#15196: Invert floating point comparisons such that no extra parity check is required. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (NCG) | Version: 8.4.3 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: | -------------------------------------+------------------------------------- Description changed by AndreasK: Old description:

This comment pretty much explains it already:

{{{ -- We have to worry about unordered operands (eg. comparisons -- against NaN). If the operands are unordered, the comparison -- sets the parity flag, carry flag and zero flag. -- All comparisons are supposed to return false for unordered -- operands except for !=, which returns true. -- -- Optimisation: we don't have to test the parity flag if we -- know the test has already excluded the unordered case: eg > -- and >= test for a zero carry flag, which can only occur for -- ordered operands. -- -- ToDo: by reversing comparisons we could avoid testing the -- parity flag in more cases. }}}

New description: This comment pretty much explains it already: {{{ -- We have to worry about unordered operands (eg. comparisons -- against NaN). If the operands are unordered, the comparison -- sets the parity flag, carry flag and zero flag. -- All comparisons are supposed to return false for unordered -- operands except for !=, which returns true. -- -- Optimisation: we don't have to test the parity flag if we -- know the test has already excluded the unordered case: eg > -- and >= test for a zero carry flag, which can only occur for -- ordered operands. -- -- ToDo: by reversing comparisons we could avoid testing the -- parity flag in more cases. }}} This would turn a sequence of {{{ jp foo jb bar jmp foo }}} into: {{{ jge foo jmp bar }}} -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15196#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15196: Invert floating point comparisons such that no extra parity check is required. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: AndreasK Type: task | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler (NCG) | Version: 8.4.3 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): Phab:D4990 Wiki Page: | -------------------------------------+------------------------------------- Changes (by AndreasK): * owner: (none) => AndreasK -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15196#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15196: Invert floating point comparisons such that no extra parity check is required. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: AndreasK Type: task | Status: patch Priority: normal | Milestone: 8.8.1 Component: Compiler (NCG) | Version: 8.4.3 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): Phab:D4990 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Krzysztof Gogolewski ): In [changeset:"6bb9bc7d3c935dcb77e0700cce28de2c9df646df/ghc" 6bb9bc7/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="6bb9bc7d3c935dcb77e0700cce28de2c9df646df" Invert FP conditions to eliminate the explicit NaN check. Summary: Optimisation: we don't have to test the parity flag if we know the test has already excluded the unordered case: eg > and >= test for a zero carry flag, which can only occur for ordered operands. By reversing comparisons we can avoid testing the parity for < and <= as well. This works since: * If any of the arguments is an NaN CF gets set. Resulting in a false result. * Since this allows us to rule out NaN we can exchange the arguments and invert the direction of the arrows. Test Plan: ci/nofib Reviewers: carter, bgamari, alpmestan Reviewed By: alpmestan Subscribers: alpmestan, simonpj, jmct, rwbarton, thomie GHC Trac Issues: #15196 Differential Revision: https://phabricator.haskell.org/D4990 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15196#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15196: Invert floating point comparisons such that no extra parity check is required. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: AndreasK Type: task | Status: closed Priority: normal | Milestone: 8.8.1 Component: Compiler (NCG) | Version: 8.4.3 Resolution: fixed | Keywords: CodeGen Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D4990 Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: merge => closed * resolution: => fixed * milestone: 8.6.2 => 8.8.1 Comment: I'm going to punt this to 8.8 to ensure it gets proper testing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15196#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler