[cfe-dev] Compile .ll file with coroutines intrinsics

[chuanqi.xcq via cfe-dev]

Hi Medhi,

It looks like a flaw to me. The .bc generated file should be compilable by invoking the clang.
I think here are two options:
 - Add an option in clang like the option `-enable-coroutines` in opt.
 - Make coroutine passes run by default in LLVM pipeline.

It depends on how stable we think Coroutine passes are now. 
It shouldn't matter to run coroutine passes for general IR. Coroutine Passes would and should do nothing when they can't find coroutine intrinsics.
In my mind, the reason why the coroutine passes got control by option is that coroutine is an experimental feature. 
So we don't want to they to break our workflow. But if we think they are stable, I think we could turn it on by default.
This may need more discussion.

Thanks,
Chuanqi
------------------------------------------------------------------
From:Mehdi AMINI <joker.eph at gmail.com>
Send Time:2021年7月9日(星期五) 10:02
To:John McCall <rjmccall at apple.com>
Cc:Xun Li <lxfind at gmail.com>; David Blaikie <dblaikie at gmail.com>; Arthur Eubanks <aeubanks at google.com>; chuanqi.xcq <yedeng.yd at linux.alibaba.com>; cfe-dev <cfe-dev at lists.llvm.org>
Subject:Re: [cfe-dev] Compile .ll file with coroutines intrinsics



On Thu, Jul 8, 2021 at 6:56 PM John McCall <rjmccall at apple.com> wrote:
On 8 Jul 2021, at 20:13, Mehdi AMINI wrote:
 > On Thu, Jul 8, 2021 at 4:40 PM Xun Li <lxfind at gmail.com> wrote:
 >
 >> Not sure if I fully understand the question. Do you have a full
 >> example of what you expect but doesn't work?
 >> Are you trying to compile C++ code that uses coroutines, or are you
 >> trying to use opt to compile an .ll file?
 >>
 >
 > I'm trying to compile IR (.ll or .bc) file: clang support IR as *input* as
 > well.
 >
 > So for example with the case you mention:
 >
 > clang -emit-llvm  -c    -O3  -Xclang -disable-llvm-passes
 > clang/test/CodeGenCoroutines/coro-symmetric-transfer-01.cpp -std=c++20 -o
 > coro.bc
 >
 > This gets me a nice coro.bc file, but I have no way to compile this with
 > clang right now, it will always crash:
 >
 > $ clang -c coro.bc
 > PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash
 > backtrace, preprocessed source, and associated run script.
 > Stack dump:
 > 0. Program arguments: clang -c coro.bc
 > 1. Code generation
 > 2. Running pass 'Function Pass Manager' on module 'coro.bc'.
 > 3. Running pass 'X86 DAG->DAG Instruction Selection' on function '@_Z3foov'
 > Segmentation fault

 Coroutines do push the boundaries of LLVM IR in the sense that
 (1) there’s a mandatory lowering sequence and (2) because of the
 complexity of that lowering, the IR prior to that sequence is
 much more of an internal representation than a stable format.
 With that said, I don’t know of any inherent reason why running
 the coroutine passes multiple times would be a problem.  It’s
 probably just some bit of coroutine bookkeeping (the attribute?)
 that we fail to remove after lowering.

 Also, the exact form of the crash is surprising; I don’t know
 why running lowering multiple times would add new things that
 ISel wouldn’t recognize.

Right now this isn't running it multiple times, it is instead never running it, the backend crashes because ISel does not know about these intrinsics (I think we should be friendlier in the failure mode, but that's another story).

I use clang to emit IR without running any pass (this is the ` -Xclang -disable-llvm-passes` part of the invocation).

 Then I'd like to use clang to "resume" compilation of this file, but clang does not allow me to have an IR input file and run these passes: this is what I'd like to fix. One option I had in the original email in this thread was to change clang to honor `-fcoroutines-ts` when the input is an IR file.

-- 
Mehdi


 John.

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