Jeff Law <l...@redhat.com> writes: > Reload has the ability to replace a pseudo with its equivalent memory > location. This is fine and good. > > Imagine: > > 1. We have a pseudo (call is pseudo A) with a read-only memory > equivalent. Pseudo A does not get a hard reg > > 2. Pseudo A crosses a call (because the memory is readonly, we will > not invalidate the equivalency) > > 3. The equivalent memory address references another pseudo (call it > pseudo B) > > 4. Pseudo B does not cross calls and is assigned a call-clobbered > hard reg. > > 5. reload replaces pseudo A with its equivalent memory form and in > doing so lengthens the lifetime of pseudo B and causes pseudo B to be > live across a call. > > Obviously this is bad. The question is where/how do we want to catch it. > > The easy solution is to go ahead and invalidate the equivalency once > we notice that pseudo A crosses a call, even though the memory > equivalent is readonly. Seems a little harsh, but it's a one line > change.
If you can spot this before reload, then you can load the memory address into a new pseudo C, and let the register allocator decide whether to save C on the stack or rematerialize it as needed. Otherwise, if you don't invalidate the equivalency, you are effectively extending the lifetime of pseudo B so that it lives across a call. If there is any register pressure, B is going to go onto the stack or is going to force something else onto the stack. At that point, you would have been better off just putting the memory address on the stack instead of pseudo B. So I think that invalidating the equivalency across a function call is probably the right thing to do. Ian
