[PATCH] lib/base64: fix -Wconstant-conversion warning in INIT_1()

[PATCH] lib/base64: fix -Wconstant-conversion warning in INIT_1()

[Rosalie Fang]

When expanding BASE64_REV_INIT() for v in 0x7c..0x7f (124..127), the
unselected ternary branch (v) - '0' + 52 evaluates to 128..131, which
exceeds SCHAR_MAX (127). Recent Clang versions diagnose implicit integer
constant conversions in unevaluated ternary arms when initializing the
s8 lookup tables:

  lib/base64.c:58:18: error: implicit conversion from 'int' to 's8'
  (aka 'signed char') changes value from 131 to -125
  [-Werror,-Wconstant-conversion]
     58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
  lib/base64.c:34:42: note: expanded from macro 'INIT_1'
     34 |                 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
        |                                              ~~~~~~~~~~^~~~

Explicitly cast the digit-branch expression to s8 to silence the
warning.

Fixes: c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables")
Assisted-by: LLM
Signed-off-by: Rosalie Fang <rosaliefang@google.com>
---
 lib/base64.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/base64.c b/lib/base64.c
index 325c7332b049..1489c048b1fd 100644
--- a/lib/base64.c
+++ b/lib/base64.c
@@ -31,7 +31,7 @@ static const char base64_tables[][65] = {
 #define INIT_1(v, ch_62, ch_63) \
 	[v] = (v) >= 'A' && (v) <= 'Z' ? (v) - 'A' \
 		: (v) >= 'a' && (v) <= 'z' ? (v) - 'a' + 26 \
-		: (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
+		: (v) >= '0' && (v) <= '9' ? (s8)((v) - '0' + 52) \
 		: (v) == (ch_62) ? 62 : (v) == (ch_63) ? 63 : -1
 
 /*
-- 
2.56.0.385.gac671c819a-goog

Re: [PATCH] lib/base64: fix -Wconstant-conversion warning in INIT_1()

[Nathan Chancellor]

Hi Rosalie,

On Tue, Nov 03, 2026 at 04:34:04PM +0000, Rosalie Fang wrote:
> When expanding BASE64_REV_INIT() for v in 0x7c..0x7f (124..127), the
> unselected ternary branch (v) - '0' + 52 evaluates to 128..131, which
> exceeds SCHAR_MAX (127). Recent Clang versions diagnose implicit integer
> constant conversions in unevaluated ternary arms when initializing the
> s8 lookup tables:
> 
>   lib/base64.c:58:18: error: implicit conversion from 'int' to 's8'
>   (aka 'signed char') changes value from 131 to -125
>   [-Werror,-Wconstant-conversion]
>      58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
>         |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
>   lib/base64.c:34:42: note: expanded from macro 'INIT_1'
>      34 |                 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
>         |                                              ~~~~~~~~~~^~~~
> 
> Explicitly cast the digit-branch expression to s8 to silence the
> warning.
> 
> Fixes: c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables")
> Assisted-by: LLM
> Signed-off-by: Rosalie Fang <rosaliefang@google.com>
> ---
>  lib/base64.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/base64.c b/lib/base64.c
> index 325c7332b049..1489c048b1fd 100644
> --- a/lib/base64.c
> +++ b/lib/base64.c
> @@ -31,7 +31,7 @@ static const char base64_tables[][65] = {
>  #define INIT_1(v, ch_62, ch_63) \
>  	[v] = (v) >= 'A' && (v) <= 'Z' ? (v) - 'A' \
>  		: (v) >= 'a' && (v) <= 'z' ? (v) - 'a' + 26 \
> -		: (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
> +		: (v) >= '0' && (v) <= '9' ? (s8)((v) - '0' + 52) \
>  		: (v) == (ch_62) ? 62 : (v) == (ch_63) ? 63 : -1
>  
>  /*
> -- 
> 2.56.0.385.gac671c819a-goog
> 

Thanks for the change. I avoided this in a different way, which has
recently been merged into a tree that should be merged into mainline
relatively soon:

  https://git.kernel.org/akpm/mm/c/3af83c11cf5990fda0bfe3959aa6391ff903ffdd

For the record, I do hope that

  https://github.com/llvm/llvm-project/pull/226775

eventually gets merged so that these workarounds will be unnecessary but
I am not sure how that will play out yet.

-- 
Cheers,
Nathan

Re: [PATCH] lib/base64: fix -Wconstant-conversion warning in INIT_1()

[Rosalie Fang]

On Tue, Nov 03, 2026 at 12:00:52PM -0700, Nathan Chancellor wrote:
> Thanks for the change. I avoided this in a different way, which has
> recently been merged into a tree that should be merged into mainline
> relatively soon:
> 
>   https://git.kernel.org/akpm/mm/c/3af83c11cf5990fda0bfe3959aa6391ff903ffdd
> 
> For the record, I do hope that
> 
>   https://github.com/llvm/llvm-project/pull/226775
> 
> eventually gets merged so that these workarounds will be unnecessary but
> I am not sure how that will play out yet.

Thanks for the pointer, Nathan! Please disregard this patch in favor of
3af83c11cf59 ("lib/base64: silence clang-24 -Wconstant-conversion with
diag pragmas").

Thanks,
Rosalie

Re: [PATCH] lib/base64: fix -Wconstant-conversion warning in INIT_1()

[David Laight]

On Tue, 3 Nov 2026 12:00:52 -0700
Nathan Chancellor <nathan@kernel.org> wrote:

> Hi Rosalie,
> 
> On Tue, Nov 03, 2026 at 04:34:04PM +0000, Rosalie Fang wrote:
> > When expanding BASE64_REV_INIT() for v in 0x7c..0x7f (124..127), the
> > unselected ternary branch (v) - '0' + 52 evaluates to 128..131, which
> > exceeds SCHAR_MAX (127). Recent Clang versions diagnose implicit integer
> > constant conversions in unevaluated ternary arms when initializing the
> > s8 lookup tables:
> > 
> >   lib/base64.c:58:18: error: implicit conversion from 'int' to 's8'
> >   (aka 'signed char') changes value from 131 to -125
> >   [-Werror,-Wconstant-conversion]
> >      58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
> >         |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
> >   lib/base64.c:34:42: note: expanded from macro 'INIT_1'
> >      34 |                 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
> >         |                                              ~~~~~~~~~~^~~~
> > 
> > Explicitly cast the digit-branch expression to s8 to silence the
> > warning.
> > 
> > Fixes: c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables")
> > Assisted-by: LLM
> > Signed-off-by: Rosalie Fang <rosaliefang@google.com>
> > ---
> >  lib/base64.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/lib/base64.c b/lib/base64.c
> > index 325c7332b049..1489c048b1fd 100644
> > --- a/lib/base64.c
> > +++ b/lib/base64.c
> > @@ -31,7 +31,7 @@ static const char base64_tables[][65] = {
> >  #define INIT_1(v, ch_62, ch_63) \
> >  	[v] = (v) >= 'A' && (v) <= 'Z' ? (v) - 'A' \
> >  		: (v) >= 'a' && (v) <= 'z' ? (v) - 'a' + 26 \
> > -		: (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
> > +		: (v) >= '0' && (v) <= '9' ? (s8)((v) - '0' + 52) \
> >  		: (v) == (ch_62) ? 62 : (v) == (ch_63) ? 63 : -1
> >  
> >  /*
> > -- 
> > 2.56.0.385.gac671c819a-goog
> >   
> 
> Thanks for the change. I avoided this in a different way, which has
> recently been merged into a tree that should be merged into mainline
> relatively soon:
> 
>   https://git.kernel.org/akpm/mm/c/3af83c11cf5990fda0bfe3959aa6391ff903ffdd

#define INIT_1(v, ch_62, ch_63) \
	[v] = (s8)((v) >= 'A' && (v) <= 'Z' ? (v) - 'A' \
		 : (v) >= 'a' && (v) <= 'z' ? (v) - 'a' + 26 \
		 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
		 : (v) == (ch_62) ? 62 : (v) == (ch_63) ? 63 : -1)
probably looks best?

> For the record, I do hope that
> 
>   https://github.com/llvm/llvm-project/pull/226775
> 
> eventually gets merged so that these workarounds will be unnecessary but
> I am not sure how that will play out yet.

Someone really doesn't like kernel code!
There are other warnings like 'cast of pointer to/from integer of a different
size' that it would be nice not to have reported in code that will be optimised
away.
At least the kernel gave up on some of the 'type limits' warnings, they
had similar problems (even inside __builtin_choose_expr()).

David

(I'm reading these unthreaded...)

Re: [PATCH] lib/base64: fix -Wconstant-conversion warning in INIT_1()

[David Laight]

On Tue,  3 Nov 2026 16:34:04 +0000
Rosalie Fang <rosaliefang@google.com> wrote:

> When expanding BASE64_REV_INIT() for v in 0x7c..0x7f (124..127), the
> unselected ternary branch (v) - '0' + 52 evaluates to 128..131, which
> exceeds SCHAR_MAX (127). Recent Clang versions diagnose implicit integer
> constant conversions in unevaluated ternary arms when initializing the
> s8 lookup tables:

Has that been reported as a compiler bug?

> 
>   lib/base64.c:58:18: error: implicit conversion from 'int' to 's8'
>   (aka 'signed char') changes value from 131 to -125
>   [-Werror,-Wconstant-conversion]
>      58 |         [BASE64_IMAP] = BASE64_REV_INIT('+', ',')
>         |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
>   lib/base64.c:34:42: note: expanded from macro 'INIT_1'
>      34 |                 : (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
>         |                                              ~~~~~~~~~~^~~~
> 
> Explicitly cast the digit-branch expression to s8 to silence the
> warning.
> 
> Fixes: c4eb7ad32eab ("lib/base64: optimize base64_decode() with reverse lookup tables")
> Assisted-by: LLM
> Signed-off-by: Rosalie Fang <rosaliefang@google.com>
> ---
>  lib/base64.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/base64.c b/lib/base64.c
> index 325c7332b049..1489c048b1fd 100644
> --- a/lib/base64.c
> +++ b/lib/base64.c
> @@ -31,7 +31,7 @@ static const char base64_tables[][65] = {
>  #define INIT_1(v, ch_62, ch_63) \
>  	[v] = (v) >= 'A' && (v) <= 'Z' ? (v) - 'A' \
>  		: (v) >= 'a' && (v) <= 'z' ? (v) - 'a' + 26 \
> -		: (v) >= '0' && (v) <= '9' ? (v) - '0' + 52 \
> +		: (v) >= '0' && (v) <= '9' ? (s8)((v) - '0' + 52) \
>  		: (v) == (ch_62) ? 62 : (v) == (ch_63) ? 63 : -1

It might save further trouble to put the (s8) cast on the entire RHS
expression.

David

>  
>  /*