I’m working on enhancing EarlyCSE to use MemorySSA and have come across the following issue due to differences in EarlyCSE and MemorySSA’s handling of !invariant.load. EarlyCSE will not currently optimize the following code by replacing %x2 with %x and removing the second load:
B1:
%x = load %p
clobber()
…
B2: // dominated by B1
%x2 = load %p !invariant.load
Sanjoy (who added the !invariant.load support to EarlyCSE) and I discussed this, and I believe we are both in agreement that this optimization should be legal. I’d like to make sure there is agreement on this and possibly clarify the LangRef wording on !invariant.load to make the legality of this transformation more clear.
Sanjoy suggested the following:
Instead of “The existence of the !invariant.load metadata on the instruction tells the optimizer and code generator that the address operand to this load points to memory which can be assumed unchanged.” we say “It is undefined behavior to invariant_load from a location that has been changed since it became dereferenceable”. In the current langref, I find “The existence” somewhat confusing, since it seems to imply that adding dead code can change the behavior of the program.
I don’t want to specify the semantics in a way that:
int* ptr = …
int k0 = *ptr; // normal load
clobber();
int k1 = *ptr; // normal loadhas a different meaning than
int* ptr = …
int k0 = *ptr; // normal load
clobber();
int k1 = *ptr; // normal load
if () {
int k2 = *ptr; // !invariant load
}That is, adding dead code should not change the behavior of the
program – the code guarded by () should be able to have
any amount of junk without breaking the program, since it does not
actually execute.
Does this seem like a clearer wording of the intended semantics?