[llvm-dev] [inline-asm][asm-goto] Supporting "asm goto" in inline assembly

[Yatsina, Marina via llvm-dev]

Linux kernel is using the “asm goto” feature, other projects probably use it as well. I think it provides motivation to support it in LLVM.
Regarding the complexity, I believe there is some infrastructure that we can at least partially reuse (the support for “indirectbr” instruction).

My focus is adding “asm goto” support, the other things are indeed completely orthogonal and came up in bugs related to Bug 9295<https://bugs.llvm.org/show_bug.cgi?id=9295>.
The reason I mentioned them in this discussion is that they seem to require IR change as well.
I thought that if we’re doing this IR change for “asm goto” it’s worth checking if there is additional useful information we want to expose except the C labels used by inline assembly.
If you prefer to separate the IR changes for “asm goto” and the IR changes that will allow exposing defined symbols, then we can focus on the “asm goto” feature alone.

Thanks,
Marina



From: Chandler Carruth [mailto:chandlerc at gmail.com]
Sent: Thursday, March 30, 2017 11:21
To: Yatsina, Marina <marina.yatsina at intel.com>; llvm-dev at lists.llvm.org; rnk at google.com; jyknight at google.com; ehsan at mozilla.com; rjmccall at apple.com; mehdi.amini at apple.com; chandlerc at gmail.com; matze at braunis.de
Subject: Re: [llvm-dev] [inline-asm][asm-goto] Supporting "asm goto" in inline assembly

On Wed, Mar 29, 2017 at 9:38 AM Yatsina, Marina via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi,

I wanted to revive this issue of supporting asm goto (Bug 9295<https://bugs.llvm.org/show_bug.cgi?id=9295>).

FWIW, my opinion is that the use cases don't really justify the complexity this adds to the compiler. I feel like embedding inline assembly into normal C control flow constructs and writing the entire thing directly give pretty reasonable options.

But maybe others really see high-value reasons to add support for this... If they do...

As was already proposed, the best way seems to be introducing new IR.
If we’re changing the IR, we should probably provide an infrastructure that solves or at least enables future support for things like:

1.      MS-style inline asm jmps and goto (Bug 24529<https://bugs.llvm.org/show_bug.cgi?id=24529>)
I *strongly* agree with the 'WONTFIX' resolution here. More than GCC's "asm goto", this feature seems much more harmful to the compiler and much less well motivated.


2.      Analyzing symbols defined/references in the inline assembly (Bug 28970<https://bugs.llvm.org/show_bug.cgi?id=28970>), taking into account module/file-scope inline assembly.
Unless we decide to address #1 in some way, this seems completely orthogonal to "asm goto". While I'd love to see a good resolution to PR28970, I don't think it makes sense to couple the two together. Among other things, none of my concerns about "asm goto" apply to simply exposing the symbols defined.


3.      Provide some information about the cost of the inline assembly? (I’m not sure if we want to couple it with this issue and if the cost should be represented in this new IR or some other way)
I suspect this too should be an orthogonal discussion.

I think we should expose a TTI-cost-analysis API and allow passing inline assembly to it. Then targets can, if they choose, actually analyze the assembly to compute a cost. But I don't think we need IR changes here and even if we do, likely orthogonal ones to "asm goto".
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