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

[Sylvain Henry]

Hi,

Indeed the optimisation level is directly queried in a few places (e.g. 
grep "optLevel" and "opt_level"). Especially in Core opt pipeline 
getCoreToDo returns:

     core_todo =
      if opt_level == 0 then
        [ static_ptrs_float_outwards,
          CoreDoSimplify max_iter
              (base_mode { sm_phase = FinalPhase
                         , sm_names = ["Non-opt simplification"] })
        , add_caller_ccs
        ]

      else {- opt_level >= 1 -} [...]

Somewhat relevant issue: https://gitlab.haskell.org/ghc/ghc/-/issues/17844


On 11/10/2021 06:08, Erdi, Gergo via ghc-devs wrote:
>
> 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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