[CJefferson]

But, I wonder how much it would reduce performance, if we only have to pick a value the first time the memory is read?

I would imagine there isn't that many cases where we are reading uninitalised memory and counting on that reading not saving a value. It would happen when reading in 8-byte blocks for alignment, but does it happen that much elsewhere?

[ablob]

if you pick a value you have to store it, and if you have to store it it might spill into memory when register allocation fails. Moving from register-only to stack/heap usage easily slows down your program by an order of magnitude or two. If this is in a hot path, which I'd argue it is since using uninitialized values seems senseless otherwise, it might have a big impact.

The only way to really know is to test this. Compilers and their optimizations depend on a lot of things. Even the order and layout of instructions can matter due to the instruction cache. You can always go and make the guarantee later on, but undoing it would be impossible.