From fe7a71c232068bc57e7e14935ff443a4a6315dac Mon Sep 17 00:00:00 2001 From: Cyril SIX Date: Wed, 31 Mar 2021 11:34:55 +0200 Subject: Big simplification of get_loop_info Another remnant of trying to devise a complicated algorithm for a problem that was, in fact, very simple: I just had to check whether the branch was within the loop body. I tested it functionally on the benchmarks: only heapsort is changed, in slightly worst (4-5%), because the old get_loop_info had done a buggy guess that proved to be lucky for that particular case. The other benchmarks are unchanged: the predictions stay the exact same. The get_loop_info could potentially be improved by having a natural loop detection that extends to outer loops (not just inner loops), though I expect the performance improvements would be very small. --- backend/Duplicateaux.ml | 127 ++++++------------------------------------------ 1 file changed, 16 insertions(+), 111 deletions(-) (limited to 'backend') diff --git a/backend/Duplicateaux.ml b/backend/Duplicateaux.ml index b3635527..7504f724 100644 --- a/backend/Duplicateaux.ml +++ b/backend/Duplicateaux.ml @@ -270,120 +270,25 @@ let get_inner_loops f code is_loop_header = ) (PTree.elements loopmap) end - -(* Returns a PTree of either None or Some b where b determines the node following the loop, for a cb instruction *) -(* It uses the fact that loops in CompCert are done by a branch (backedge) instruction followed by a cb *) +(* Returns a PTree of either None or Some b where b determines the node in the loop body, for a cb instruction *) let get_loop_info f is_loop_header bfs_order code = - debug "GET LOOP INFO\n"; - debug "==================================\n"; let loop_info = ref (PTree.map (fun n i -> None) code) in - let mark_path n lbody = - let cb_info = ref PTree.empty in - let visited = ref (PTree.map (fun n i -> false) code) in - (* Returns true if there is a path from src to dest (not involving jumptables) *) - (* Mark nodes as visited along the way *) - let explore src dest = - debug "Trying to dive a path from %d to %d\n" (P.to_int src) (P.to_int dest); - (* Memoizing the results to avoid exponential blow-up *) - let memory = ref PTree.empty in - let rec explore_rec src = - debug "explore_rec %d vs %d... " (P.to_int src) (P.to_int dest); - if (P.to_int src) == (P.to_int dest) then (debug "FOUND\n"; true) - else if (get_some @@ PTree.get src !visited) then (debug "VISITED... :( \n"; false) - (* if we went out of the innermost loop *) - else if (not @@ List.mem src lbody) then (debug "Out of innermost...\n"; false) - else begin - let inst = get_some @@ PTree.get src code in - visited := PTree.set src true !visited; - match rtl_successors inst with - | [] -> false - | [s] -> explore_wrap s - | [s1; s2] -> let snapshot_visited = ref !visited in begin - debug "\t\tSplit at %d: either %d or %d\n" (P.to_int src) (P.to_int s1) (P.to_int s2); - (* Remembering that we tried the ifso node *) - cb_info := PTree.set src true !cb_info; - match explore_wrap s1 with - | true -> ( - visited := !snapshot_visited; - match explore_wrap s2 with - | true -> begin - (* Both paths lead to a loop: we cannot predict the CB - * (but the explore still succeeds) *) - cb_info := PTree.remove src !cb_info; - true - end - | false -> true (* nothing to do, the explore succeeded *) - ) - | false -> begin - cb_info := PTree.set src false !cb_info; - match explore_wrap s2 with - | true -> true - | false -> (cb_info := PTree.remove src !cb_info; false) - end - end - | _ -> false - end - and explore_wrap src = begin - match PTree.get src !memory with - | Some b -> b - | None -> - let result = explore_rec src in - (memory := PTree.set src result !memory; result) - end in explore_wrap src - (* Goes forward until a CB is encountered - * Returns None if no CB was found, or Some the_cb_node - * Marks nodes as visited along the way *) - in let rec advance_to_cb src = - if (get_some @@ PTree.get src !visited) then None - else begin - visited := PTree.set src true !visited; - match get_some @@ PTree.get src code with - | Inop s | Iop (_, _, _, s) | Iload (_,_,_,_,_,s) | Istore (_,_,_,_,s) | Icall (_,_,_,_,s) - | Ibuiltin (_,_,_,s) -> advance_to_cb s - | Icond _ -> Some src - | Ijumptable _ | Itailcall _ | Ireturn _ -> None - end - in begin - debug "Attempting to find natural loop from HEAD %d..\n" (P.to_int n); - match advance_to_cb n with - | None -> (debug "\tNo CB found\n") - | Some s -> ( debug "\tFound a CB! %d\n" (P.to_int s); - match get_some @@ PTree.get s !loop_info with - | None | Some _ -> begin - match get_some @@ PTree.get s code with - | Icond (_, _, n1, n2, _) -> ( - let b1 = explore n1 n in - let b2 = explore n2 n in - if (b1 && b2) then - debug "\tBoth paths lead back to the head: NONE\n" - else if (b1 || b2) then begin - if b1 then begin - debug "\tTrue path leads to the head: TRUE\n"; - loop_info := PTree.set s (Some true) !loop_info; - end else if b2 then begin - debug "\tFalse path leads to the head: FALSE\n"; - loop_info := PTree.set s (Some false) !loop_info - end; - debug "\tSetting other CBs encountered..\n"; - List.iter (fun (cb, dir) -> - debug "\t\t%d is %B\n" (P.to_int cb) dir; - loop_info := PTree.set cb (Some dir) !loop_info - ) (PTree.elements !cb_info) - end else - debug "\tNo path leads back to the head: NONE\n" - ) - | _ -> failwith "\tNot an Icond\n" - end - (* | Some _ -> ( debug "already loop info there\n" ) FIXME - we don't know yet whether a branch to a loop head is a backedge or not *) - ) - end + let mark_iloop iloop = + List.iter (fun n -> + match get_some @@ PTree.get n code with + | Icond (_, _, ifso, ifnot, _) -> + let b1 = List.mem ifso iloop.body in + let b2 = List.mem ifnot iloop.body in + if (b1 && b2) then () + else if (b1 || b2) then begin + if b1 then loop_info := PTree.set n (Some true) !loop_info + else if b2 then loop_info := PTree.set n (Some false) !loop_info + end + | _ -> () + ) iloop.body in let iloops = get_inner_loops f code is_loop_header in - begin - List.iter (fun il -> mark_path il.head il.body) iloops; - (* List.iter mark_path @@ List.filter (fun n -> get_some @@ PTree.get n is_loop_header) bfs_order; *) - debug "==================================\n"; - !loop_info - end + List.iter mark_iloop iloops; + !loop_info (* Remark - compared to the original Branch Prediction for Free paper, we don't use the store heuristic *) let get_directions f code entrypoint = begin -- cgit