-O* does more than what's in optLevelFlags?
Gergo.Erdi at sc.com
Mon Oct 11 07:54:21 UTC 2021
I've done some digging into this, and it turns out the DynFlag's `optLevel` itself is used at some places, most notably when creating the main [CoreToDo]. So turning on all these flags on their own doesn't equal setting -On for the right "n"; in fact, currently setting most of these flags does NOTHING on its own unless -On with n>=1 is *also* passed on the command line, and there is no command line flag to *only* turn on Core optimizations in the abstract, without actually turning any specific ones on.
Is this a documentation bug, an implementation bug (as in, if any of the relevant opts are set, then the CoreToDos should always include the optimization steps selected), or a design bug (there is no way to support this meaningfully)?
From: Erdi, Gergo
Sent: Monday, October 11, 2021 12:09 PM
To: 'GHC' <ghc-devs at haskell.org>
Cc: Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com>
Subject: -O* does more than what's in optLevelFlags?
What is set by -O* that is not included in optLevelFlags?
I would have thought that setting all the flags implied by, e.g., -O1, would be the same as setting -O1 itself. But this is not the case! Here are all the flags for O1 from optLevelFlags:
And here are the ones that are set by O0 (the default) but not by O1:
So I expected that the following two invocations of GHC would be equivalent:
1. ghc -O1
2. ghc -fdo-lambda-eta-expansion -fdo-eta-reduction -fllvm-tbaa -fcall-arity -fexitification -fcase-merge -fcase-folding -fcmm-elim-common-blocks -fcmm-sink -fcmm-static-pred -fcse -fstg-cse -fenable-rewrite-rules -ffloat-in -ffull-laziness -fignore-asserts -floopification -fblock-layout-cfg -fspecialise -fcross-module-specialise -finline-generics -fstrictness -funbox-small-strict-fields -fcpr-anal -fworker-wrapper -fsolve-constant-dicts -fnum-constant-folding -fno-ignore-interface-pragmas -fno-omit-interface-pragmas
However, just by observing the output of -dshow-passes, I can see that while -O1 applies all these optimizations, the second version does NOT, even though I have turned on each and every one of them one by one.
Looking at compiler/GHC/Driver/Session.hs, it is not at all clear that -O* should do more than just setting the flags from optLevelFlags. What other flags are implied by -O*?
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