llvm dev - Jun 2018 - Naming clash: -DCLS=n and CLS in code

Tim Northover via llvm-dev wrote on 06/19/2018 07:55 PM:
> On Tue, 19 Jun 2018 at 18:22, U.Mutlu via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> I think such "reserved" names like CLS that can be passed to
the compiler(s),
>> should be avoided as identifiers for naming variables, constants, enum
>> members, macros etc.
>
> Why are you passing that argument in the first place? The compiler
> completely ignores it.
CLS stands for cacheline size. It is an important parameter
for optimization of the generated code, at least with gcc/g++.
-DCLS=n should have the same importance like for example -DNDEBUG.
> I sort of assume some library makes use of it, but if so that library
> is certainly not required while compiling Clang and is actually
> obscure enough that it's not even obvious from Googling. So it's a
bit
> of a stretch to claim the token is reserved.
Yes, it might not be required for the Average Joe user, but that's not me
:-)
Let me wish you a Good Morning!...  :-)

> > Why are you passing that argument in the first place? The compiler
> > completely ignores it.
>
> CLS stands for cacheline size. It is an important parameter
> for optimization of the generated code, at least with gcc/g++.
> -DCLS=n should have the same importance like for example -DNDEBUG.
The GCC source doesn't appear to mention it, and neither does any
documentation I can find. NDEBUG on the other hand is in both the C
and C++ standards.

I still think there's some weird library out there that interprets it
the way you're suggesting, and Clang is not that weird library.

Cheers.

Tim.

Tim Northover wrote on 06/19/2018 08:54 PM:
>>> Why are you passing that argument in the first place? The compiler
>>> completely ignores it.
>>
>> CLS stands for cacheline size. It is an important parameter
>> for optimization of the generated code, at least with gcc/g++.
>> -DCLS=n should have the same importance like for example -DNDEBUG.
>
> The GCC source doesn't appear to mention it, and neither does any
> documentation I can find. NDEBUG on the other hand is in both the C
> and C++ standards.
>
> I still think there's some weird library out there that interprets it
> the way you're suggesting, and Clang is not that weird library.
This is from a famous article series titled "What every programmer should
know
about memory":
   gcc -DCLS=$(getconf LEVEL1_DCACHE_LINESIZE) ...

https://lwn.net/Articles/255364/
http://mosermichael.github.io/cstuff/all/blog/2015/12/11/wepskn.html
https://www.dcl.hpi.uni-potsdam.de/teaching/numasem/slides/NUMASem_Matrix_multiplication.pdf

You can find more examples by searching for "DCLS getconf
LEVEL1_DCACHE_LINESIZE".