[llvm-dev] RFC: Need One True Way to check for -Oz/-Os (minsize, optsize) in passes...

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 20 01:29:17 PST 2017

On Fri, Jan 20, 2017 at 1:15 AM James Molloy <James.Molloy at arm.com> wrote:

> Hey Chandler,
> Thanks for bringing this up. As I see it:
> The advantage of (1) is that it allows easier exploration of IR - I can
> run the optimizer in -O2, optsize and minsize mode all without going back
> to clang.
> The advantage of (2) is that it works with LTO… and I think that must
> trump all other concerns, as you said. In fact, the advantage of (1) I just
> stated could be gained by simply “opt -force-attribute=*:minsize” (do we
> have a wildcard match for -force-attribute? if we don’t, we should). So
> while unwieldy, it does work (and ideally should be documented!)

If we go with (2), I'm happy to work on a mechanism in the opt tool to
essentially automate this process to implement '-Oz' so that it won't be
any harder than it is today. But I suspect it would be better to spell the
option differently to make it clear that it will *transform* the IR to get
this, not just a mode setting. Maybe a dedicated pass that is essentially
the force attribute you describe, but canned for forcing these attributes
globally when not already present on functions.
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