On Wed, 2024-04-10 at 11:49 +0200, stefan@xxxxxxxxx wrote: > > But I keep considering this as a bug. And clang behaves correctly! No it is not. Both compilers are correct as anything can happen for an undefined behavior. > > https://godbolt.org/z/az8WqboET > > typedef unsigned long long int u64; > typedef unsigned int u32; > typedef unsigned short u16; > > u64 foo(u16 *a, u16 *b) { > u32 x = *a * *b; > u64 r = x; > return r >> 31; > } > > gcc yields > > foo: > xor eax, eax > ret In this case GCC can assume (*a * *b) must be in [0, 2147483647] because otherwise there is an undefined behavior. Thus r is in [0, 2147483647], so r >> 31 must be 0. -- Xi Ruoyao <xry111@xxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
