alt Hacker NewsThis series was a good explanation for me of why treating UB this way is genuinely useful: https://blog.llvm.org/2011/05/what-every-c-programmer-should...
Being able to assume certain things don't happen is powerful when you're writing optimisations, not doing that would have a real performance cost
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benj111 • yesterday at 2:36 PM
Right. But to take the first example, the value of initialised memory.
It's undefined so it doesn't have to be zeroed therefore increasing efficiency.
But it's also UB so if you do know that memory contains something, you can't take advantage of that because it's UB. Having it UB is fine. It's the compilers assuming UB can't happen and optimising it away.
> Being able to assume certain things don't happen is powerful when you're writing optimisations, not doing that would have a real performance cost
A few of those are significant performance gains, the majority are not.
Emitting the instruction for a NULL pointer dereference is effectively no more costly than not emitting that instruction.
It's the code removal that's killing me.