#15126: Opportunity to compress common info table representation. -------------------------------------+------------------------------------- Reporter: AndreasK | Owner: (none) Type: task | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Keywords: CodeGen | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I've looked at a lot of GHC produced assembly recently and noticed that most info tables describing stacks have the form: {{{ .align 8 .long SDjR_srt-(block_cHmk_info)+296 .long 0 .quad 6151 .quad 4294967326 }}} I haven't managed to dig fully into the description however some observations: * I noticed that the second .long directive almost always ends up being zero. * When figuring out what is what I realized the first quad (describing the pointers) is almost never fully used. * The last entrie (closure type + ?), here `4294967326` also seems quite repetitive given the size reserved. So I looked in detail at spectral/simple: * There are 2012 info tables of this sort with all of them having a zero in the second long. * We also reserve 8 byte for the stack layout. However only a single of these tables requires more than 4 byte. The compiled module has a size of 276384 Bytes, with 16092 being redundant: * 4 bytes for 0 * 4 bytes unused stack description * times 2012 info tables. That is an overhead of 5,8% which seems like quite a lot to me. The questions where to put that information is a different one. But only looking at the data and not how it is used tagging the pointer to the SRT table seems like a possibility. The info table description `4294967326` also appeared over 1k times. Maybe it's possible to come up with a more efficient encoding there as well. I didn't give it much thought yet since I don't have the time to do anything about it in the near future. But putting it here in case anyone is interested or looks into this in the future. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15126 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler