#2731: Avoid unnecessary evaluation when unpacking constructors -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 8.5 Resolution: | Keywords: CodeGen Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by AndreasK): * version: 6.8.3 => 8.5 Comment: If anyone wonders this is still the case in GHC-8.4 / HEAD. Changing the type of `T` to `data T = MkT !(Bool,Bool)` also doesn't change anything. Cmm Code. {{{ {offset c1pY: if ((Sp + -8) < SpLim) (likely: False) goto c1q2; else goto c1q3; c1q2: R2 = R2; R1 = foo_closure; call (stg_gc_fun)(R2, R1) args: 8, res: 0, upd: 8; c1q3: I64[Sp - 8] = c1pV; R1 = R2; Sp = Sp - 8; if (R1 & 7 != 0) goto c1pV; else goto c1pW; c1pW: call (I64[R1])(R1) returns to c1pV, args: 8, res: 8, upd: 8; c1pV: I64[Sp] = c1q1; R1 = P64[R1 + 7]; if (R1 & 7 != 0) goto c1q1; else goto c1q5; c1q5: call (I64[R1])(R1) returns to c1q1, args: 8, res: 8, upd: 8; c1q1: R1 = P64[R1 + 7] & (-8); Sp = Sp + 8; call (I64[R1])(R1) args: 8, res: 0, upd: 8; } }}} To solve this we would need to track the evaluation requirements of binders in STG. Something it seems we currently don't do. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/2731#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler