#8082: Ordering of assembly blocks affects performance -----------------------------------+---------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (NCG) | Version: 7.6.3 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 (amd64) Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -----------------------------------+---------------------------------- Comment (by jstolarek): It is best to view attached assembly dumps with: {{{ diff -y --suppress-common-lines reverse-complem-HEAD.asm​ reverse-complem- bool-primops.asm​ | less }}} Or some other visual diff program. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8082#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8082: Ordering of assembly blocks affects performance -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (NCG) | Version: 7.6.3 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: Runtime | (amd64) performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by thomie): * failure: None/Unknown => Runtime performance bug -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8082#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8082: Ordering of assembly blocks affects performance -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (NCG) | Version: 7.6.3 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: Runtime | (amd64) performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by carter): could that be a branch prediction issue? (plus perhaps a matter of memory locality in the instruction cache?) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8082#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8082: Ordering of assembly blocks affects performance -------------------------------------+------------------------------------- Reporter: jstolarek | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (NCG) | Version: 7.6.3 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: Runtime | (amd64) performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by Joachim Breitner ): In [changeset:"de1160be047790afde4ec76de0a81ba3be0c73fa/ghc"]: {{{ #!CommitTicketReference repository="ghc" revision="de1160be047790afde4ec76de0a81ba3be0c73fa" Refactor the story around switches (#10137) This re-implements the code generation for case expressions at the Stg → Cmm level, both for data type cases as well as for integral literal cases. (Cases on float are still treated as before). The goal is to allow for fancier strategies in implementing them, for a cleaner separation of the strategy from the gritty details of Cmm, and to run this later than the Common Block Optimization, allowing for one way to attack #10124. The new module CmmSwitch contains a number of notes explaining this changes. For example, it creates larger consecutive jump tables than the previous code, if possible. nofib shows little significant overall improvement of runtime. The rather large wobbling comes from changes in the code block order (see #8082, not much we can do about it). But the decrease in code size alone makes this worthwhile. ``` Program Size Allocs Runtime Elapsed TotalMem Min -1.8% 0.0% -6.1% -6.1% -2.9% Max -0.7% +0.0% +5.6% +5.7% +7.8% Geometric Mean -1.4% -0.0% -0.3% -0.3% +0.0% ``` Compilation time increases slightly: ``` -1 s.d. ----- -2.0% +1 s.d. ----- +2.5% Average ----- +0.3% ``` The test case T783 regresses a lot, but it is the only one exhibiting any regression. The cause is the changed order of branches in an if-then-else tree, which makes the hoople data flow analysis traverse the blocks in a suboptimal order. Reverting that gets rid of this regression, but has a consistent, if only very small (+0.2%), negative effect on runtime. So I conclude that this test is an extreme outlier and no reason to change the code. Differential Revision: https://phabricator.haskell.org/D720 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8082#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler