#8971: Native Code Generator 7.8.1 RC2 is not as optimized as 7.6.3... --------------------------------------------+------------------------------ Reporter: GordonBGood | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (NCG) | Version: 7.8.1-rc2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime performance bug | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by awson): Replying to [comment:8 GordonBGood]:
This bug has nothing to do with the difference between the LLVM and the NCG compiler backends; it it well known that NCG isn't that efficient for tight loops. For this bug use case code, NCG has never combined the memory read, the 'and' operation, and the memory write into one read/modify/write instruction whereas LLVM does, which explains the difference between them.
Oh, I know this. But in the last days I fought with a pile of bugs in 64-bit Windows GHC and I saw a lot of NCG-produced assembly that was bad far beyond what you've described above. Off the top of my head, when I worked with #8974, I saw this: {{{ call suspendThread addq $40,%rsp subq $8,%rsp subq $32,%rsp movq %rax,%rbx movl $0,%eax call performMajorGC addq $40,%rsp movq %rbx,%rcx subq $8,%rsp subq $32,%rsp movl $0,%eax call resumeThread }}} produced by NCG, versus {{{ callq suspendThread movq %rax, %rsi callq performMajorGC movq %rsi, %rcx callq resumeThread }}} produced by LLVM. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8971#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler