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Add debug section to the gen_dev crate readme

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Ahmad Sattar 2023-03-08 23:33:44 +01:00
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@ -86,6 +86,57 @@ This is the general procedure I follow with some helpful links:
1. If things aren't working, reach out on zulip. Get advice, maybe even pair.
1. Make a PR.
## Debugging x86_64 backend output
While working on the x86_64 backend it may be useful to inspect the assembly output of a given piece of Roc code. With the right tools, you can do this rather easily. You'll need `objdump` to follow along.
We'll try to explore the x86 assembly output of some lines of Roc code:
```elixir
app "dbg"
provides [main] to "."
main =
(List.len [1]) + 41
```
If this file exists somewhere in the repo as `dbg.roc`, we'll be able to compile an object file by issuing the following command:
```console
# `cargo run --` can be replaces with calling the compiled `roc` cli binary.
$ cargo run -- build --dev main.roc --no-link
```
Which will produce a minimal `dbg.o` object file containing the output assembly code. This object file can be inspected by using `objdump` in the following way:
```console
$ objdump -M intel -dS dbg.o
dbg.o: file format elf64-x86-64
Disassembly of section .text.700000006:
0000000000000000 <List_len_1>:
0: 55 push rbp
1: 48 89 e5 mov rbp,rsp
4: 48 8b 85 18 00 00 00 mov rax,QWORD PTR [rbp+0x18]
b: 5d pop rbp
c: c3 ret
Disassembly of section .text.400000013:
0000000000000000 <Num_add_1>:
0: 55 push rbp
# .. more output ..
Disassembly of section .text.1000000000:
0000000000000000 <roc__main_1_exposed>:
0: 55 push rbp
# .. more output ..
```
The output lines contain the hexadecimal representation of the x86 opcodes and fields followed by the `intel` assembly syntax. This setup is very useful for figuring out the causes of invalid pointer references (or equivalent) when running the resulting x86 assembly.
## Helpful Resources
- [Compiler Explorer](https://godbolt.org/) -