64-to-32 warning

[Ian Rogers]

The clang warning -Wshorten-64-to-32 can be useful to catch
inadvertent truncation. In some instances this truncation can lead to
changing the sign of a result, for example, truncation to return an
int to fit a sort routine. Silence the warning by making the implicit
truncation explicit.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 include/linux/bitmap.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 595217b7a6e7..4395e0a618f4 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -442,7 +442,7 @@ static __always_inline
 unsigned int bitmap_weight(const unsigned long *src, unsigned int nbits)
 {
 	if (small_const_nbits(nbits))
-		return hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
+		return (int)hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
 	return __bitmap_weight(src, nbits);
 }
 
-- 
2.49.0.504.g3bcea36a83-goog

Re: [PATCH v1 2/5] bitmap: Silence a clang -Wshorten-64-to-32 warning

[Arnd Bergmann]

On Thu, Apr 3, 2025, at 18:56, Ian Rogers wrote:
> The clang warning -Wshorten-64-to-32 can be useful to catch
> inadvertent truncation. In some instances this truncation can lead to
> changing the sign of a result, for example, truncation to return an
> int to fit a sort routine. Silence the warning by making the implicit
> truncation explicit.


>  unsigned int bitmap_weight(const unsigned long *src, unsigned int nbits)
>  {
>  	if (small_const_nbits(nbits))
> -		return hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
> +		return (int)hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
>  	return __bitmap_weight(src, nbits);
>  }

I don't understand this one. hweight_long() and bitmap_weight()
both return unsigned value, so why do you need to cast this to
a signed value to avoid a signedness problem?

hweight_long() should never return anything larger than 64ul
anyway, which is way outside of the range where it would get
sign-extended.

A more logical change to me would be to make hweight_long()
and bitmap_weight() have the same return type, either
'unsigned long' or 'unsigned int'.

      Arnd

Re: [PATCH v1 2/5] bitmap: Silence a clang -Wshorten-64-to-32 warning

[Ian Rogers]

On Thu, Apr 3, 2025 at 10:49 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Thu, Apr 3, 2025, at 18:56, Ian Rogers wrote:
> > The clang warning -Wshorten-64-to-32 can be useful to catch
> > inadvertent truncation. In some instances this truncation can lead to
> > changing the sign of a result, for example, truncation to return an
> > int to fit a sort routine. Silence the warning by making the implicit
> > truncation explicit.
>
>
> >  unsigned int bitmap_weight(const unsigned long *src, unsigned int nbits)
> >  {
> >       if (small_const_nbits(nbits))
> > -             return hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
> > +             return (int)hweight_long(*src & BITMAP_LAST_WORD_MASK(nbits));
> >       return __bitmap_weight(src, nbits);
> >  }
>
> I don't understand this one. hweight_long() and bitmap_weight()
> both return unsigned value, so why do you need to cast this to
> a signed value to avoid a signedness problem?
>
> hweight_long() should never return anything larger than 64ul
> anyway, which is way outside of the range where it would get
> sign-extended.
>
> A more logical change to me would be to make hweight_long()
> and bitmap_weight() have the same return type, either
> 'unsigned long' or 'unsigned int'.

I don't disagree but the scope of that change would be much larger.
Yury has expressed concern over needing to update printf modifiers. I
was aiming for the minimal change that silences clang's
-Wshorten-64-to-32 warning.

Thanks,
Ian

>       Arnd

[PATCH v1 3/5] bitops: Silence a clang -Wshorten-64-to-32 warning

[Ian Rogers]

The clang warning -Wshorten-64-to-32 can be useful to catch
inadvertent truncation. In some instances this truncation can lead to
changing the sign of a result, for example, truncation to return an
int to fit a sort routine. Silence the warning by making the implicit
truncation explicit.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 include/asm-generic/bitops/fls64.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/asm-generic/bitops/fls64.h b/include/asm-generic/bitops/fls64.h
index 866f2b2304ff..9ad3ff12f454 100644
--- a/include/asm-generic/bitops/fls64.h
+++ b/include/asm-generic/bitops/fls64.h
@@ -21,7 +21,7 @@ static __always_inline int fls64(__u64 x)
 	__u32 h = x >> 32;
 	if (h)
 		return fls(h) + 32;
-	return fls(x);
+	return fls((__u32)x);
 }
 #elif BITS_PER_LONG == 64
 static __always_inline int fls64(__u64 x)
-- 
2.49.0.504.g3bcea36a83-goog

Re: [PATCH v1 3/5] bitops: Silence a clang -Wshorten-64-to-32 warning

[Arnd Bergmann]

On Thu, Apr 3, 2025, at 18:57, Ian Rogers wrote:
> The clang warning -Wshorten-64-to-32 can be useful to catch
> inadvertent truncation. In some instances this truncation can lead to
> changing the sign of a result, for example, truncation to return an
> int to fit a sort routine. Silence the warning by making the implicit
> truncation explicit.

The fls64() function only seems to deal with unsigned values, so
I don't see how it would change the sign.

> diff --git a/include/asm-generic/bitops/fls64.h 
> b/include/asm-generic/bitops/fls64.h
> index 866f2b2304ff..9ad3ff12f454 100644
> --- a/include/asm-generic/bitops/fls64.h
> +++ b/include/asm-generic/bitops/fls64.h
> @@ -21,7 +21,7 @@ static __always_inline int fls64(__u64 x)
>  	__u32 h = x >> 32;
>  	if (h)
>  		return fls(h) + 32;
> -	return fls(x);
> +	return fls((__u32)x);
>  }

Maybe this would be clearer with an explicit upper_32_bits()/
lower_32_bits() instead of the cast and the shift?

      Arnd

Re: [PATCH v1 3/5] bitops: Silence a clang -Wshorten-64-to-32 warning

[Ian Rogers]

On Thu, Apr 3, 2025 at 10:43 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Thu, Apr 3, 2025, at 18:57, Ian Rogers wrote:
> > The clang warning -Wshorten-64-to-32 can be useful to catch
> > inadvertent truncation. In some instances this truncation can lead to
> > changing the sign of a result, for example, truncation to return an
> > int to fit a sort routine. Silence the warning by making the implicit
> > truncation explicit.
>
> The fls64() function only seems to deal with unsigned values, so
> I don't see how it would change the sign.

You are right. I was trying to motivate in the message why building
with -Wshorten-64-to-32 is a good thing, and in this case we're making
an implicit cast explicit.

> > diff --git a/include/asm-generic/bitops/fls64.h
> > b/include/asm-generic/bitops/fls64.h
> > index 866f2b2304ff..9ad3ff12f454 100644
> > --- a/include/asm-generic/bitops/fls64.h
> > +++ b/include/asm-generic/bitops/fls64.h
> > @@ -21,7 +21,7 @@ static __always_inline int fls64(__u64 x)
> >       __u32 h = x >> 32;
> >       if (h)
> >               return fls(h) + 32;
> > -     return fls(x);
> > +     return fls((__u32)x);
> >  }
>
> Maybe this would be clearer with an explicit upper_32_bits()/
> lower_32_bits() instead of the cast and the shift?

It feels a little overkill to me, but if others prefer it then it is a
minor change.

Thanks,
Ian

>       Arnd

[PATCH v1 4/5] math64: Silence a clang -Wshorten-64-to-32 warning

[Ian Rogers]

The clang warning -Wshorten-64-to-32 can be useful to catch
inadvertent truncation. In some instances this truncation can lead to
changing the sign of a result, for example, truncation to return an
int to fit a sort routine. Silence the warning by making the implicit
truncation explicit.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 include/linux/math64.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/math64.h b/include/linux/math64.h
index 6aaccc1626ab..f32fcb2a2331 100644
--- a/include/linux/math64.h
+++ b/include/linux/math64.h
@@ -179,7 +179,7 @@ static __always_inline u64 mul_u64_u64_shr(u64 a, u64 mul, unsigned int shift)
 #ifndef mul_u64_u32_shr
 static __always_inline u64 mul_u64_u32_shr(u64 a, u32 mul, unsigned int shift)
 {
-	u32 ah = a >> 32, al = a;
+	u32 ah = a >> 32, al = (u32)a;
 	u64 ret;
 
 	ret = mul_u32_u32(al, mul) >> shift;
-- 
2.49.0.504.g3bcea36a83-goog

[PATCH v1 5/5] hash.h: Silence a clang -Wshorten-64-to-32 warning

[Ian Rogers]

The clang warning -Wshorten-64-to-32 can be useful to catch
inadvertent truncation. In some instances this truncation can lead to
changing the sign of a result, for example, truncation to return an
int to fit a sort routine. Silence the warning by making the implicit
truncation explicit.

Signed-off-by: Ian Rogers <irogers@google.com>
---
 include/linux/hash.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/hash.h b/include/linux/hash.h
index 38edaa08f862..ecc8296cb397 100644
--- a/include/linux/hash.h
+++ b/include/linux/hash.h
@@ -75,7 +75,7 @@ static __always_inline u32 hash_64_generic(u64 val, unsigned int bits)
 {
 #if BITS_PER_LONG == 64
 	/* 64x64-bit multiply is efficient on all 64-bit processors */
-	return val * GOLDEN_RATIO_64 >> (64 - bits);
+	return (u32)(val * GOLDEN_RATIO_64 >> (64 - bits));
 #else
 	/* Hash 64 bits using only 32x32-bit multiply. */
 	return hash_32((u32)val ^ __hash_32(val >> 32), bits);
-- 
2.49.0.504.g3bcea36a83-goog