-O* does more than what's in optLevelFlags?

[Erdi, Gergo]

PUBLIC

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:

Opt_DoLambdaEtaExpansion
Opt_DoEtaReduction
Opt_LlvmTBAA
Opt_CallArity
Opt_Exitification
Opt_CaseMerge
Opt_CaseFolding
Opt_CmmElimCommonBlocks
Opt_CmmSink
Opt_CmmStaticPred
Opt_CSE
Opt_StgCSE
Opt_EnableRewriteRules
Opt_FloatIn
Opt_FullLaziness
Opt_IgnoreAsserts
Opt_Loopification
Opt_CfgBlocklayout
Opt_Specialise
Opt_CrossModuleSpecialise
Opt_InlineGenerics
Opt_Strictness
Opt_UnboxSmallStrictFields
Opt_CprAnal
Opt_WorkerWrapper
Opt_SolveConstantDicts
Opt_NumConstantFolding

And here are the ones that are set by O0 (the default) but not by O1:

Opt_IgnoreInterfacePragmas
Opt_OmitInterfacePragmas

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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