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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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