Robert Dewar <[EMAIL PROTECTED]> writes: > Ian Lance Taylor wrote: > > > The new option -fstrict-overflow tells gcc that it can assume the > > strict signed overflow semantics prescribed by the language standard. > > This option is enabled by default at -O2 and higher. Using > > -fno-strict-overflow will tell gcc that it can not assume that signed > > overflow is undefined behaviour. The general effect of using this > > option will be that signed overflow will become implementation > > defined. This will disable a number of generally harmless > > optimizations, but will not have the same effect as -fwrapv. > > Can you share the implementation definition (implementation defined > generally means that the implementation must define what it does). > This seems awfully vague.
You're right, I shouldn't have said "implementation defined." What will happen with -fno-strict-overflow is whatever the processor ISA happens to do when a signed arithmetic operation overflows. For ordinary machines it will just wrap. It is intentionally vague. If you need non-vague semantics, you should use -fwrapv. -fno-strict-overflow is intended to provide the vague semantics which C compilers have historically provided, in support of existing code. -fstrict-overflow provides the reasonably precise semantics of the language standard. The options are generally analogous to -fstrict-aliasing and -fno-strict-aliasing. Ian
