d8.exe built for ARM64EC crashes immediately when compiling a sample JS file

[The Gorge]

ARM64EC:

ARM64EC (Emulation Compatible) is a Windows ABI from Microsoft that allows x64 and ARM64 code to run together in the same process. (Reference: Microsoft ARM64EC documentation)

We are trying to build V8 (commit 50d767cfc52, Chromium 104.0.5112.102) with ARM64EC support.

Since ARM64EC requires a newer toolchain, we upgraded Clang 15 → Clang 21 and Visual Studio → 17.14.6 / Windows SDK 26100.

To make the code compatible with the newer Clang, we applied minor fixes based on the compilation errors reported.

These changes build and run correctly for:

• x64 works (d8.exe hello.js runs fine)
• ARM64 works (d8.exe hello.js runs fine)
• ARM64EC builds successfully, but d8.exe hello.js immediately crashes:

C:\code\chromium_git\chromium\src\out\Debug_GN_arm64ec>d8.exe hello.js

 ==== C stack trace ===============================

        (No symbol) [0x0000024F9792CD00]
        $iexit_thunk$cdecl$i8$i8i8i8i8i8i8 [0x00007FFA40A6A6B8+28]

This issue occurs only with ARM64EC builds, while x64 and ARM64 builds work correctly under the same toolchain and with the minor code adjustments required for newer Clang. 

Reference issue:

• A related Chromium issue reporting an ARM64EC crash in d8.exe: Chromium issue 440947107

Could you give me some suggestions about this issue? Thanks.

[The Gorge]

New observation:

• When running with the --jitless flag, d8.exe is able to execute a sample JavaScript file successfully under ARM64EC.
• This seems to suggest that the crash occurs only when JIT is enabled.
• Could this be related to missing or incomplete ARM64EC-specific implementation in V8’s JIT backend?

Any pointers on where to start looking would be greatly appreciated.