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

[Yatsina, Marina via llvm-dev]

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
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