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Diffstat (limited to 'lib/compiler/src/beam_ssa_bool.erl')
| -rw-r--r-- | lib/compiler/src/beam_ssa_bool.erl | 70 |
1 files changed, 70 insertions, 0 deletions
diff --git a/lib/compiler/src/beam_ssa_bool.erl b/lib/compiler/src/beam_ssa_bool.erl index 8fb87391c5..5b81ca2be1 100644 --- a/lib/compiler/src/beam_ssa_bool.erl +++ b/lib/compiler/src/beam_ssa_bool.erl @@ -1072,7 +1072,60 @@ redirect_phi(Phi, Args, SuccFail, G0, St) -> redirect_phi_1(PhiVtx, [{#b_literal{val=false},FalseExit}, {#b_var{}=SuccBool,_BoolExit}], SuccFail, G0, St) -> + %% This was most likely an `andalso` in the source code. BoolVtx = get_vertex(SuccBool, St), + + %% We must be careful when rewriting guards that reference boolean + %% expressions defined before the guard. Here is an example: + %% + %% Bool = Z =:= false, + %% if + %% X =:= Y andalso Bool -> ok; + %% true -> error + %% end. + %% + %% Slightly simplified, the SSA code will look like this: + %% + %% 10: Bool = bif:'=:=' _2, `false` + %% br ^11 + %% + %% 11: B = bif:'=:=' X, Y + %% br B, ^20, ^30 + %% + %% 20: br ^40 + %% 30: br ^40 + %% + %% 40: Phi = phi { `true`, ^20 }, { Bool, ^30 } + %% br Phi, ^100, ^200 + %% + %% 100: ret `ok` + %% 200: ret `error' + %% + %% The usual rewriting of the phi node will result in the following + %% SSA code: + %% + %% 10: Bool = bif:'=:=' _2, `false` + %% br Bool, ^100, ^200 + %% + %% 11: B = bif:'=:=' X, Y + %% br B, ^100, ^200 + %% + %% 20: br ^40 + %% 30: br ^40 + %% + %% 40: Phi = phi { `true`, ^20 }, { Bool, ^30 } + %% br Phi, ^100, ^200 + %% + %% 100: ret `ok` + %% 200: ret `error' + %% + %% Block 11 is no longer reachable; thus, the X =:= Y test has been dropped. + %% To avoid dropping tests, we should check whether if there is a path from + %% 10 to block 20. If there is, the optimization in its current form is not + %% safe. + %% + ensure_disjoint_paths(G0, BoolVtx, FalseExit), + [FalseOut] = beam_digraph:out_edges(G0, FalseExit), G1 = beam_digraph:del_edge(G0, FalseOut), case SuccFail of @@ -1090,6 +1143,11 @@ redirect_phi_1(PhiVtx, [{#b_literal{val=true},TrueExit}, {fail,Fail}, G0, St) -> %% This was probably an `orelse` in the source code. BoolVtx = get_vertex(SuccBool, St), + + %% See the previous clause for an explanation of why we + %% must ensure that paths are disjoint. + ensure_disjoint_paths(G0, BoolVtx, TrueExit), + [TrueOut] = beam_digraph:out_edges(G0, TrueExit), G1 = beam_digraph:del_edge(G0, TrueOut), G2 = beam_digraph:add_edge(G1, TrueExit, PhiVtx, next), @@ -1119,6 +1177,18 @@ digraph_bool_def(G) -> Ds = [{Dst,Vtx} || {Vtx,#b_set{dst=Dst}} <- Vs], maps:from_list(Ds). + +%% ensure_disjoint_paths(G, Vertex1, Vertex2) -> ok. +%% Ensure that there is no path from Vertex1 to Vertex2 in +%% either direction. (It is probably overkill to test both +%% directions, but better safe than sorry.) + +ensure_disjoint_paths(G, V1, V2) -> + case beam_digraph:is_path(G, V1, V2) orelse beam_digraph:is_path(G, V2, V1) of + true -> not_possible(); + false -> ok + end. + %%% %%% Shortcut branches that branch to other branches. %%% |