On 2018-07-19 16:53:11 +0200, Joerg Schilling wrote: > Vincent Lefevre <vincent-o...@vinc17.net> wrote: > > > The problem is not just the warning. If t is signed, > > > > ((t)(~((t)0) << (sizeof (t)*CHAR_BIT - 1))) > > > > will yield undefined behavior due to overflow. This means that > > compilers may generate code that shows a behavior different from > > what you expect. Not just recent compilers. Compilers could already > > do this 20 years ago without a warning. Perhaps some did. Perhaps > > some users got crashes and other erratic behavior due to that, but > > did not know the cause or did not notice. > > A compiler that creates other than the expected behavior would need to create > intentionally buggy code. > > The question was to create working code that neither creates a warning with > newer nor with older compilers. Do you have such code?
There was my answer on stackoverflow: https://stackoverflow.com/questions/5617925/maximum-values-for-time-t-struct-timespec/39782264#39782264 To apply the same idea here: #include <stdio.h> #define CHAR_BIT 8 #define TYPE_MSBVAL1(t) ((t)(~((t)0) << (sizeof (t)*CHAR_BIT - 1))) #define TYPE_MSBVAL2(t) (2 * -((t) 1 << (sizeof(t) * CHAR_BIT - 2))) int main (void) { printf ("%d\n", TYPE_MSBVAL1(int)); printf ("%d\n", TYPE_MSBVAL2(int)); printf ("%ld\n", TYPE_MSBVAL1(long)); printf ("%ld\n", TYPE_MSBVAL2(long)); return 0; } Contrary to TYPE_MSBVAL1, TYPE_MSBVAL2 doesn't involve undefined behavior (assuming two's complement). -- Vincent Lefèvre <vinc...@vinc17.net> - Web: <https://www.vinc17.net/> 100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/> Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
