[Bug libc/14210] warning when using FD_SET() with -Wsign-conversion and _FORTIFY_SOURCE

[ppluzhnikov at google dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14210

--- Comment #4 from Paul Pluzhnikov <ppluzhnikov at google dot com> 2012-06-09 16:49:09 UTC ---
(In reply to comment #3)
> i think that version though generates worse code than casting the
> signed value to an unsigned and leaving all the checks to doing a single
> compare.

In my x86_64 build, the code for __fdelt_chk is identical before/after the
patch:

(gdb) disas __fdelt_chk
Dump of assembler code for function __fdelt_chk:
   0x00000000000fb910 <+0>:    cmp    $0x3ff,%rdi
   0x00000000000fb917 <+7>:    ja     0xfb921 <__fdelt_chk+17>
   0x00000000000fb919 <+9>:    mov    %rdi,%rax
   0x00000000000fb91c <+12>:    shr    $0x6,%rax
   0x00000000000fb920 <+16>:    retq   
   0x00000000000fb921 <+17>:    push   %rax
   0x00000000000fb922 <+18>:    callq  0xf9a60 <__GI___chk_fail>
End of assembler dump.

The other part -- macro expansion -- is a compile-time check, so it doesn't
matter.

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