Stream: cranelift

Topic: Helping with rustc backend

osa1 (May 27 2020 at 08:23):

Hey @bjorn3 -- I've been studying cranelift recently and as you know implemented a backend with it for my toy compiler. I'm wondering if there are any issues that I could help with in your cranelift codegen now. What would be a good starter for me?

bjorn3 (May 27 2020 at 09:11):

https://github.com/bjorn3/rustc_codegen_cranelift/issues/920 is an easy to fix issue.

bjorn3 (May 27 2020 at 09:14):

Fixing https://github.com/bjorn3/rustc_codegen_cranelift/blob/7031c96fb3dc829586676d55e4d451f2c8de88bd/src/abi/mod.rs#L443 should also be relatively easy. You can use tcx.codegen_fn_attrs to determine if the called function is cold and then insert the current block into fx.cold_blocks if the called function is cold. You will probably also need to pass the current block to the codegen_terminator_call function.

bjorn3 (May 27 2020 at 09:15):

@osa1 If you have any questions, let me now.

osa1 (May 27 2020 at 10:19):

Perfect, thanks @bjorn3 . The FIXME you linked looks easy to fix, I'll be working on it. I'm guessing there isn't an issue for it since you linked the source instead of issue?

bjorn3 (May 27 2020 at 10:20):

:+1: There is indeed no issue for it.

osa1 (May 27 2020 at 13:59):

@bjorn3 I'm getting the "cold" attribute of the callee, how do I update the current cranelift block with that information now?

osa1 (May 27 2020 at 14:00):

I searched in documentation for "cold", but I don't see anything relevant.

bjorn3 (May 27 2020 at 14:06):

@osa1 fx.cold_blocks is a HashSet containing all cold blocks. You will need to add a new argument to codegen_terminator_call to pass the current block first.

bjorn3 (May 27 2020 at 14:07):

The concept of "cold" is not part of Cranelift. Instead cg_clif keeps track of it itself and has an optimization pass to move all cold blocks to the end of the function.

osa1 (May 27 2020 at 14:10):

OK done ,it builds. Is there a way to test this now?

bjorn3 (May 27 2020 at 14:16):

There is no testing infrastructure other than to make sure things compile and run fine. You can verify it manually by compiling something like

fn main() {
    let a = true;
    if !a {
        a();
    } else {
        loop {}
    }
}

#[cold]
fn a() {}

if cg_clif was built in debug mode, the target/out/clif dir will contain .clif files for every function. There should be a .unopt.clif file and a .opt.clif file containing main. The .opt.clif file should contain the call to a after the loop, while .unopt.clif should contain it before the loop.

bjorn3 (May 27 2020 at 14:19):

I opened https://github.com/bjorn3/rustc_codegen_cranelift/issues/1020 for making it possible to show the clif ir when cg_clif is compiled in release mode.

osa1 (May 29 2020 at 03:34):

Hey @bjorn3 . I'm testing my code before submitting a PR. How do I make sure cg_clif was built in debug mode? It's currently not generating and .clif files as far as I can see (find . -name "*.clif")

osa1 (May 29 2020 at 03:35):

Just removed --release argument from ./test.sh, I'll try again now

osa1 (May 29 2020 at 03:37):

Even in debug mode I don't get any .clif output:

$ CHANNEL="debug" ../rustc_codegen_cranelift/cargo.sh run
rustc_codegen_cranelift is build for rustc 1.45.0-nightly (46e85b432 2020-05-24) but the default rustc version is rustc 1.45.0-nightly (a74d1862d 2020-05-14).
Using rustc 1.45.0-nightly (46e85b432 2020-05-24).
   Compiling cranelift_test v0.1.0 (/home/omer/rust/cranelift_test)
[cranelift_test                : codegen mono items] start
[cranelift_test                : codegen mono items] end time: 7.835233ms
    Finished dev [unoptimized + debuginfo] target(s) in 0.20s
     Running `target/x86_64-unknown-linux-gnu/debug/cranelift_test`
^C
$ find . -name "*.clif"
$

osa1 (May 29 2020 at 03:40):

The PR is here: https://github.com/bjorn3/rustc_codegen_cranelift/pull/1021

osa1 (May 29 2020 at 03:50):

Ohhh.. the outputs are generated in the rustc_codegen_cranelift directory, not in the current project's directory :-O

osa1 (May 29 2020 at 03:58):

Nice, my patch seems to work!

osa1 (May 29 2020 at 04:08):

@bjorn3 what does this line mean in clif output: v25 -> v24 I'd expect this to mean "move v25 to v24", but I think it's actually "move v24 to v25"? "v25" is never used in this output (only appears in this line) so I'm not sure.

osa1 (May 29 2020 at 04:26):

@bjorn3 another PR arrived: https://github.com/bjorn3/rustc_codegen_cranelift/pull/1022 again, I'm unsure about how to format these files as rustfmt isn't working

osa1 (May 29 2020 at 04:27):

Another: https://github.com/bjorn3/rustc_codegen_cranelift/pull/1023

bjorn3 (May 29 2020 at 09:02):

osa1 said:

bjorn3 what does this line mean in clif output: v25 -> v24 I'd expect this to mean "move v25 to v24", but I think it's actually "move v24 to v25"? "v25" is never used in this output (only appears in this line) so I'm not sure.

This means that v25 is an alias for v24. Every time Cranelift sees v25 it will pretend that it was actually v24. These are created by cranelift-frontend to avoid having to walk over the full clif ir to replace all occurrences of v25 with v24.

osa1 (May 29 2020 at 09:03):

Thanks! When is an alias introduced exactly?

osa1 (May 29 2020 at 09:05):

Found a relevant comment:

                // Here all the predecessors use a single value to represent our variable
                // so we don't need to have it as a block argument.
                // We need to replace all the occurrences of val with pred_val but since
                // we can't afford a re-writing pass right now we just declare an alias.
                // Resolve aliases eagerly so that we can check for cyclic aliasing,
                // which can occur in unreachable code.

osa1 (May 29 2020 at 09:16):

OK now that all is merged I'll be working on https://github.com/bjorn3/rustc_codegen_cranelift/issues/920

Joey Gouly (May 29 2020 at 09:21):

osa1 said:

bjorn3 what does this line mean in clif output: v25 -> v24 I'd expect this to mean "move v25 to v24", but I think it's actually "move v24 to v25"? "v25" is never used in this output (only appears in this line) so I'm not sure.

This is an alias. EDIT: Didn't see this was already answered, zulip noob

osa1 (May 30 2020 at 15:50):

Hey @bjorn3 -- currently looking at #920. What is the issue here exactly? Does the out-of-space error happens in generated code, or in the compiler during compilation? (or when linking?)

bjorn3 (May 30 2020 at 16:05):

@osa1 If you run out of disk space during the compilation, cg_clif will panic when trying to write files. Instead it should just use sess.fatal to report the error and abort compilation. This can happen during most of the unwraps in https://github.com/bjorn3/rustc_codegen_cranelift/blob/master/src/archive.rs. It can also happen at https://github.com/bjorn3/rustc_codegen_cranelift/blob/1e70c51f60abc544d9a325c77474190f1719e560/src/driver/aot.rs#L52 and https://github.com/bjorn3/rustc_codegen_cranelift/blob/1e70c51f60abc544d9a325c77474190f1719e560/src/driver/aot.rs#L232. This issue can happen for all IO errors, but on my system that is mostly running out of disk space.

Last updated: Dec 23 2024 at 12:05 UTC