During codegen we do not yet know the indexes that will be used for
called functions. Therefore, we store the offset into the in-memory
code where the index is needed with a pointer to the Decl and use this
data to insert the proper indexes while writing the binary in the flush
function.
Before this commit the wasm backend worked similarly to elf. As
functions were generated they were written directly to the output file
and existing code was shifted around in the file as necessary. This
approach had several disadvantages:
- Large amounts of padding in the output were necessary to avoid
expensive copying of data within the file.
- Function/type/global/etc indexes were required to be known at the time
of preforming codegen, which severely limited the flexibility of where
code could be placed in the binary
- Significant complexity to track the state of the output file through
incremental updates
This commit takes things in a different direction. Code is incrementally
compiled into in-memory buffers and the entire binary is rewritten using
these buffers on flush. This has several advantages:
- Significantly smaller resulting binaries
- More performant resulting binaries due to lack of indirection
- Significantly simpler compiler code
- Indexes no longer need to be known before codegen. We can track where
Decls must be referenced by index insert the proper indexes while
writing the code in the flush() function. This is not yet implemented
but is planned for the next commit.
The main disadvantage is of course increased memory usage in order to
store these buffers of generated code.
This commit write out Mach-O header in the linker's `flush`
method. The header currently only populates the magic number,
filetype, and cpu info.
Signed-off-by: Jakub Konka <kubkon@jakubkonka.com>
* Implemented all R-type arithmetic/logical instructions
* Implemented all I-type arithmetic/logical instructions
* Implemented all load and store instructions
* Implemented all of RV64I except FENCE
Functions which are free'd are not immediately removed from the binary
as this would cause a shifting of function indexes. Instead, they hang
around until they can be overwritten by a new function. This means that
the types associated with these dead functions must also remain until
the function is overwritten to avoid a type mismatch.
Exports now have a dirty flag and are rewritten on flush if this flag
has been set.
A couple other minor changes have been made based on Andrew's review.
Thus far, we only generate the type, function, export, and code
sections. These are sufficient to generate and export simple functions.
Codegen is currently hardcoded to `i32.const 42`, the main goal of this
commit is to create infrastructure for the container format which will
work with incremental compilation.
This is part of an ongoing effort to reduce size of in-memory AST. This
enum flattening pattern is widespread throughout the self-hosted
compiler.
This is a API breaking change for consumers of the self-hosted parser.
