[cfe-dev] [RFC] Clearing Clang AST before running backend optimizations/codegen to save memory

David Blaikie via cfe-dev cfe-dev at lists.llvm.org
Tue Sep 21 16:27:54 PDT 2021

On Tue, Sep 21, 2021 at 4:25 PM Arthur Eubanks <aeubanks at google.com> wrote:

> But yeah - we do have the https://reviews.llvm.org/D4234
>> "LocTrackingOnly" mode which looks like it could be used for Rpass
>> diagnostics, for instance. (& removing use of the AST from the LLVM
>> diagnostic system might help make it more consistent behavior even when
>> doing LTO or other separations between AST parsing and IR transformations)
> Thanks for the pointer to that. I was getting confused because
> "-Rpass=foo" was triggering LocTrackingOnly but "-Rpass" wasn't, which I've
> fixed. So -Rpass is no longer a concern.
> So we just have to worry about non-Rpass diagnostics, e.g.
> -fwarn-stack-size. All of these only use the function name for source
> locations. We can either just give up and have worse diagnostics for these,
> meaning no demangling and no source location per diagnostic, or create a
> side table of these right before codegen as you suggested. For now I'll go
> with creating a side table to preserve the status quo, hopefully its
> construction runtime is not measurable.
> Inline asm is special in that it carries around a source location token
> even in the IR, so we don't need to go through the AST to find a source
> location which is nice

That all sounds pretty good to me - thanks for looking into all these nooks
and crannies!
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