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authorBjörn Gustavsson <[email protected]>2018-11-27 16:05:23 +0100
committerBjörn Gustavsson <[email protected]>2019-01-14 13:36:01 +0100
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Introduce subtraction of types
Introduce subtraction of types to allow some redundant tests to be eliminated. Consider this function: foo(L) when is_list(L) -> case L of [_|_] -> non_empty; [] -> empty end. After entering the body of the function, L is known to be either a cons cell or nil (otherwise the is_list/1 guard would have failed). If the L is not a cons cell, it must be nil. Therefore, the test for nil in the second clause of the case can be eliminated. Here is the SSA code with some additonal comments for the function before the optimization: function t:foo(_0) { 0: @ssa_bool = bif:is_list _0 br @ssa_bool, label 4, label 3 4: %% _0 is now a list (cons or nil). @ssa_bool:8 = is_nonempty_list _0 br @ssa_bool:8, label 9, label 7 9: ret literal non_empty 7: %% _0 is not a cons (or we wouldn't be here). %% Subtracting cons from the previously known type list %% gives that _0 must be nil. @ssa_bool:10 = bif:'=:=' _0, literal [] br @ssa_bool:10, label 11, label 6 11: ret literal empty 6: _6 = put_tuple literal case_clause, _0 %% t.erl:5 @ssa_ret:12 = call remote (literal erlang):(literal error)/1, _6 ret @ssa_ret:12 3: _9 = put_list _0, literal [] %% t.erl:4 @ssa_ret:13 = call remote (literal erlang):(literal error)/2, literal function_clause, _9 ret @ssa_ret:13 } Type subtraction gives us that _0 must be nil in block 7, allowing us to remove the comparison of _0 with nil. The code for the function can be simplified to: function t:foo(_0) { 0: @ssa_bool = bif:is_list _0 br @ssa_bool, label 4, label 3 4: @ssa_bool:8 = is_nonempty_list _0 br @ssa_bool:8, label 9, label 11 9: ret literal non_empty 11: ret literal empty 3: _9 = put_list _0, literal [] %% t.erl:4 @ssa_ret:13 = call remote (literal erlang):(literal error)/2, literal function_clause, _9 ret @ssa_ret:13 }
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