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-rw-r--r--lib/compiler/src/beam_block.erl17
1 files changed, 11 insertions, 6 deletions
diff --git a/lib/compiler/src/beam_block.erl b/lib/compiler/src/beam_block.erl
index a8cfdffdf3..85d332c56e 100644
--- a/lib/compiler/src/beam_block.erl
+++ b/lib/compiler/src/beam_block.erl
@@ -262,12 +262,17 @@ opt_move_1(R, [{set,[D],[R],move}|Is0], Acc) ->
{yes,Is} -> opt_move_rev(D, Acc, Is);
no -> not_possible
end;
-opt_move_1({x,_}, [{set,_,_,{alloc,_,_}}|_], _) ->
- %% The optimization is not possible. If the X register is not
- %% killed by allocation, the optimization would not be safe.
- %% If the X register is killed, it means that there cannot
- %% follow a 'move' instruction with this X register as the
- %% source.
+opt_move_1(_R, [{set,_,_,{alloc,_,_}}|_], _) ->
+ %% The optimization is either not possible or not safe.
+ %%
+ %% If R is an X register killed by allocation, the optimization is
+ %% not safe. On the other hand, if the X register is killed, there
+ %% will not follow a 'move' instruction with this X register as
+ %% the source.
+ %%
+ %% If R is a Y register, the optimization is still not safe
+ %% because the new target register is an X register that cannot
+ %% safely pass the alloc instruction.
not_possible;
opt_move_1(R, [{set,_,_,_}=I|Is], Acc) ->
%% If the source register is either killed or used by this