Re: PROPOSAL: Extend inline asm syntax with size spec

[Richard Biener]

On October 7, 2018 6:09:30 PM GMT+02:00, Nadav Amit <namit@xxxxxxxxxx> wrote:
>at 2:18 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>
>> Hi people,
>> 
>> this is an attempt to see whether gcc's inline asm heuristic when
>> estimating inline asm statements' cost for better inlining can be
>> improved.
>> 
>> AFAIU, the problematic arises when one ends up using a lot of inline
>> asm statements in the kernel but due to the inline asm cost
>estimation
>> heuristic which counts lines, I think, for example like in this here
>> macro:
>> 
>>
>https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Ftorvalds%2Flinux.git%2Ftree%2Farch%2Fx86%2Finclude%2Fasm%2Fcpufeature.h%23n162&amp;data=02%7C01%7Cnamit%40vmware.com%7C6db1258c65ea45bbe4ea08d62c35ceec%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C1%7C0%7C636745007006838299&amp;sdata=iehl%2Fb8h%2BZE%2Frqb4qjac19WekSgOObc9%2BM1Jto1VgF4%3D&amp;reserved=0
>> 
>> the resulting code ends up not inlining the functions themselves
>which
>> use this macro. I.e., you see a CALL <function> instead of its body
>> getting inlined directly.
>> 
>> Even though it should be because the actual instructions are only a
>> couple in most cases and all those other directives end up in another
>> section anyway.
>> 
>> The issue is explained below in the forwarded mail in a larger detail
>> too.
>> 
>> Now, Richard suggested doing something like:
>> 
>> 1) inline asm ("...")
>> 2) asm ("..." : : : : <size-expr>)
>> 3) asm ("...") __attribute__((asm_size(<size-expr>)));
>> 
>> with which user can tell gcc what the size of that inline asm
>statement
>> is and thus allow for more precise cost estimation and in the end
>better
>> inlining.
>> 
>> And FWIW 3) looks pretty straight-forward to me because attributes
>are
>> pretty common anyways.
>> 
>> But I'm sure there are other options and I'm sure people will have
>> better/different ideas so feel free to chime in.
>
>Thanks for taking care of it. I would like to mention a second issue,
>since
>you may want to resolve both with a single solution: not inlining
>conditional __builtin_constant_p(), in which there are two code-paths -
>one
>for constants and one for variables.
>
>Consider for example the Linux kernel ilog2 macro, which has a
>condition
>based on __builtin_constant_p() (
>https://elixir.bootlin.com/linux/v4.19-rc7/source/include/linux/log2.h#L160
>). The compiler mistakenly considers the âheavyâ code-path that is
>supposed
>to be evaluated only in compilation time to evaluate the code size.

But this is a misconception about __builtin_constant_p. It doesn't guard sth like 'constexpr' regions. If you try to use it with those semantics you'll fail (appearantly you do). 

Of course IPA CP code size estimates when seeing a constant fed to bcp might be not optimal, that's another issue of course. 

Richard. 

>This
>causes the kernel to consider functions such as kmalloc() as âbigâ.
>kmalloc() is marked with always_inline attribute, so instead the
>calling
>functions, such as kzalloc() are not inlined.
>
>When I thought about hacking gcc to solve this issue, I considered an
>intrinsic that would override the cost of a given statement. This
>solution
>is not too nice, but may solve both issues.
>
>In addition, note that AFAIU the impact of a wrong cost of code
>estimation
>can also impact loop and other optimizations.
>
>Regards,
>Nadav