4 ms·

Based on a quick look through https://bugzilla.mozilla.org/show_bug.cgi?id=1319203 https://bugzilla.mozilla.org/show_bug.cgi?id=1319203 it looks like the entry

Based on a quick look through https://bugzilla.mozilla.org/show_bug.cgi?id=1319203 https://bugzilla.mozilla.org/show_bug.cgi?id=1319203 it looks like the entry stub is generated by the JIT itself. So no, the entry stub is not written in C++. It gets output by the JIT as part of the generated jitcode. Specifically, see the GenerateJitEntry bits in https://bug1319203.bmoattachments.org/attachment.cgi?id=8949103 https://bug1319203.bmoattachments.org/attachment.cgi?id=8949... -- the masm.whatever() calls are calls into an architecture-dependent assembler that will output the relevant machine instructions. There is still an out-of-line codepath that does call into a C++-implemented entry stub in some cases when the argument conversions involved are too complicated to do them directly in the JIT-generated code.

mathw 8y ago
Thanks for this, I was also quite confused as to why the "jumping through C++" stages were so expensive. Thinking about what you've said, it makes a lot more sense that you're moving between two worlds and marshalling arguments back and forth in a variety of expensive ways. It's a shame Lin didn't explain that a little more in an otherwise excellent article, but I guess she was focussing on the JS/JIT/WASM story.