JS: add GHC.JS.Optimizer, remove RTS.Printer, add Linker.Opt

This MR changes some simple optimizations and is a first step in re-architecting
the JS backend pipeline to add the optimizer. In particular it:

- removes simple peep hole optimizations from `GHC.StgToJS.Printer` and removes that module
- adds module `GHC.JS.Optimizer`
- defines the same peep hole opts that were removed only now they are `Syntax -> Syntax` transformations rather than `Syntax -> JS code` optimizations
- hooks the optimizer into code gen
- adds FuncStat and ForStat constructors to the backend.

Working Ticket:
- #22736

Related MRs:
- MR !10142
- MR !10000

-------------------------
Metric Decrease:
    CoOpt_Read
    ManyAlternatives
    PmSeriesS
    PmSeriesT
    PmSeriesV
    T10421
    T12707
    T13253
    T13253-spj
    T15164
    T17516
    T18140
    T18282
    T18698a
    T18698b
    T18923
    T1969
    T19695
    T20049
    T3064
    T5321FD
    T5321Fun
    T783
    T9198
    T9233
    T9630
-------------------------

4 files changed, 116 insertions, 8 deletions

diff --git a/testsuite/tests/javascript/opt/all.T b/testsuite/tests/javascript/opt/all.T
new file mode 100644
index 0000000000..35bdb51ee2
--- /dev/null
+++ b/testsuite/tests/javascript/opt/all.T
@@ -0,0 +1,4 @@
+# These are JavaScript-specific tests for the JS backend optimizer
+setTestOpts(when(not(js_arch()),skip))
+
+test('deadCodeElim', normal, compile_and_run, ['-package ghc'])
diff --git a/testsuite/tests/javascript/opt/deadCodeElim.hs b/testsuite/tests/javascript/opt/deadCodeElim.hs
new file mode 100644
index 0000000000..70f6aa6a74
--- /dev/null
+++ b/testsuite/tests/javascript/opt/deadCodeElim.hs
@@ -0,0 +1,96 @@
+
+import GHC.JS.Optimizer
+import GHC.JS.Syntax
+import GHC.JS.Unsat.Syntax (Ident (..))
+
+import GHC.Data.FastString
+
+double_return :: JStat
+double_return = BlockStat [ ReturnStat (SatInt 0)
+                          , ReturnStat (SatInt 1)
+                          ]
+
+double_return_opt :: JStat
+double_return_opt = (BlockStat [ReturnStat (SatInt 0)])
+
+in_func :: JStat
+in_func = AssignStat (jvar (fsLit "foo")) AssignOp (ValExpr (JFunc [] double_return))
+
+in_func_opt :: JStat
+in_func_opt = AssignStat (jvar (fsLit "foo")) AssignOp (ValExpr (JFunc [] double_return_opt))
+
+nested_blocks :: JStat
+nested_blocks = BlockStat [ double_return <> double_return
+                          , double_return
+                          ] <> double_return
+
+nested_blocks_opt :: JStat
+nested_blocks_opt = double_return_opt
+
+global_func :: JStat
+global_func = FuncStat (TxtI (fsLit "bar")) [] double_return
+
+global_func_opt :: JStat
+global_func_opt = FuncStat (TxtI (fsLit "bar")) [] double_return_opt
+
+func_with_locals :: JStat
+func_with_locals = AssignStat (jvar (fsLit "foo"))
+                   AssignOp
+                   (ValExpr (JFunc []
+                            (BlockStat [ AssignStat (jvar (fsLit "one")) AssignOp (SatInt 2)
+                                       , AssignStat (jvar (fsLit "two")) AssignOp (SatInt 3)
+                                       , ApplStat (jvar (fsLit "f")) [(SatInt 100)]
+                                       , ReturnStat (SatInt 0)
+                                       , ReturnStat (SatInt 1)
+                                       ])))
+
+func_with_locals_opt :: JStat
+func_with_locals_opt = AssignStat (jvar (fsLit "foo"))
+                       AssignOp
+                       (ValExpr (JFunc []
+                                 (BlockStat [ AssignStat (jvar (fsLit "one")) AssignOp (SatInt 2)
+                                            , AssignStat (jvar (fsLit "two")) AssignOp (SatInt 3)
+                                            , ApplStat (jvar (fsLit "f")) [(SatInt 100)]
+                                            , ReturnStat (SatInt 0)
+                                            ])))
+
+-- This one comes straight from MR10260 where we noticed the optimizer was not catching the redundant return
+bignum_test :: JStat
+bignum_test = DeclStat (TxtI $ fsLit "h$ghczmbignumZCGHCziNumziIntegerziintegerToInt64zh_e")
+              (Just (ValExpr $ JFunc [] $ BlockStat [ DeclStat (TxtI $ fsLit "h$$ghczmbignumZCGHCziNumziIntegerzids_s_2f9e") (Just (jvar $ fsLit "h$r2"))
+                                                    , ApplStat (jvar $ fsLit "h$p1") [jvar $ fsLit "h$$ghczmbignumZCGHCziNumziInteger_99"]
+                                                    , ReturnStat (ApplExpr (jvar $ fsLit "h$e") [jvar $ fsLit "h$$ghczmbignumZCGHCziNumziIntegerzids_s_2f9e"])
+                                                    , ReturnStat (ApplExpr (jvar $ fsLit "h$rs") [])]))
+
+bignum_test_opt :: JStat
+bignum_test_opt =
+  DeclStat (TxtI $ fsLit "h$ghczmbignumZCGHCziNumziIntegerziintegerToInt64zh_e")
+              (Just (ValExpr $ JFunc [] $ BlockStat [ DeclStat (TxtI $ fsLit "h$$ghczmbignumZCGHCziNumziIntegerzids_s_2f9e") (Just (jvar $ fsLit "h$r2"))
+                                                    , ApplStat (jvar $ fsLit "h$p1") [jvar $ fsLit "h$$ghczmbignumZCGHCziNumziInteger_99"]
+                                                    , ReturnStat (ApplExpr (jvar $ fsLit "h$e") [jvar $ fsLit "h$$ghczmbignumZCGHCziNumziIntegerzids_s_2f9e"])
+                                                    ]))
+
+bignum_test_2 :: JStat
+bignum_test_2 = BlockStat [FuncStat (TxtI $ fsLit "h$$ghczmbignumZCGHCziNumziInteger_99") [] (BlockStat [DeclStat (TxtI $ fsLit "h$ghczmbignumZCGHCziNumziIntegerziintegerToInt64zh_e")
+              (Just (ValExpr $ JFunc [] $ BlockStat [ DeclStat (TxtI $ fsLit "h$$ghczmbignumZCGHCziNumziIntegerzids_s_2f9e") (Just (jvar $ fsLit "h$r2"))
+                                                    , ApplStat (jvar $ fsLit "h$p1") [jvar $ fsLit "h$$ghczmbignumZCGHCziNumziInteger_99"]
+                                                    , ReturnStat (ApplExpr (jvar $ fsLit "h$e") [jvar $ fsLit "h$$ghczmbignumZCGHCziNumziIntegerzids_s_2f9e"])
+                                                    , ReturnStat (ApplExpr (jvar $ fsLit "h$rs") [])]))])]
+
+bignum_test_opt_2 :: JStat
+bignum_test_opt_2 = BlockStat [FuncStat (TxtI $ fsLit "h$$ghczmbignumZCGHCziNumziInteger_99") [] (BlockStat [DeclStat (TxtI $ fsLit "h$ghczmbignumZCGHCziNumziIntegerziintegerToInt64zh_e")
+              (Just (ValExpr $ JFunc [] $ BlockStat [ DeclStat (TxtI $ fsLit "h$$ghczmbignumZCGHCziNumziIntegerzids_s_2f9e") (Just (jvar $ fsLit "h$r2"))
+                                                    , ApplStat (jvar $ fsLit "h$p1") [jvar $ fsLit "h$$ghczmbignumZCGHCziNumziInteger_99"]
+                                                    , ReturnStat (ApplExpr (jvar $ fsLit "h$e") [jvar $ fsLit "h$$ghczmbignumZCGHCziNumziIntegerzids_s_2f9e"])
+                                                    ]))])]
+
+main :: IO ()
+main = mapM_ print
+       [ jsOptimize double_return == double_return_opt
+       , jsOptimize in_func       == in_func_opt
+       , jsOptimize nested_blocks == nested_blocks_opt
+       , jsOptimize global_func   == global_func_opt
+       , jsOptimize func_with_locals == func_with_locals_opt
+       , jsOptimize bignum_test == bignum_test_opt
+       , jsOptimize bignum_test_2 == bignum_test_opt_2
+       ]
diff --git a/testsuite/tests/javascript/opt/deadCodeElim.stdout b/testsuite/tests/javascript/opt/deadCodeElim.stdout
new file mode 100644
index 0000000000..672e08f95c
--- /dev/null
+++ b/testsuite/tests/javascript/opt/deadCodeElim.stdout
@@ -0,0 +1,7 @@
+True
+True
+True
+True
+True
+True
+True
diff --git a/testsuite/tests/linters/notes.stdout b/testsuite/tests/linters/notes.stdout
index 51fef76584..aed259caf9 100644
--- a/testsuite/tests/linters/notes.stdout
+++ b/testsuite/tests/linters/notes.stdout
@@ -7,7 +7,7 @@ ref    compiler/GHC/Core/Opt/Simplify/Utils.hs:1343:37:     Note [Gentle mode]
 ref    compiler/GHC/Core/Opt/Specialise.hs:1790:28:     Note [Arity decrease]
 ref    compiler/GHC/Core/TyCo/Rep.hs:1556:31:     Note [What prevents a constraint from floating]
 ref    compiler/GHC/Driver/Main.hs:1762:34:     Note [simpleTidyPgm - mkBootModDetailsTc]
-ref    compiler/GHC/Driver/Session.hs:3993:49:     Note [Eta-reduction in -O0]
+ref    compiler/GHC/Driver/Session.hs:4062:49:     Note [Eta-reduction in -O0]
 ref    compiler/GHC/Hs/Expr.hs:194:63:     Note [Pending Splices]
 ref    compiler/GHC/Hs/Expr.hs:1736:87:     Note [Lifecycle of a splice]
 ref    compiler/GHC/Hs/Expr.hs:1772:7:     Note [Pending Splices]
@@ -15,6 +15,7 @@ ref    compiler/GHC/Hs/Extension.hs:146:5:     Note [Strict argument type constr
 ref    compiler/GHC/Hs/Pat.hs:143:74:     Note [Lifecycle of a splice]
 ref    compiler/GHC/HsToCore/Pmc/Solver.hs:858:20:     Note [COMPLETE sets on data families]
 ref    compiler/GHC/HsToCore/Quote.hs:1476:7:     Note [How brackets and nested splices are handled]
+ref    compiler/GHC/JS/Optimizer.hs:206:7:     Note [Unsafe JavaScript optimizations]
 ref    compiler/GHC/Stg/Unarise.hs:442:32:     Note [Renaming during unarisation]
 ref    compiler/GHC/StgToCmm.hs:106:18:     Note [codegen-split-init]
 ref    compiler/GHC/StgToCmm.hs:109:18:     Note [pipeline-split-init]
@@ -25,14 +26,14 @@ ref    compiler/GHC/Tc/Gen/HsType.hs:2621:7:     Note [Matching a kind signature
 ref    compiler/GHC/Tc/Gen/Pat.hs:176:20:     Note [Typing patterns in pattern bindings]
 ref    compiler/GHC/Tc/Gen/Pat.hs:1127:7:     Note [Matching polytyped patterns]
 ref    compiler/GHC/Tc/Gen/Sig.hs:81:10:     Note [Overview of type signatures]
-ref    compiler/GHC/Tc/Gen/Splice.hs:357:16:     Note [How brackets and nested splices are handled]
-ref    compiler/GHC/Tc/Gen/Splice.hs:532:35:     Note [PendingRnSplice]
-ref    compiler/GHC/Tc/Gen/Splice.hs:656:7:     Note [How brackets and nested splices are handled]
-ref    compiler/GHC/Tc/Gen/Splice.hs:889:11:     Note [How brackets and nested splices are handled]
+ref    compiler/GHC/Tc/Gen/Splice.hs:356:16:     Note [How brackets and nested splices are handled]
+ref    compiler/GHC/Tc/Gen/Splice.hs:531:35:     Note [PendingRnSplice]
+ref    compiler/GHC/Tc/Gen/Splice.hs:655:7:     Note [How brackets and nested splices are handled]
+ref    compiler/GHC/Tc/Gen/Splice.hs:888:11:     Note [How brackets and nested splices are handled]
 ref    compiler/GHC/Tc/Instance/Family.hs:474:35:     Note [Constrained family instances]
 ref    compiler/GHC/Tc/Module.hs:711:15:     Note [Extra dependencies from .hs-boot files]
 ref    compiler/GHC/Tc/Solver/Rewrite.hs:1008:7:     Note [Stability of rewriting]
-ref    compiler/GHC/Tc/TyCl.hs:1120:6:     Note [Unification variables need fresh Names]
+ref    compiler/GHC/Tc/TyCl.hs:1124:6:     Note [Unification variables need fresh Names]
 ref    compiler/GHC/Tc/Types.hs:692:33:     Note [Extra dependencies from .hs-boot files]
 ref    compiler/GHC/Tc/Types.hs:1423:47:     Note [Care with plugin imports]
 ref    compiler/GHC/Tc/Types/Constraint.hs:255:34:     Note [NonCanonical Semantics]
@@ -46,8 +47,8 @@ ref    hadrian/src/Expression.hs:145:30:     Note [Linking ghc-bin against threa
 ref    linters/lint-notes/Notes.hs:32:29:     Note [" <> T.unpack x <> "]
 ref    linters/lint-notes/Notes.hs:69:22:     Note [...]
 ref    testsuite/config/ghc:272:10:     Note [WayFlags]
-ref    testsuite/driver/testlib.py:160:10:     Note [Why is there no stage1 setup function?]
-ref    testsuite/driver/testlib.py:164:2:     Note [Why is there no stage1 setup function?]
+ref    testsuite/driver/testlib.py:165:10:     Note [Why is there no stage1 setup function?]
+ref    testsuite/driver/testlib.py:169:2:     Note [Why is there no stage1 setup function?]
 ref    testsuite/mk/boilerplate.mk:267:2:     Note [WayFlags]
 ref    testsuite/tests/indexed-types/should_fail/ExtraTcsUntch.hs:30:27:     Note [Extra TcS Untouchables]
 ref    testsuite/tests/perf/should_run/all.T:8:6:     Note [Solving from instances when interacting Dicts]