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Chore: Update V8 flag initialization to use V8 API #458

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Chore: Update V8 flag initialization to use V8 API #458

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a9843a6
Set flags via the v8 API, instead of directly modifying them
rachgreen33 Sep 16, 2025
785835d
Merge branch 'main' into v8_flags_from_string
rachgreen33 Sep 16, 2025
fbec45d
Continue using the liftoff disable flag, but this time via the v8 API
rachgreen33 Sep 16, 2025
965ccd7
Fixed the includes statements.
rachgreen33 Sep 16, 2025
d1c7aa2
Fixed includes ordering.
rachgreen33 Sep 17, 2025
e061828
Added a comment explaining why we are disabling liftoff.
rachgreen33 Sep 17, 2025
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11 changes: 7 additions & 4 deletions src/v8/v8.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -28,9 +28,10 @@

#include "include/proxy-wasm/limits.h"

#include "absl/strings/str_format.h"
#include "include/v8-initialization.h"
#include "include/v8-version.h"
#include "include/v8.h"
#include "src/flags/flags.h"
#include "src/wasm/c-api.h"
#include "wasm-api/wasm.hh"

Expand All @@ -42,10 +43,12 @@ wasm::Engine *engine() {
static wasm::own<wasm::Engine> engine;

std::call_once(init, []() {
::v8::internal::v8_flags.liftoff = false;
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But why? It was written years ago, but at the time loading Wasm module wouldn't complete before all compilation phases completed, so Liftoff compilation was wasteful. Is that not the case anymore?

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Thanks for the callout on this. I discussed with Matt and we do want to keep this disabled - Updated.

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@mpwarres mpwarres Sep 17, 2025 •
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Keeping it SGTM, but I think it would be helpful to add a comment that provides some context on why we set it. Internal bug b/278233034 contains some pointers to past discussions. I'd suggest something like: "Disable Liftoff compiler to force optimized JIT up-front."

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Again, this is based on tests from years ago, but TurboFan was always blocking loading of Wasm modules, and this flag doesn't affect that. What it does is that it skips Liftoff compilation that would be discarded in favor of TurboFan anyway... It would be good to verify that's still the case, though.

There are some docs for V8 here: https://v8.dev/docs/wasm-compilation-pipeline

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We want Turbofan to block on loading of Wasm modules, right? Thanks for the docs link, it aligns with discussion above: --no-liftoff sets the "TurboFan only" mode. If liftoff is enabled, calls can immediately use the compiled code, and then re-compilation with TurboFan is triggered in the background only after a certain threshold of calls.

::v8::internal::v8_flags.wasm_max_mem_pages =
PROXY_WASM_HOST_MAX_WASM_MEMORY_SIZE_BYTES / PROXY_WASM_HOST_WASM_MEMORY_PAGE_SIZE_BYTES;
std::string args = absl::StrFormat("--wasm_max_mem_pages=%u --no-liftoff",
PROXY_WASM_HOST_MAX_WASM_MEMORY_SIZE_BYTES /
PROXY_WASM_HOST_WASM_MEMORY_PAGE_SIZE_BYTES);
::v8::V8::SetFlagsFromString(args.c_str(), args.size());
::v8::V8::EnableWebAssemblyTrapHandler(true);

engine = wasm::Engine::make();
});

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