#10699: Regression: panic with custom rewrite rules on primops -------------------------------------+------------------------------------- Reporter: thomie | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: #10555 Differential Revisions: | -------------------------------------+------------------------------------- The following program results in a panic with ghc-7.10.2 (and also HEAD with the patches for #10528 and #10595 applied), but not with ghc-7.10.1. {{{#!haskell {-# LANGUAGE MagicHash #-} module T10555b where import GHC.Prim {-# RULES "double commute left *" forall x1 x2 x3. (*##) x1 ((*##) x2 x3) = (*##) ((*##) x2 x3) x1 #-} {-# RULES "double **4" forall x . x **## 4.0## = let xx = x *## x in xx *## xx #-} }}} {{{ $ ghc-7.10.2 -fforce-recomp -O T10555b.hs ghc: panic! (the 'impossible' happened) (GHC version 7.10.2 for x86_64-unknown-linux): Simplifier ticks exhausted When trying RuleFired double commute left * To increase the limit, use -fsimpl-tick-factor=N (default 100) If you need to do this, let GHC HQ know, and what factor you needed To see detailed counts use -ddump-simpl-stats Total ticks: 4004 }}} I find this suprising, because there isn't anything to rewrite yet, is there? These are the prerequisites to trigger the bug: * both rules are needed * the function (`*##`) should be a primop If I replace the function (`*##`) by `f` below, I don't get a panic, but I do get the following warning: {{{ RULE left-hand side too complicated to desugar Optimised lhs: case f x2 x3 of wild_00 { __DEFAULT -> f x1 wild_00 } Orig lhs: case f x2 x3 of wild_00 { __DEFAULT -> f x1 wild_00 } }}} {{{#!haskell {-# NOINLINE f #-} f :: Double# -> Double# -> Double# f = undefined }}} Note that the first rewrite rule (`"double commute left *"`) is buggy by itself, since it will loop on `times4` (with any compiler version): {{{#!haskell times4 :: Double -> Double times4 (D# x) = D# ((x *## x) *## (x *## x)) }}} So I'm not quite sure if there is a actually a bug in GHC here, but I don't understand what's going on either. These examples are extracted from the [https://hackage.haskell.org/package /fast-math fast-math] package. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10699 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler