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Commit fe7a71c2 authored by Cyril SIX's avatar Cyril SIX
Browse files

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.
parent f7365bc7
......@@ -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
......
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