No-op assignment I64[BaseReg + 784] = I64[BaseReg + 784]; is generated into optimized Cmm

Hello, on SPARC and I also guess on PPC it's possible to get following line in optimized Cmm code:

I64[BaseReg + 784] = I64[BaseReg + 784];

this line is then translated by NCG wasting 5 isns on SPARC at least. Don't know PPC. I'm not sure if this is possible to duplicate this on i386 due to fewer regs. Generally speaking you need to have 32bit target with more regs available. Interesting fact is that such line is not presented in non-optimized Cmm, but is presented in optimized one. Both optimized and non-optimized Cmms attached. Both get from compiling T7507 testcase by stage1 compiler on SPARC:

/home/karel/vcs/ghc-src/ghc-sparc-reg_ncg/inplace/bin/ghc-stage1 -fforce-recomp -dcore-lint -dcmm-lint -dno-debug-output -no-user-package-db -rtsopts -fno-ghci-history -c T7507.hs  -O -ddump-opt-cmm > T7507.opt-cmm-sparc-reg-ncg

Of course non-opt Cmm is got by -ddump-cmm instead of -ddump-opt-cmm. If you need more simplified testcase, then following code is usable too:

module Main where

import Data.Int

main = print ( ( 2 ^ 6 ) :: Int64 )

Trac metadata

Trac field	Value
Version	7.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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