[GHC] #14971: Use appropriatly sized comparison instruction for small values.
#14971: Use appropriatly sized comparison instruction for small values. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: (CodeGen) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC currently defaults all comparisons originating from Cmm switch statements to 64bit on x64. This incurs a small overhead in instruction size. Fixing this manually gave a speedup of ~1,5% in microbenchmarks. In detail we generate Cmm of the form: {{{ _s8Dg::P64 = R1; _c8EF::P64 = _s8Dg::P64 & 7; switch [1 .. 2] _c8EF::P64 { case 1 : goto c8Ey; case 2 : goto c8EC; } }}} Which results in assembly like: {{{ andl $7,%ebx cmpq $1,%rbx }}} It's obvious that c8EF fits into a byte, but is sized up to 64 bits. Changing this would enable us to use cmpl instead of cmpq and save us a byte on each comparison. While this isn't major in my microbenchmarks it resultet in a speedup of ~1,5% for such constructs in inner loops. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14971 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14971: Use appropriatly sized comparison instruction for small values. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: (CodeGen) | Resolution: | Keywords: CodeGen Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * keywords: => CodeGen Comment: Sounds good! 1.5% is a lot. How did you get that figure? Do you have a patch :-)? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14971#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14971: Use appropriatly sized comparison instruction for small values. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: (CodeGen) | Resolution: | Keywords: CodeGen Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by AndreasK): Replying to [comment:1 simonpj]:
Sounds good! 1.5% is a lot. How did you get that figure?
Indeed it's more than I would have expected. But these are micro benchmarks so it's hard to predict how much of that will remain when running regular programs. Given that cache behaviour and cpu bottlecks would be different there. I've compiled two criterion benchmarks and changed the assembly by Hand. One example was: {{{ module Main (main) where import Criterion.Main mapInt :: Int -> Int mapInt 1 = 111 mapInt 2 = 211 mapInt 3 = 311 {-# NOINLINE sumAndLookup #-} sumAndLookup :: [Int] -> Int sumAndLookup = sum . map mapInt main = do let value = map (\x -> 1 + x `mod` 3) [1..30] print (sumAndLookup value) defaultMain [ bgroup "opSize" [ bench "caseOfThree" $ whnf sumAndLookup value] ] }}}
Do you have a patch :-)?
Not yet. I submitted a project proposal for Summer of Code which includes writing a patch for this. But if that should not work out I still expect to get around to it eventually. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14971#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14971: Use appropriatly sized comparison instruction for small values. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: (CodeGen) | Resolution: | Keywords: CodeGen Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by vanessamchale): * cc: vanessa.mchale@… (added) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14971#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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