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Merge pull request rust-lang#444 from GuillaumeGomez/improve-docs
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@antoyo
antoyo authored Feb 17, 2024
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252 changes: 29 additions & 223 deletions Readme.md
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Expand Up @@ -17,6 +17,18 @@ A secondary goal is to check if using the gcc backend will provide any run-time
**This requires a patched libgccjit in order to work.
You need to use my [fork of gcc](https://github.com/antoyo/gcc) which already includes these patches.**

```bash
$ cp config.example.toml config.toml
```

If don't need to test GCC patches you wrote in our GCC fork, then the default configuration should
be all you need. You can update the `rustc_codegen_gcc` without worrying about GCC.

### Building with your own GCC version

If you wrote a patch for GCC and want to test it without this backend, you will need
to do a few more things.

To build it (most of these instructions come from [here](https://gcc.gnu.org/onlinedocs/jit/internals/index.html), so don't hesitate to take a look there if you encounter an issue):

```bash
Expand Down Expand Up @@ -51,18 +63,17 @@ $ make check-jit RUNTESTFLAGS="-v -v -v jit.exp=jit.dg/test-asm.cc"

**Put the path to your custom build of libgccjit in the file `config.toml`.**

If you followed the instructions exactly as written (ie, you have created a `gcc-build` folder
where gcc is built), the only thing you need to do is:
You now need to set the `gcc-path` value in `config.toml` with the result of this command:

```bash
$ cp config.example.toml config.toml
$ dirname $(readlink -f `find . -name libgccjit.so`)
```

But if you did something different, you also need to set the `gcc-path` value in `config.toml` with
the result of this command:
and to comment the `download-gccjit` setting:

```bash
$ dirname $(readlink -f `find . -name libgccjit.so`)
```toml
gcc-path = "[MY PATH]"
# download-gccjit = true
```

Then you can run commands like this:
Expand Down Expand Up @@ -128,224 +139,19 @@ $ LIBRARY_PATH="[gcc-path value]" LD_LIBRARY_PATH="[gcc-path value]" rustc +$(ca
<dd>Dump a C-like representation to /tmp/gccjit_dumps and enable debug info in order to debug this C-like representation.</dd>
</dl>

## Licensing

While this crate is licensed under a dual Apache/MIT license, it links to `libgccjit` which is under the GPLv3+ and thus, the resulting toolchain (rustc + GCC codegen) will need to be released under the GPL license.

However, programs compiled with `rustc_codegen_gcc` do not need to be released under a GPL license.

## Debugging

Sometimes, libgccjit will crash and output an error like this:

```
during RTL pass: expand
libgccjit.so: error: in expmed_mode_index, at expmed.h:249
0x7f0da2e61a35 expmed_mode_index
../../../gcc/gcc/expmed.h:249
0x7f0da2e61aa4 expmed_op_cost_ptr
../../../gcc/gcc/expmed.h:271
0x7f0da2e620dc sdiv_cost_ptr
../../../gcc/gcc/expmed.h:540
0x7f0da2e62129 sdiv_cost
../../../gcc/gcc/expmed.h:558
0x7f0da2e73c12 expand_divmod(int, tree_code, machine_mode, rtx_def*, rtx_def*, rtx_def*, int)
../../../gcc/gcc/expmed.c:4335
0x7f0da2ea1423 expand_expr_real_2(separate_ops*, rtx_def*, machine_mode, expand_modifier)
../../../gcc/gcc/expr.c:9240
0x7f0da2cd1a1e expand_gimple_stmt_1
../../../gcc/gcc/cfgexpand.c:3796
0x7f0da2cd1c30 expand_gimple_stmt
../../../gcc/gcc/cfgexpand.c:3857
0x7f0da2cd90a9 expand_gimple_basic_block
../../../gcc/gcc/cfgexpand.c:5898
0x7f0da2cdade8 execute
../../../gcc/gcc/cfgexpand.c:6582
```

To see the code which causes this error, call the following function:

```c
gcc_jit_context_dump_to_file(ctxt, "/tmp/output.c", 1 /* update_locations */)
```
This will create a C-like file and add the locations into the IR pointing to this C file.
Then, rerun the program and it will output the location in the second line:
```
libgccjit.so: /tmp/something.c:61322:0: error: in expmed_mode_index, at expmed.h:249
```
Or add a breakpoint to `add_error` in gdb and print the line number using:
```
p loc->m_line
p loc->m_filename->m_buffer
```
To print a debug representation of a tree:
```c
debug_tree(expr);
```

(defined in print-tree.h)

