llvm dev - Mar 2021 - [RFC] Create llvm/Support/InternalLimits.h

Hello,

There are internal limits within LLVM that are fundamental to how LLVM operates
but are defined in awkward places when it comes to library layering. I'd
like to propose that we fix that with a simple "InternalLimits.h" file
that contains a few essential constants.

For example, the maximum alignment supported by LLVM is defined by CodeGen but
clang's Sema needs to know that value to enforce that users stay within
bounds. That being said, clang's Sema doesn't need or want to depend on
CodeGen for various good reasons.

If we create llvm/Support/InternalLimits.h and define the alignment limit there,
then LLVM's CodeGen and clang's Sema can both include that file without
Sema creating a dependency on CodeGen.

Thoughts?
Dave

On Fri, Mar 5, 2021 at 9:05 AM David Zarzycki via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hello,
>
> There are internal limits within LLVM that are fundamental to how LLVM
operates but are defined in awkward places when it comes to library layering.
I'd like to propose that we fix that with a simple
"InternalLimits.h" file that contains a few essential constants.
>
> For example, the maximum alignment supported by LLVM is defined by CodeGen
but clang's Sema needs to know that value to enforce that users stay within
bounds. That being said, clang's Sema doesn't need or want to depend on
CodeGen for various good reasons.
>
> If we create llvm/Support/InternalLimits.h and define the alignment limit
there, then LLVM's CodeGen and clang's Sema can both include that file
without Sema creating a dependency on CodeGen.
>
> Thoughts?
Thank you for looking into this! I think that sounds like a reasonable approach.

~Aaron
> Dave
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Sounds good to me. Depending on the "limits" we might want to put it
in
llvm/IR/ though.
Eventually we might want to have multiple (and all included form 
Frontend/InternalLimits.h) but let's see.

~ Johannes


On 3/5/21 8:05 AM, David Zarzycki via llvm-dev wrote:
> Hello,
>
> There are internal limits within LLVM that are fundamental to how LLVM
operates but are defined in awkward places when it comes to library layering.
I'd like to propose that we fix that with a simple
"InternalLimits.h" file that contains a few essential constants.
>
> For example, the maximum alignment supported by LLVM is defined by CodeGen
but clang's Sema needs to know that value to enforce that users stay within
bounds. That being said, clang's Sema doesn't need or want to depend on
CodeGen for various good reasons.
>
> If we create llvm/Support/InternalLimits.h and define the alignment limit
there, then LLVM's CodeGen and clang's Sema can both include that file
without Sema creating a dependency on CodeGen.
>
> Thoughts?
> Dave
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Seems like a reasonable idea, but we want to be careful to define which 
internal limits are in scope, and which are not.

For instance, ValueTracking has a bunch of internal scope limits for 
analysis purpose.  Moving those into some generic location would seem 
like a mistake.

Your example of alignment is a good one.

As a idea, what if we specifically restricted this to internal limits of 
the IR representation?  Or is that too narrow?  What other specific 
limits are you thinking of?

Philip

On 3/5/21 6:05 AM, David Zarzycki via llvm-dev wrote:
> Hello,
>
> There are internal limits within LLVM that are fundamental to how LLVM
operates but are defined in awkward places when it comes to library layering.
I'd like to propose that we fix that with a simple
"InternalLimits.h" file that contains a few essential constants.
>
> For example, the maximum alignment supported by LLVM is defined by CodeGen
but clang's Sema needs to know that value to enforce that users stay within
bounds. That being said, clang's Sema doesn't need or want to depend on
CodeGen for various good reasons.
>
> If we create llvm/Support/InternalLimits.h and define the alignment limit
there, then LLVM's CodeGen and clang's Sema can both include that file
without Sema creating a dependency on CodeGen.
>
> Thoughts?
> Dave
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev