llvm dev - Jan 2021 - [RFC] Enabling opt's -enable-new-pm by default with ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER

With many of the new PM issues ironed out, I'd like to turn on opt's
-enable-new-pm flag by default when we build LLVM
with ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER on.

-enable-new-pm essentially translates the legacy way of specifying opt
passes (e.g. -instcombine) into the new way of specifying passes (e.g.
-passes=instcombine), making tests that use `opt -instcombine` run under
the new PM, unless specifically opted out via `-enable-new-pm=0`.

One issue is that until we enable the new PM by default, testing locally
may not show issues that pop up until a bot
with ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER on complains after committing,
since basically everybody has ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER off
locally. Hopefully the amount of time between conditionally turning on
opt's -enable-new-pm and turning on the new PM by default everywhere is
fairly small and this won't be too big of an issue. I haven't seen this
discrepancy appear too often when looking at existing passing tests failing
when -enable-new-pm is turned on.
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> On Jan 21, 2021, at 21:02, Arthur Eubanks via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> With many of the new PM issues ironed out, I'd like to turn on
opt's -enable-new-pm flag by default when we build LLVM with
ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER on.
> 
That sounds like a great idea & sounds good to me, as long as all current
tests pass with the new behavior and there’s a public bot that uses
ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER=true.

Thanks everyone involved in ironing out the remaining issues!
> -enable-new-pm essentially translates the legacy way of specifying opt
passes (e.g. -instcombine) into the new way of specifying passes (e.g.
-passes=instcombine), making tests that use `opt -instcombine` run under the new
PM, unless specifically opted out via `-enable-new-pm=0`.
> 
> One issue is that until we enable the new PM by default, testing locally
may not show issues that pop up until a bot with
ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER on complains after committing, since
basically everybody has ENABLE_NEW_EXPERIMENTAL_PASS_MANAGER off locally.
Hopefully the amount of time between conditionally turning on opt's
-enable-new-pm and turning on the new PM by default everywhere is fairly small
and this won't be too big of an issue. I haven't seen this discrepancy
appear too often when looking at existing passing tests failing when
-enable-new-pm is turned on.
I think that’s fine, the same issue exists for a lot of different options. IIUC
to reproduce a failure, it should be enough to pass `-enable-new-pm` to the
failing test locally?

Cheers,
Florian