[llvm-dev] [inline-asm][asm-goto] Supporting "asm goto" in inline assembly
John McCall via llvm-dev
llvm-dev at lists.llvm.org
Tue Apr 4 11:44:06 PDT 2017
> On Apr 4, 2017, at 2:12 PM, Matthias Braun <matze at braunis.de> wrote:
> My two cents:
>
> - I think inline assembly should work even if the compiler cannot parse the contents. This would rule out msvc inline assembly (or alternatively put all the parsing and interpretation burden on the frontend), but would work with gcc asm goto which specifies possible targets separately.
> - Supporting control flow in inline assembly by allowing jumps out of an assembly block seems natural to me.
> - Jumping into an inline assembly block seems like an unnecessary feature to me.
> - To have this working in lib/CodeGen we would need an alternative opcode with the terminator flag set. (There should also be opportunities to remodel some instruction flags in the backend, to be part of the MachineInstr instead of the opcode, but that is an orthogonal discussion to this)
> - I don't foresee big problems in CodeGen, we should take a look on how computed goto is implementation to find ways to reference arbitrary basic blocks.
> - The register allocator fails when the terminator instruction also writes a register which is subsequently spilled (none of the existing targets does that, but you could specify this situation in inline assembly).
> - I'd always prefer intrinsics over inline assembly. Hey, why don't we add a -Wassembly that warns on inline assembly usage and is enabled by default...
> - I still think inline assembly is valuable for new architecture bringup/experimentation situations.
To me, this feels like a great example of "we really wanted a language feature, but we figured out that we could hack it in using inline assembly in a way that's ultimately significantly harder for the compiler to support than a language feature, and now it's your problem." I agree with Chandler that we should just design and implement the language feature.
I would recommend:
if (__builtin_patchable_branch("section name")) {
trace();
}
==>
%0 = call i1 @llvm.patchable_branch(i8* @sectionNameString)
br %0, ...
where @llvm.patchable_branch has the semantics of appending whatever patching information is necessary to the given section such that, if you apply the patch, it will change the result of the call from 0 to 1. That can then typically be pattern-matched in the backend to get the optimal codegen.
If I might recommend a better ABI for the patching information: consider using a pair of relative pointers, one from the patching information to the patchable instruction, and one from the patchable instruction to the new target. That would allow the patching information to be relocated at zero cost.
The actual details of how to apply the patch, and what the inline patchable-instruction sequence needs to be in order to accept the patch, would be target-specific. The documented motivating example seems to assume that a single nop is always big enough, which is pretty questionable.
This feature could be made potentially interesting to e.g. JIT authors by allowing the patching information to be embellished with additional information to identify the source branch.
John.
>
> - Matthias
>
>> On Apr 4, 2017, at 9:26 AM, Chandler Carruth via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>> On Tue, Apr 4, 2017 at 6:07 AM Yatsina, Marina via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 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 <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 tracefunction. 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/ <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 <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.
>> Thanks, this is exactly the kind of discussion that I think will help make progress here.
>>
>> I think this feature makes a lot of sense and is a really nice feature. However, I think implementing it with inline assembly imposes a lot of really unfortunate constraints on compilation -- it requires asm goto, pushsection and popsection, etc.
>>
>> I would much rather provide a much more direct way to represent a patchable nop and the addresses of label within a function. For example, I could imagine something like:
>>
>> ```
>> if (0) { trace_call: /* code to call the trace function */ }
>> patch: __builtin_patchable_nop()
>> __builtin_save_labels(trace_call, patch)
>> ```
>>
>> But someone can probably design a much better way to represent this in Clang. The advantages I see here (admittedly, mostly for the implementation in Clang and LLVM):
>>
>> 1) It allows Clang and LLVM to model this with running an assembler over anything.
>> 2) It doesn't require new terminators in LLVM's IR
>> 3) We already have intrinsics in LLVM's IR that could easily be extended to produce a nop.
>> 4) It would be portable -- each backend could select an appropriate sized nop to patch a jump into
>>
>> Would this make sense?
>>
>> <>
>> 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 <mailto:chandlerc at gmail.com>]
>> Sent: Thursday, March 30, 2017 23:22
>> To: Yatsina, Marina <marina.yatsina at intel.com <mailto:marina.yatsina at intel.com>>; llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>; rnk at google.com <mailto:rnk at google.com>; jyknight at google.com <mailto:jyknight at google.com>; ehsan at mozilla.com <mailto:ehsan at mozilla.com>; rjmccall at apple.com <mailto:rjmccall at apple.com>; mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>; matze at braunis.de <mailto:matze at braunis.de>; Tayree, Coby <coby.tayree at intel.com <mailto: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
>>
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