To print a debug representation of a gimple struct:

```c
debug_gimple_stmt(gimple_struct)
```
To get the `rustc` command to run in `gdb`, add the `--verbose` flag to `cargo build`.
To have the correct file paths in `gdb` instead of `/usr/src/debug/gcc/libstdc++-v3/libsupc++/eh_personality.cc`:
Maybe by calling the following at the beginning of gdb:
```
set substitute-path /usr/src/debug/gcc /path/to/gcc-repo/gcc
```
TODO(antoyo): but that's not what I remember I was doing.
### `failed to build archive` error
When you get this error:
```
error: failed to build archive: failed to open object file: No such file or directory (os error 2)
```
That can be caused by the fact that you try to compile with `lto = "fat"`, but you didn't compile the sysroot with LTO.
(Not sure if that's the reason since I cannot reproduce anymore. Maybe it happened when forgetting setting `FAT_LTO`.)
### ld: cannot find crtbegin.o
When compiling an executable with libgccijt, if setting the `*LIBRARY_PATH` variables to the install directory, you will get the following errors:
## Extra documentation

```
ld: cannot find crtbegin.o: No such file or directory
ld: cannot find -lgcc: No such file or directory
ld: cannot find -lgcc: No such file or directory
libgccjit.so: error: error invoking gcc driver
```
To fix this, set the variables to `gcc-build/build/gcc`.
### How to debug GCC LTO
Run do the command with `-v -save-temps` and then extract the `lto1` line from the output and run that under the debugger.
### How to send arguments to the GCC linker
```
CG_RUSTFLAGS="-Clink-args=-save-temps -v" ../y.sh cargo build
```
### How to see the personality functions in the asm dump
```
CG_RUSTFLAGS="-Clink-arg=-save-temps -v -Clink-arg=-dA" ../y.sh cargo build
```
### How to see the LLVM IR for a sysroot crate
```
cargo build -v --target x86_64-unknown-linux-gnu -Zbuild-std
# Take the command from the output and add --emit=llvm-ir
```
### To prevent the linker from unmangling symbols
Run with:
```
COLLECT_NO_DEMANGLE=1
```
### How to use a custom-build rustc
* Build the stage2 compiler (`rustup toolchain link debug-current build/x86_64-unknown-linux-gnu/stage2`).
* Clean and rebuild the codegen with `debug-current` in the file `rust-toolchain`.
### How to install a forked git-subtree
Using git-subtree with `rustc` requires a patched git to make it work.
The PR that is needed is [here](https://github.com/gitgitgadget/git/pull/493).
Use the following instructions to install it:
```bash
git clone git@github.com:tqc/git.git
cd git
git checkout tqc/subtree
make
make install
cd contrib/subtree
make
cp git-subtree ~/bin
```

Then, do a sync with this command:

```bash
PATH="$HOME/bin:$PATH" ~/bin/git-subtree push -P compiler/rustc_codegen_gcc/ ../rustc_codegen_gcc/ sync_branch_name
cd ../rustc_codegen_gcc
git checkout master
git pull
git checkout sync_branch_name
git merge master
```

To send the changes to the rust repo:

```bash
cd ../rust
git pull origin master
git checkout -b subtree-update_cg_gcc_YYYY-MM-DD
PATH="$HOME/bin:$PATH" ~/bin/git-subtree pull --prefix=compiler/rustc_codegen_gcc/ https://github.com/rust-lang/rustc_codegen_gcc.git master
git push

# Immediately merge the merge commit into cg_gcc to prevent merge conflicts when syncing from rust-lang/rust later.
PATH="$HOME/bin:$PATH" ~/bin/git-subtree push -P compiler/rustc_codegen_gcc/ ../rustc_codegen_gcc/ sync_branch_name
```

TODO: write a script that does the above.

https://rust-lang.zulipchat.com/#narrow/stream/301329-t-devtools/topic/subtree.20madness/near/258877725

### How to use [mem-trace](https://github.com/antoyo/mem-trace)

`rustc` needs to be built without `jemalloc` so that `mem-trace` can overload `malloc` since `jemalloc` is linked statically, so a `LD_PRELOAD`-ed library won't a chance to intercept the calls to `malloc`.
More specific documentation is available in the [`doc`](./doc) folder:

### How to generate GIMPLE
* [Common errors](./doc/errors.md)
* [Debugging GCC LTO](./doc/debugging-gcc-lto.md)
* [Debugging libgccjit](./doc/debugging-libgccjit.md)
* [Git subtree sync](./doc/subtree.md)
* [List of useful commands](./doc/tips.md)
* [Send a patch to GCC](./doc/sending-gcc-patch.md)

If you need to check what gccjit is generating (GIMPLE), then take a look at how to
generate it in [gimple.md](./doc/gimple.md).

### How to build a cross-compiling libgccjit

#### Building libgccjit

* Follow the instructions on [this repo](https://github.com/cross-cg-gcc-tools/cross-gcc).

#### Configuring rustc_codegen_gcc

* Run `./y.sh prepare --cross` so that the sysroot is patched for the cross-compiling case.
* Set the path to the cross-compiling libgccjit in `gcc-path` (in `config.toml`).
* Make sure you have the linker for your target (for instance `m68k-unknown-linux-gnu-gcc`) in your `$PATH`. Currently, the linker name is hardcoded as being `$TARGET-gcc`. Specify the target when building the sysroot: `./y.sh build --target-triple m68k-unknown-linux-gnu`.
* Build your project by specifying the target: `OVERWRITE_TARGET_TRIPLE=m68k-unknown-linux-gnu ../y.sh cargo build --target m68k-unknown-linux-gnu`.

