Cannot run a snapshot with --preview-dart-2

[natebosch]

If I create a snapshot without --preview-dart-2 and try to run it with --preview-dart-2 I get some long stack traces starting with:

Crash when formatting: [Encoding] Unable to decode bytes as UTF-8.
RangeError (index): Invalid value: Not in range 0..5792, inclusive: 5813
#0      _StringBase.[] (dart:core/runtime/libstring_patch.dart:233:55)
#1      Source.getTextLine (package:kernel/ast.dart:5695:42)
#2      getSourceLine (package:front_end/src/fasta/messages.dart:27:9)

If I create a snapshot with --preview-dart-2 and try to run it with --preview-dart-2 I get a bunch of garbage output that make it look like it's trying to parser the binary file as dart text source then a stack trace.

If I create a snapshot with --preview-dart-2 and try to run it without any VM arguments I get a sensible error:

Snapshot not compatible with the current VM configuration: the snapshot requires 'release strong' but the VM has 'release no-strong'

[vsmenon]

Do we need this fixed to unblock #32188 ?

[vsmenon]

@a-siva @mraleph

[natebosch]

Do we need this fixed to unblock #32188 ?

We can make some progress by running from source always for Dart 2 mode but we'll be paying a performance cost.

[keertip]

@natebosch, in Dart 2 appjit snapshots is what works, no script snapshots. Did you try an appjit?

[natebosch]

No we have not tried appjit snapshots. This will require some work in pub, I'm not sure how significant.

Is there reason we aren't supporting non-jit snapshots?

[keertip]

In dart 2, the kernel file is the equivalent of script snapshot. You can generate kernel files for the vm using the gen_kernel tool and then pass this to the VM instead of source.

[natebosch]

I can't find gen_kernel in the dev.48 sdk bin directory - where are we shipping it?

[keertip]

@a-siva @alexmarkov, we are not shipping the gen_kernel tool in the sdk build. Could you add this?

[dgrove]

cc @mit-mit we should think about whether gen_kernel is the right name for this tool.

[jakobr-google]

So, um, why do we need a different tool? Why doesn't dart --preview-dart-2 --snapshot=<snapshot-file> <dart-file> produce a snapshot in the right format?

[jensjoha]

Script snapshotting through kernel _used_to work...

Now though I get this:

$ pkg/vm/tool/dart2 --snapshot=foo.snapshot hello.dart

$ ls -lha foo.snapshot
-rw-r----- 1 jensj primarygroup 847 Apr 20 12:22 foo.snapshot

$ out/ReleaseX64/dart foo.snapshot
Snapshot not compatible with the current VM configuration: the snapshot requires 'release strong' but the VM has 'release no-strong'

$ out/ReleaseX64/dart --strong foo.snapshot                            
Snapshot not compatible with the current VM configuration: the snapshot requires 'release no-strong' but the VM has 'release strong'

[mit-mit]

Big +1 for making it work again, and not introducing yet another tool.

[jensjoha]

Interestingly this still works:

$ pkg/front_end/tool/fasta compile hello.dart

$ out/ReleaseX64/dart --snapshot=foo.snapshot hello.dart.dill

$ ls -lha foo.snapshot
-rw-r----- 1 jensj primarygroup 452K Apr 20 15:39 foo.snapshot

$ out/ReleaseX64/dart foo.snapshot
Hello, world!

Updated last command...

[vsmenon]

@a-siva - assigning to you. We need this to create and run Dart 2 snapshots for pub, test, etc.

[mraleph]

@natebosch do we need this to work only when building SDK or do we need this to work from SDK itself too?

[natebosch]

We need this to work externally to the SDK.

pub builds snapshots of binaries to run them, and if we're going to have to pay a ~10 second penalty for running from source we'll really want to keep doing that so we only pay the parse cost once.

Similarly in build_runner we currently rely on starting an Isolate on a dart source file we wrote. If there is a startup performance penalty we need to start using a snapshot there too.

