On 11/29/2015 09:24 AM, Ajit Kumar Agarwal wrote:
I agree with the above. To add up on the above, we only require to calculate the set of objects ( SSA_NAMES) that are live at the birth or the header of the loop. We don't need to calculate the live through the Loop considering Live in and Live out of all the basic blocks of the Loop. This is because the set of objects (SSA_NAMES) That are live-in at the birth or header of the loop will be live-in at every node in the Loop. If a v live out at the header of the loop then the variable is live-in at every node in the Loop. To prove this, Consider a Loop L with header h such that The variable v defined at d is live-in at h. Since v is live at h, d is not part of L. This follows from the dominance property, i.e. h is strictly dominated by d. Furthermore, there exists a path from h to a use of v which does not go through d. For every node of the loop, p, since the loop is strongly connected Component of the CFG, there exists a path, consisting only of nodes of L from p to h. Concatenating those two paths prove that v is live-in and live-out Of p. On top of live-in at the birth or header of the loop as proven above, if we calculate the Live out of the exit block of the block and Live-in at the destination Edge of the exit block of the loops. This consider the liveness outside of the Loop. The above two cases forms the basis of better estimator for register pressure as far as LICM is concerned. If you agree with the above, I will implement add the above in the patch for register_used estimates for better estimate of register pressure for LICM.
Yes, I think we're in agreement. jeff
