[RFC] Elide Runtime Polyfill Checks By Adding Closure Typings w. -fstatic-builtins

[sebholl]

Polyfills for ESNext functions are included in many modules. These polyfill functions slow down otherwise native functionality despite often being on critical paths (e.g. object spreads) and so reducing runtime work in these hot-paths is low-hanging fruit.

Typically, these functions would be hard to reason with as JS runtime allows almost all implementations to be overridden. Handily, Hermes already provides a -fstatic-builtins mode that allows us to assert that particular built-ins are frozen.

This PR attempts to take advantage of this to statically reason out any of the if (Object.keys) or typeof Object.keys === 'function' checks, which eliminates never-executed branches, and elides runtime conditional checks.

We do this in LowerBuiltinCalls, where we assert that these built-ins are of type closure and re-run the basic InstSimplify and SimplifyCFG. Empirically, these optimization re-runs at this BCGen level seem to have improved byte-code output notwithstanding.

Test Plan

Runtime execution was shown to have minimal-to-no difference.

Version	Byte-Code Line Count
w. Changes	2,424,160
No Changes	2,431,258

A common babel transform is interopRequireWildcard, which simplifies as follows:

 Function<>(# params, # registers, # symbols):
 Offset in debug table: source #, scope #, textified callees #
  CreateEnvironment #
@@ -1155073,11 +1152371,7 @@ Offset in debug table: source #, scope #, textified callees #
 #:
  NewObject #
  TryGetById #, #, #, "Object"
- GetById #, #, #, "defineProperty"
- JmpFalse #, #
- TryGetById #, #, #, "Object"
  GetById #, #, #, "getOwnPropertyDes"...
-#:
  Mov #, #
  LoadConstUndefined #
  GetPNameList #, #, #, #

An example of the improve byte-code produced where two entire branches are removed from object-shim:

-Function<shimObjectKeys>(1 params, 15 registers, 0 symbols):
-Offset in debug table: source 0x2c3d7b, scope 0x0000, textified callees 0x0000
-    CreateEnvironment r1
-    GetGlobalObject   r0
-    TryGetById        r2, r0, 1, "Object"
-    GetById           r2, r2, 2, "keys"
-    JmpTrue           L1, r2
-    TryGetById        r3, r0, 1, "Object"
-    GetEnvironment    r2, 0
-    LoadFromEnvironment r2, r2, 3
-    PutById           r3, r2, 1, "keys"
-    Jmp               L2
+Function<shimObjectKeys>(1 params, 14 registers, 0 symbols):
+Offset in debug table: source 0x2c27f6, scope 0x0000, textified callees 0x0000
+    CreateEnvironment r0
+    CreateClosure     r4, r0, Function<>
+    LoadConstUndefined r3
+    LoadConstUInt8    r2, 1
+    LoadConstUInt8    r1, 2
+    Call3             r1, r4, r3, r2, r1
+    JmpTrue           L1, r1
+    GetGlobalObject   r1
+    TryGetById        r1, r1, 1, "Object"
+    CreateClosure     r0, r0, Function<keys>
+    PutById           r1, r0, 1, "keys"
 L1:
-    CreateClosure     r5, r1, Function<>
-    LoadConstUndefined r4
-    LoadConstUInt8    r3, 1
-    LoadConstUInt8    r2, 2
-    Call3             r2, r5, r4, r3, r2
-    JmpTrue           L2, r2
-    TryGetById        r2, r0, 1, "Object"
-    CreateClosure     r1, r1, Function<keys>
-    PutById           r2, r1, 1, "keys"
-L2:
+    GetGlobalObject   r0
     TryGetById        r0, r0, 1, "Object"
     GetById           r0, r0, 2, "keys"
-    JmpTrue           L3, r0
-    GetEnvironment    r1, 0
-    LoadFromEnvironment r0, r1, 3
-L3:
     Ret               r0

Finally, automated tests:

$ cmake --build ../build --target check-hermes all -j 4
[103/104] Running the Hermes regression tests
Testing Time: 22.90s
  Expected Passes    : 1767
  Unsupported Tests  : 62