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. This might become an issue if successive hoistings will end up hoisting the expression "too far" resulting in long range hoisting ? 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. 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 ? We could do that by "marking" blocks during first hoisting pass (or before hoisting / PRE), that intersect with AVAIL_OUT of at least one successor and use that info to check the heuristic during successive hoisting passes ? Thanks, Prathamesh
