Yatsina, Marina via llvm-dev
2017-Mar-30 14:44 UTC
[llvm-dev] [inline-asm][asm-goto] Supporting "asm goto" in inline assembly
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". --------------------------------------------------------------------- Intel Israel (74) Limited This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170330/97425d6b/attachment.html>
Joerg Sonnenberger via llvm-dev
2017-Mar-30 19:38 UTC
[llvm-dev] [inline-asm][asm-goto] Supporting "asm goto" in inline assembly
On Thu, Mar 30, 2017 at 02:44:32PM +0000, Yatsina, Marina via llvm-dev wrote:> 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.I haven't seen anything in all of pkgsrc. Just as data point. Joerg
Chandler Carruth via llvm-dev
2017-Mar-30 20:22 UTC
[llvm-dev] [inline-asm][asm-goto] Supporting "asm goto" in inline assembly
Just responding to the motivation stuff as that remains an open question: On Thu, Mar 30, 2017 at 4:44 PM Yatsina, Marina <marina.yatsina at intel.com> wrote:> Linux kernel is using the “asm goto” feature, >But your original email indicated they have an alternative code path for compilers that don't support it? What might be compelling would be if there are serious performance problems when using the other code path that cannot be addressed by less invasive (and more general) improvements to LLVM. If this is the *only* way to get comparable performance from the Linux Kernel, then I think that might be an interesting discussion. But it would take a very careful and detailed analysis of why IMO.> other projects probably use it as well. >This is entirely possible, but I'd like to understand which projects and why they use it rather than any of the alternatives before we impose the implementation complexity on LLVM. At least that's my two cents. -Chandler -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170330/25c527cd/attachment.html>
Yatsina, Marina via llvm-dev
2017-Apr-04 13:07 UTC
[llvm-dev] [inline-asm][asm-goto] Supporting "asm goto" in inline assembly
Asm goto feature was introduces to GCC in order to optimize the support for tracepoints in Linux kernel (it can be used for other things that do nop patching). GCC documentation describes their motivating example here: https://gcc.gnu.org/onlinedocs/gcc-4.8.4/gcc/Extended-Asm.html #define TRACE1(NUM) \ do { \ asm goto ("0: nop;" \ ".pushsection trace_table;" \ ".long 0b, %l0;" \ ".popsection" \ : : : : trace#NUM); \ if (0) { trace#NUM: trace(); } \ } while (0) #define TRACE TRACE1(__COUNTER__) In this example (which in fact inspired the asm goto feature) we want on rare occasions to call the trace function; on other occasions we'd like to keep the overhead to the absolute minimum. The normal code path consists of a single nop instruction. However, we record the address of this nop together with the address of a label that calls the trace function. This allows the nop instruction to be patched at run time to be an unconditional branch to the stored label. It is assumed that an optimizing compiler moves the labeled block out of line, to optimize the fall through path from the asm. Here is the Linux kernel RFC which discusses the old C way of implementing it and the performance issues that were noticed. It also states some performance numbers of the old C code vs. the asm goto: https://lwn.net/Articles/350714/ This LTTng (Linux Trace Toolkit Next Generation) presentation talks about using this feature as a way of optimize static tracepoints (slides 3-4) https://www.computer.org/cms/ComputingNow/HomePage/2011/0111/rW_SW_UsingTracing.pdf This presentation also mentions that a lot of other Linux applications use this tracing mechanism. I believe we already have much of the infrastructure in place (using the indirecbr instruction infrastructure). We do need to make sure MachineBlockPlacement optimizes the fall through path to make sure we can gain the performance for the nop patching. Thanks, Marina From: Chandler Carruth [mailto:chandlerc at gmail.com] Sent: Thursday, March 30, 2017 23:22 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; matze at braunis.de; Tayree, Coby <coby.tayree at intel.com> Subject: Re: [llvm-dev] [inline-asm][asm-goto] Supporting "asm goto" in inline assembly Just responding to the motivation stuff as that remains an open question: On Thu, Mar 30, 2017 at 4:44 PM Yatsina, Marina <marina.yatsina at intel.com<mailto:marina.yatsina at intel.com>> wrote: Linux kernel is using the “asm goto” feature, But your original email indicated they have an alternative code path for compilers that don't support it? What might be compelling would be if there are serious performance problems when using the other code path that cannot be addressed by less invasive (and more general) improvements to LLVM. If this is the *only* way to get comparable performance from the Linux Kernel, then I think that might be an interesting discussion. But it would take a very careful and detailed analysis of why IMO. other projects probably use it as well. This is entirely possible, but I'd like to understand which projects and why they use it rather than any of the alternatives before we impose the implementation complexity on LLVM. At least that's my two cents. -Chandler --------------------------------------------------------------------- Intel Israel (74) Limited This e-mail and any attachments may contain confidential material for the sole use of the intended recipient(s). Any review or distribution by others is strictly prohibited. If you are not the intended recipient, please contact the sender and delete all copies. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170404/9e4e52e6/attachment.html>
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