[jensjoha]

Disclaimer: I'm not suggesting we do this exactly as such, but the following hack (or proof of concept or whatnot) at least works on a hello world script:

0. Tried on top of 1f7fa3e
1. Revert da14cbd.
2. Apply the following patch:

diff --git a/runtime/bin/dfe.cc b/runtime/bin/dfe.cc
index 6bccb8de40..c6387b8e1b 100644
--- a/runtime/bin/dfe.cc
+++ b/runtime/bin/dfe.cc
@@ -230,6 +230,30 @@ void* DFE::CompileAndReadScript(const char* script_uri,
       strong ? platform_strong_dill : platform_dill;
   intptr_t platform_binary_size =
       strong ? platform_strong_dill_size : platform_dill_size;
+
+  {
+    const uint8_t* buffer = NULL;
+    intptr_t buffer_length = -1;
+    const uint8_t** kernel_ir = &buffer;
+    intptr_t* kernel_ir_size = &buffer_length;
+
+    *kernel_ir = NULL;
+    *kernel_ir_size = -1;
+    void* script_file = DartUtils::OpenFileUri(script_uri, false);
+    if (script_file != NULL) {
+      const uint8_t* buffer = NULL;
+      DartUtils::ReadFile(&buffer, kernel_ir_size, script_file);
+      DartUtils::CloseFile(script_file);
+      if (*kernel_ir_size != 0 && buffer != NULL) {
+        if (DartUtils::SniffForMagicNumber(buffer, *kernel_ir_size) ==
+            DartUtils::kSnapshotMagicNumber) {
+          Dart_EnterScope();
+          return Dart_LoadScriptFromSnapshot(buffer, *kernel_ir_size);
+        }
+      }
+    }
+  }
+
   Dart_KernelCompilationResult result = Dart_CompileToKernel(
       sanitized_uri, platform_binary, platform_binary_size);
   switch (result.status) {
diff --git a/runtime/vm/dart_api_impl.cc b/runtime/vm/dart_api_impl.cc
index eff8af86ba..b80c021237 100644
--- a/runtime/vm/dart_api_impl.cc
+++ b/runtime/vm/dart_api_impl.cc
@@ -5129,9 +5129,7 @@ DART_EXPORT Dart_Handle Dart_LoadScript(Dart_Handle url,
   }
   Library& library = Library::Handle(Z, I->object_store()->root_library());
   if (!library.IsNull()) {
-    const String& library_url = String::Handle(Z, library.url());
-    return Api::NewError("%s: A script has already been loaded from '%s'.",
-                         CURRENT_FUNC, library_url.ToCString());
+    return Api::NewHandle(T, library.raw());
   }
   if (line_offset < 0) {
     return Api::NewError("%s: argument 'line_offset' must be positive number",

3. Try this:

$ time pkg/vm/tool/dart2 hello.dart
Hello, world!

real    0m0.401s
user    0m0.552s
sys     0m0.112s

$ rm foo.snapshot

$ time pkg/vm/tool/dart2 --snapshot=foo.snapshot hello.dart

real    0m0.512s
user    0m0.744s
sys     0m0.140s

$ ls -la foo.snapshot
-rw-r----- 1 jensj primarygroup 847 Apr 23 10:43 foo.snapshot

$ time pkg/vm/tool/dart2 foo.snapshot
Hello, world!

real    0m0.193s
user    0m0.276s
sys     0m0.080s

Again: I'm not saying we should do it like this, but maybe this can give someone else a starting point.

Update: I just tried on dart2js, and that doesn't work with the above. It appears that many dart: libraries are included in the snapshot and it thus hits ASSERT(Library::LookupLibrary(thread, lib_url) == Library::null()); when calling Library::Register.

[a-siva]

CL https://dart-review.googlesource.com/c/sdk/+/52740 out for review

sivalinuxmach[sdk]>out/ReleaseX64/dart --preview-dart-2 --snapshot=/tmp/hello.snapshot /tmp/hello.dart
sivalinuxmach[sdk]>out/ReleaseX64/dart /tmp/hello.snapshot
Hello World
sivalinuxmach[sdk]>

[dgrove]

Any update on this, @a-siva ?

[dgrove]

CL https://dart-review.googlesource.com/c/sdk/+/52740 has landed. Can this be closed?

[a-siva]

Yes the CL has landed and stuck, closing issue.

[kevmoo]

4b1180a – landed in -dev.53