cmmImplementSwitchPlans creates duplicate blocks

Given this c-- code *after* common block elimination and *before* cmmImplementSwitchPlans:

    ... 
cfzc:
   _sfh3::P64 = R1;
   _cfB7::I64 = %MO_UU_Conv_W32_W64(I32[I64[_sfh3::P64 - 1] - 4]);
   switch [0 .. 20] _cfB7::I64 {
     case 2 : goto cfAR;
     case 4 : goto cfAV;
     case 6 : goto cfAZ;
     default: goto cfAC;
   }
cfAZ:
   _sfh6::I64 = I64[_sfh3::P64 + 7];
   _sfgp::I64 = 1;
   goto sfgo;
cfAV:
   _sfh5::I64 = I64[_sfh3::P64 + 7];
   _sfgp::I64 = 1;
   goto sfgo;
cfAR:
   _sfh4::I64 = I64[_sfh3::P64 + 7];
   _sfgp::I64 = 1;
   goto sfgo;
sfgo:
  ... 

Everything is fine, the blocks cfAZ, cfAV and cfAR are all a little different.

But after cmmImplementSwitchPlans the code becomes:

    ...   
cfzc:
    _sfgl::I64 = I64[Sp + 24];
    _sfgn::P64 = P64[Sp + 16];
    _cfB7::I64 = %MO_UU_Conv_W32_W64(I32[I64[R1 - 1] - 4]);
    if (_cfB7::I64 < 5) goto ufBe; else goto ufBg;
ufBe:
    if (_cfB7::I64 < 4) goto ufBf; else goto cfAV;
ufBf:
    if (_cfB7::I64 != 2) goto cfAC; else goto cfAR;
cfAR:
    _sfgp::I64 = 1;
    goto sfgo;
cfAV:
    _sfgp::I64 = 1;
    goto sfgo;
ufBg:
    if (_cfB7::I64 != 6) goto cfAC; else goto cfAZ;
cfAZ:
    _sfgp::I64 = 1;
    goto sfgo;
sfgo:
    ... 

We can now see that cfAZ, cfAV and cfAR are code duplicates!

Now, https://github.com/ghc/ghc/blob/master/compiler/cmm/CmmPipeline.hs#L72 states

...
       ----------- Eliminate common blocks -------------------------------------
       g <- {-# SCC "elimCommonBlocks" #-}
            condPass Opt_CmmElimCommonBlocks elimCommonBlocks g
                          Opt_D_dump_cmm_cbe "Post common block elimination"

       -- Any work storing block Labels must be performed _after_
       -- elimCommonBlocks

       g <- {-# SCC "createSwitchPlans" #-}
            runUniqSM $ cmmImplementSwitchPlans dflags g
       dump Opt_D_dump_cmm_switch "Post switch plan" g
...

For some reason I don't understand cmmImplementSwitchPlans is run *after* elimCommonBlocks. Although it seems elimCommonBlocks can deal with the constructs introduced in cmmImplementSwitchPlans.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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CC
Operating system
Architecture