If the target is not yet supported by the Rust compiler, create a [target specification file](https://docs.rust-embedded.org/embedonomicon/custom-target.html) (note that the `arch` specified in this file must be supported by the rust compiler).
Then, you can use it the following way:

* Add the target specification file using `--target` as an **absolute** path to build the sysroot: `./y.sh build --target-triple m68k-unknown-linux-gnu --target $(pwd)/m68k-unknown-linux-gnu.json`
* Build your project by specifying the target specification file: `OVERWRITE_TARGET_TRIPLE=m68k-unknown-linux-gnu ../y.sh cargo build --target path/to/m68k-unknown-linux-gnu.json`.

If you get the following error:
## Licensing

```
/usr/bin/ld: unrecognised emulation mode: m68kelf
```
While this crate is licensed under a dual Apache/MIT license, it links to `libgccjit` which is under the GPLv3+ and thus, the resulting toolchain (rustc + GCC codegen) will need to be released under the GPL license.

Make sure you set `gcc-path` (in `config.toml`) to the install directory.
However, programs compiled with `rustc_codegen_gcc` do not need to be released under a GPL license.
3 changes: 3 additions & 0 deletions doc/debugging-gcc-lto.md
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# How to debug GCC LTO

Run do the command with `-v -save-temps` and then extract the `lto1` line from the output and run that under the debugger.
74 changes: 74 additions & 0 deletions doc/debugging-libgccjit.md
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# Debugging libgccjit

Sometimes, libgccjit will crash and output an error like this:

```
during RTL pass: expand
libgccjit.so: error: in expmed_mode_index, at expmed.h:249
0x7f0da2e61a35 expmed_mode_index
../../../gcc/gcc/expmed.h:249
0x7f0da2e61aa4 expmed_op_cost_ptr
../../../gcc/gcc/expmed.h:271
0x7f0da2e620dc sdiv_cost_ptr
../../../gcc/gcc/expmed.h:540
0x7f0da2e62129 sdiv_cost
../../../gcc/gcc/expmed.h:558
0x7f0da2e73c12 expand_divmod(int, tree_code, machine_mode, rtx_def*, rtx_def*, rtx_def*, int)
../../../gcc/gcc/expmed.c:4335
0x7f0da2ea1423 expand_expr_real_2(separate_ops*, rtx_def*, machine_mode, expand_modifier)
../../../gcc/gcc/expr.c:9240
0x7f0da2cd1a1e expand_gimple_stmt_1
../../../gcc/gcc/cfgexpand.c:3796
0x7f0da2cd1c30 expand_gimple_stmt
../../../gcc/gcc/cfgexpand.c:3857
0x7f0da2cd90a9 expand_gimple_basic_block
../../../gcc/gcc/cfgexpand.c:5898
0x7f0da2cdade8 execute
../../../gcc/gcc/cfgexpand.c:6582
```

To see the code which causes this error, call the following function:

```c
gcc_jit_context_dump_to_file(ctxt, "/tmp/output.c", 1 /* update_locations */)
```
This will create a C-like file and add the locations into the IR pointing to this C file.
Then, rerun the program and it will output the location in the second line:
```
libgccjit.so: /tmp/something.c:61322:0: error: in expmed_mode_index, at expmed.h:249
```
Or add a breakpoint to `add_error` in gdb and print the line number using:
```
p loc->m_line
p loc->m_filename->m_buffer
```
To print a debug representation of a tree:
```c
debug_tree(expr);
```

(defined in print-tree.h)

To print a debug representation of a gimple struct:

```c
debug_gimple_stmt(gimple_struct)
```
To get the `rustc` command to run in `gdb`, add the `--verbose` flag to `cargo build`.
To have the correct file paths in `gdb` instead of `/usr/src/debug/gcc/libstdc++-v3/libsupc++/eh_personality.cc`:
Maybe by calling the following at the beginning of gdb:
```
set substitute-path /usr/src/debug/gcc /path/to/gcc-repo/gcc
```
TODO(antoyo): but that's not what I remember I was doing.
27 changes: 27 additions & 0 deletions doc/errors.md
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# Common errors

This file lists errors that were encountered and how to fix them.

### `failed to build archive` error

When you get this error:

```
error: failed to build archive: failed to open object file: No such file or directory (os error 2)
```

That can be caused by the fact that you try to compile with `lto = "fat"`, but you didn't compile the sysroot with LTO.
(Not sure if that's the reason since I cannot reproduce anymore. Maybe it happened when forgetting setting `FAT_LTO`.)

### ld: cannot find crtbegin.o

When compiling an executable with libgccijt, if setting the `*LIBRARY_PATH` variables to the install directory, you will get the following errors:

```
ld: cannot find crtbegin.o: No such file or directory
ld: cannot find -lgcc: No such file or directory
ld: cannot find -lgcc: No such file or directory
libgccjit.so: error: error invoking gcc driver
```

To fix this, set the variables to `gcc-build/build/gcc`.
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