On Thu, 6 May 2021, Prathamesh Kulkarni wrote: > Hi Richard, > I was just wondering if second (and higher) order hoistings may defeat > the "AVAIL_OUT in at least one successor heuristic" ? > > For eg: > bb2: > if (cond1) goto bb3 else goto bb4; > > bb3: > if (cond2) goto bb5 else goto bb6; > > bb5: > return x + y; > > bb6: > return x + y; > > bb4: > if (cond3) goto bb7 else goto bb8; > > bb7: > return x + y; > > bb8: > return x + y; > > In above CFG, IIUC, we will end up hoisting x + y in bb2 even if bb3 > or bb4 don't originally > contain x + y since during first hoisting pass x + y will be hoisted > from bb5, bb6 into bb3, > and likewise from bb7, bb8 into bb4 and during second hoisting pass it > will hoist x + y into bb2, since x + y is now available in bb3 and > bb4.
But that's intended - the logic is _supposed_ to do "everything at once", thus repeated runs of hoisting should not hoist more. Which is also why we no longer iterate hoist insertion. > This might become an issue if successive hoistings will end up > hoisting the expression "too far" resulting in long range hoisting ? I think this "long range hoisting" argument is a red herring... > Also if we're hoisting from post-dom block in which case the > expression may not be truly ANTIC, and eventually end up being > inserted into a successor block by successive hoisting passes. But this shouldn't happen - can you show a testcase where it does? > I was wondering if we should check that the expression is "originally" > in AVAIL_OUT in at least one successor of block to avoid considering > expressions inserted by hoisting / PRE ? No, why should we? Richard.
