#8199: Get rid of HEAP_ALLOCED ----------------------------+---------------------------------------------- Reporter: ezyang | Owner: ezyang Type: feature | Status: new request | Milestone: 7.10.1 Priority: normal | Version: 7.7 Component: | Keywords: Compiler | Architecture: Unknown/Multiple Resolution: | Difficulty: Project (more than a week) Operating System: | Blocked By: 2841 Unknown/Multiple | Related Tickets: Type of failure: | None/Unknown | Test Case: | Blocking: | ----------------------------+---------------------------------------------- Changes (by ezyang): * blockedby: => 2841 Old description:
This bug is to track progress of removing HEAP_ALLOCED from GHC, promising faster GC, as long as we can keep the cost of indirections down.
The relevant wiki article: http://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/HeapAlloced ; we are implementing method 2. Version 2 of the patchset is probably correct.
Improvements possible:
* Don't pay for a double indirection when -fPIC is turned on. Probably the easiest way to do this is to *not* bake in the indirections into compiled code when it is -fPIC'd, and instead scribble over the GOT. However, I don't know how to go backwards from a symbol to a GOT entry, so we might need some heinous assembly hacks to get this working.
* Closure tables should have their indirections short-circuited by the initialization code.
* We are paying an indirection when a GC occurs when the closure is not in R1. According to the wiki page, technically this is not needed, but I don't know how to eliminate references to the closure itself from stg_gc_fun.
* Properly handle the Windows DLL case (e.g. SRTs); this should be simpler now that everything is an indirection.
* Bikeshed hs_main API changes (because closures are not valid prior to RTS initialization, so you have to pass in an indirection instead)
* ~~Improve static indirection and stable pointer registration, avoiding binary bloat from `__attribute(constructor)__` stubs.~~ After discussing this with some folks, it seems that there isn't really a portable way to do this when we are using the system dynamic linker to load libraries at startup. The problem is that we need to somehow get a list of all the GHC-compiled libraries which got loaded, and really the easiest way to get that is to just build it ourselves.
* ~~Need to implement a new megablock tracking structure so we can free/check for lost blocks~~. Now that efficient lookup is not necessary, perhaps we can write-optimize the megablock tracking structures.
New description: This bug is to track progress of removing HEAP_ALLOCED from GHC, promising faster GC, as long as we can keep the cost of indirections down. The relevant wiki article: http://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/HeapAlloced ; we are implementing method 2. Version 2 of the patchset is probably correct. Improvements possible: * Don't pay for a double indirection when -fPIC is turned on. Probably the easiest way to do this is to *not* bake in the indirections into compiled code when it is -fPIC'd, and instead scribble over the GOT. However, I don't know how to go backwards from a symbol to a GOT entry, so we might need some heinous assembly hacks to get this working. * Closure tables should have their indirections short-circuited by the initialization code. But maybe it is not worth the cost of telling the RTS about the closure tables (also, they would need to be made writeable). * We are paying an indirection when a GC occurs when the closure is not in R1. According to the wiki page, technically this is not needed, but I don't know how to eliminate references to the closure itself from stg_gc_fun. * Properly handle the Windows DLL case (e.g. SRTs). We will probably have to reorganize how the indirections are laid out. * Bikeshed hs_main API changes (because closures are not valid prior to RTS initialization, so you have to pass in an indirection instead) * ~~Make it work for GHCi linking of static objects.~~ Blocked on #2841, I have it working for ELF, and can make it work for other platforms as soon as I get relevant machines. * ~~Improve static indirection and stable pointer registration, avoiding binary bloat from `__attribute(constructor)__` stubs.~~ After discussing this with some folks, it seems that there isn't really a portable way to do this when we are using the system dynamic linker to load libraries at startup. The problem is that we need to somehow get a list of all the GHC-compiled libraries which got loaded, and really the easiest way to get that is to just build it ourselves. * ~~Need to implement a new megablock tracking structure so we can free/check for lost blocks~~. Now that efficient lookup is not necessary, perhaps we can write-optimize the megablock tracking structures. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8199#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler