Move let expressions into sequences

The Core Erlang compiler simplifies let expressions by moving them
into their argument when it is another let or a case expression.
It can then perform other optimizations such as removing the let
expressions altogether, sometimes saving BEAM registers later in
the process.

This commit teaches sys_core_fold how to move let expressions into
sequence arguments.

1 files changed, 19 insertions, 0 deletions

diff --git a/lib/compiler/src/sys_core_fold.erl b/lib/compiler/src/sys_core_fold.erl
index cda3f7d81e..6b0ae87172 100644
--- a/lib/compiler/src/sys_core_fold.erl
+++ b/lib/compiler/src/sys_core_fold.erl
@@ -2342,6 +2342,25 @@ move_let_into_expr(#c_let{vars=Lvs0,body=Lbody0}=Let,
 	    Case#c_case{arg=Cexpr,clauses=[Ca,Cb]};
 	{_,_,_} -> impossible
     end;
+move_let_into_expr(#c_let{vars=Lvs0,body=Lbody0}=Let,
+		   #c_seq{arg=Sarg0,body=Sbody0}=Seq, Sub0) ->
+    %%
+    %% let <Lvars> = do <Seq-arg>
+    %%                  <Seq-body>
+    %% in <Let-body>
+    %%
+    %%       ==>
+    %%
+    %% do <Seq-arg>
+    %%    let <Lvars> = <Seq-body>
+    %%    in <Let-body>
+    %%
+    Sarg = body(Sarg0, Sub0),
+    Sbody1 = body(Sbody0, Sub0),
+    {Lvs,Sbody,Sub} = let_substs(Lvs0, Sbody1, Sub0),
+    Lbody = body(Lbody0, Sub),
+    Seq#c_seq{arg=Sarg,body=Let#c_let{vars=Lvs,arg=core_lib:make_values(Sbody),
+				      body=Lbody}};
 move_let_into_expr(_Let, _Expr, _Sub) -> impossible.
 
 is_failing_clause(#c_clause{body=B}) ->