[mksnapshot] Add v8_enable_fast_mksnapshot

The v8_enable_fast_mksnapshot gn flag reduces time spent in mksnapshot
on x64 debug builds from 19s to 6s by disabling far jump rewrites and
register allocation verification. This flag should only be used locally
for development.

Bug: v8:6688
Change-Id: I02e8546a6a329b9cb377b95ab586d5857a3c6731
Reviewed-on: https://chromium-review.googlesource.com/632258Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47573}

BUILD.gn

@@ -58,6 +58,9 @@ declare_args() {
   # Enable slow dchecks.
   v8_enable_slow_dchecks = false
 
+  # Enable fast mksnapshot runs.
+  v8_enable_fast_mksnapshot = false
+
   # Enable code-generation-time checking of types in the CodeStubAssembler.
   v8_enable_verify_csa = false
 
@@ -801,6 +804,13 @@ action("run_mksnapshot") {
     sources += [ v8_embed_script ]
     args += [ rebase_path(v8_embed_script, root_build_dir) ]
   }
+
+  if (v8_enable_fast_mksnapshot) {
+    args += [
+      "--no-turbo-rewrite-far-jumps",
+      "--no-turbo-verify-allocation",
+    ]
+  }
 }
 
 action("v8_dump_build_config") {

src/compiler/code-assembler.cc

@@ -154,12 +154,14 @@ Handle<Code> CodeAssembler::GenerateCode(CodeAssemblerState* state) {
 
   RawMachineAssembler* rasm = state->raw_assembler_.get();
   Schedule* schedule = rasm->Export();
+
   JumpOptimizationInfo jump_opt;
+  bool should_optimize_jumps =
+      rasm->isolate()->serializer_enabled() && FLAG_turbo_rewrite_far_jumps;
 
   Handle<Code> code = Pipeline::GenerateCodeForCodeStub(
       rasm->isolate(), rasm->call_descriptor(), rasm->graph(), schedule,
-      state->flags_, state->name_,
-      rasm->isolate()->serializer_enabled() ? &jump_opt : nullptr);
+      state->flags_, state->name_, should_optimize_jumps ? &jump_opt : nullptr);
 
   if (jump_opt.is_optimizable()) {
     jump_opt.set_optimizing();

src/flag-definitions.h

@@ -449,6 +449,8 @@ DEFINE_BOOL(turbo_store_elimination, true,
 DEFINE_BOOL(trace_store_elimination, false, "trace store elimination")
 DEFINE_BOOL(turbo_experimental, false,
             "enable crashing features, for testing purposes only")
+DEFINE_BOOL(turbo_rewrite_far_jumps, true,
+            "rewrite far to near jumps (ia32,x64)")
 
 // Flags to help platform porters
 DEFINE_BOOL(minimal, false,