Hi. Thanks for the note. The file under discussion came from GNULIB (I believe) so I'm adding bug-gnulib and will let that team comment on this.
Given that it's not an issue on commonly used CPUs, I don't see this as a high priority issue either way. Thanks, Arnold Samy Mahmoudi <samy.mahmo...@gmail.com> wrote: > Hello, > > Compiling with the option -Wshift-negative-value outputs the following > warning: > > missing_d/mktime.c:82:22: warning: left shift of negative value > [-Wshift-negative-value] > : ~ (time_t) 0 << (sizeof (time_t) * CHAR_BIT - 1)) > > In relation to gcc PR c/65179, Martin Sebor wrote: > > "Shifting a negative value by a positive number of bits does have a natural > meaning (i.e., shifting the bit pattern the same way as unsigned). The > reason why it's undefined in C and C++ is because some processors don't > shift the sign bit out and may raise an overflow when a one bit is shifted > into the sign position (typically those that provide an arithmetic left > shift). But most processors implement a logical left shift and behave the > same way for signed operands as for unsigned. The result of a left shift of > a negative number computed by GCC matches that of hardware that doesn't > differentiate between arithmetic and logical left shifts (which is all the > common CPUs, including ARM, MIPS, PowerPC, x86), so the only value in > diagnosing it is portability to rare CPUs or to compilers that behave > differently than GCC (if there are any)." > > On most platforms, the attached patch does not provide any functional > change. > > Besides, do you think using intmax_t and uintmax_t could results in a > portability loss ? > > Best regards, > Samy Mahmoudi ------------------------------- diff --git a/missing_d/mktime.c b/missing_d/mktime.c index d394ef17..16a944d3 100644 --- a/missing_d/mktime.c +++ b/missing_d/mktime.c @@ -79,7 +79,7 @@ #ifndef TIME_T_MIN #define TIME_T_MIN (0 < (time_t) -1 ? (time_t) 0 \ - : ~ (time_t) 0 << (sizeof (time_t) * CHAR_BIT - 1)) + : (time_t) (intmax_t) ~ ((uintmax_t) ~ (time_t) 0 >> 1)) #endif #ifndef TIME_T_MAX #define TIME_T_MAX (~ (time_t) 0 - TIME_T_MIN)
