Re: [PATCH] byteswap: port better to limited platforms

[Paul Eggert]

On 2024-05-17 16:51, Bruno Haible wrote:
> -------------------------- foo.c --------------------------
> unsigned long long x = 0xff00000000000000;
> -----------------------------------------------------------
>
> $ gcc -Wall -S foo.c
> foo.c:1: warning: integer constant is too large for ‘long’ type

I don't see how that would produce incorrect code for byteswap.h.

If memory serves, GCC formerly warned about integer constants that had a 
"surprising" type, for a definition of "surprising" that was too 
generous so that there were too many false alarms.

The longer story is C89 lacked 'long long'. Also, in C89 on a 32-bit int 
platform the decimal integer literal 2147483648 had type 'unsigned int' 
because 2147483648 fits in 'unsigned int' but not plain 'int'. Although 
this misfeature was fixed in C99 it was present in many compilers for 
some time after that. (It still seems to be present in MS Visual Studio! 
See 
<https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-2-c4146?view=msvc-170>.)

To help write portable code, GCC formerly warned about integer constants 
that might not be portable to C89, but it then went ahead and produced 
correct code.

The need for these warnings went away long ago (except perhaps for 
Microsoft...) but they can be annoying so I installed the attached patch 
to pacify older GCCs.


PS. We could simplify the code by removing all uses of __builtin_bswap64 
etc. Although these uses are present only to improve performance, they 
do not improve performance with GCC 14 x86-64 with -O2.

0001-byteswap-pacify-GCC-4.4.7-and-older.patch
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