Append .aot to .wasm as a custom section named "aot"

[lum1n0us]

No description provided.

[yamt]

how do you plan to use the section?

[lum1n0us]

a pre_compiled wasm plugin for Envoy

[yamt]

a pre_compiled wasm plugin for Envoy

how does it load and execute the aot section?
i'm asking because i have alignment concerns.

[lum1n0us]

thanks for reminding. in a nutshell, envoy will load aot custom section and pass the content(in a string_view) to WAMR. https://github.com/proxy-wasm/proxy-wasm-cpp-host/blob/master/src/wasm.cc#L293

[yamt]

thanks for reminding. in a nutshell, envoy will load aot custom section and pass the content(in a string_view) to WAMR. https://github.com/proxy-wasm/proxy-wasm-cpp-host/blob/master/src/wasm.cc#L293

i suppose that it's safer to make some padding so that the start offset of section_content within the file will have the max alignment we can use in our aot format. (4 byte?)

[lum1n0us]

the section layout likes:

| section id(0) | section length | section name | section content(.aot) |

IIUC, please correct me if wrong, the 4 byte padding goes to the beginning of section content, and the consumer(WAMR or embedder) needs to ignore the 4 padding.

[yamt]

the section layout likes:

| section id(0) | section length | section name | section content(.aot) |

IIUC, please correct me if wrong, the 4 byte padding goes to the beginning of section content, and the consumer(WAMR or embedder) needs to ignore the 4 padding.

yea, i guess it's simplest to add the padding at the top of the custom section body.
like:

 | section id(0) | section length | section name | padding len | padding (zeros) | section content(.aot) |

note that the implementation might be a bit tricky as "section length" is variable lengh (leb128) and it affects the padding length.
you might want to use a redundant encoding of leb128. (eg. always use 5 byte)