Thanks, I agree with most of your last email, just replying to one thing: > > ->rf does because of data flow causality, ->ppo does because of > > program structure, so that makes sense to be ->hb. > > > > IMHO, ->rfi should as well, because it is embodying a flow of data, so > > that is a bit confusing. It would be great to clarify more perhaps > > with an example about why ->rfi cannot be ->hb, in the > > "happens-before" section. > > Maybe. We do talk about store forwarding, and in fact the ppo section > already says: > > ------------------------------------------------------------------------ > R ->dep W ->rfi R', > > where the dep link can be either an address or a data dependency. In > this situation we know it is possible for the CPU to execute R' before > W, because it can forward the value that W will store to R'. > ------------------------------------------------------------------------ Thank you for pointing this out! In the text that follows this, in this paragraph: <quote> where the dep link can be either an address or a data dependency. In this situation we know it is possible for the CPU to execute R' before W, because it can forward the value that W will store to R'. But it cannot execute R' before R, because it cannot forward the value before it knows what that value is, or that W and R' do access the same location. </quote> The "in this situation" should be clarified that the "situation" is a data-dependency. Only in the case of data-dependency, the ->rfi cannot cause misordering if I understand it correctly. However, that sentence does not mention data-dependency explicitly. Or let me know if I missed something? Thanks, - Joel
