https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30484
--- Comment #17 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 24 Aug 2021, vincent-gcc at vinc17 dot net wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=30484 > > --- Comment #16 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> --- > The issue is that the source code assuming -fno-wrapv may be more complex, > thus > giving slower generated code. Here's an example, which consists in adding 3 > signed integers, for which the user knows that the sum is representable, so > that the only issue is a potential integer overflow in the first addition. > I've > used GCC 11.2.0 on x86_64. > > With -fwrapv, the integer overflow is well-defined as wrapping, so that the > user can write: > > int f (int a, int b, int c) > { > return a + b + c; > } > > The generated code with -O3 -fwrapv has 2 instructions (the 2 additions): > > addl %edx, %esi > leal (%rsi,%rdi), %eax > > But without -fwrapv, one needs to make sure that one doesn't get any integer > overflow. Assume that the user knows that there is a single negative number > among the 3 integers, so that using this negative number in the first addition > will avoid an integer overflow. So the user can write: > > int f (int a, int b, int c) > { > if (b < 0) > return a + b + c; > else > return a + c + b; > } > > The generated code with -O3 has 6 instructions: > > leal (%rdi,%rdx), %eax > addl %esi, %edi > addl %edx, %edi > addl %esi, %eax > testl %esi, %esi > cmovs %edi, %eax True. The user could have written the following though: int f (int a, int b, int c) { return (unsigned)a + b + c; } or alternatively int f (int a, int b, int c) { return (long)a + b + c; } both of which produce optimal code. > In theory, the compiler could normally optimize to produce the same code as > with the source that assumes -fwrapv (here, a + b + c and a + c + b are > obviously equivalent on a typical processor), but in practice, this is often > not the case as shown above.
