From 364caf0e19e570d00cfd03d9dd3fcda21fb2e459 Mon Sep 17 00:00:00 2001 From: Gordon Henriksen Date: Sat, 29 Sep 2007 02:13:43 +0000 Subject: Collector is the base class for garbage collection code generators. This version enhances the previous patch to add root initialization as discussed here: http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070910/053455.html Collector gives its subclasses control over generic algorithms: unsigned NeededSafePoints; //< Bitmask of required safe points. bool CustomReadBarriers; //< Default is to insert loads. bool CustomWriteBarriers; //< Default is to insert stores. bool CustomRoots; //< Default is to pass through to backend. bool InitRoots; //< If set, roots are nulled during lowering. It also has callbacks which collectors can hook: /// If any of the actions are set to Custom, this is expected to /// be overriden to create a transform to lower those actions to /// LLVM IR. virtual Pass *createCustomLoweringPass() const; /// beginAssembly/finishAssembly - Emit module metadata as /// assembly code. virtual void beginAssembly(Module &M, std::ostream &OS, AsmPrinter &AP, const TargetAsmInfo &TAI) const; virtual void finishAssembly(Module &M, CollectorModuleMetadata &CMM, std::ostream &OS, AsmPrinter &AP, const TargetAsmInfo &TAI) const; Various other independent algorithms could be implemented, but were not necessary for the initial two collectors. Some examples are listed here: http://llvm.org/docs/GarbageCollection.html#collector-algos git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42466 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/CodeGen/README.txt | 38 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) (limited to 'lib/CodeGen/README.txt') diff --git a/lib/CodeGen/README.txt b/lib/CodeGen/README.txt index 8767191320..8bd9a7af0d 100644 --- a/lib/CodeGen/README.txt +++ b/lib/CodeGen/README.txt @@ -148,3 +148,41 @@ It's not always a good idea to choose rematerialization over spilling. If all the load / store instructions would be folded then spilling is cheaper because it won't require new live intervals / registers. See 2003-05-31-LongShifts for an example. + +//===---------------------------------------------------------------------===// + +Instead of unconditionally inserting a null initializer for every GC root when +Collector::InitRoots is set, the collector infrastructure should get a little +bit smarter and perform a trivial DSE of the initial basic block up to the +first safe point. + +//===---------------------------------------------------------------------===// + +With a copying garbage collector, derived pointers must not be retained across +collector safe points; the collector could move the objects and invalidate the +derived pointer. This is bad enough in the first place, but safe points can +crop up unpredictably. Consider: + + %array = load { i32, [0 x %obj] }** %array_addr + %nth_el = getelementptr { i32, [0 x %obj] }* %array, i32 0, i32 %n + %old = load %obj** %nth_el + %z = div i64 %x, %y + store %obj* %new, %obj** %nth_el + +If the i64 division is lowered to a libcall, then a safe point will (must) +appear for the call site. If a collection occurs, %array and %nth_el no longer +point into the correct object. + +The fix for this is to copy address calculations so that dependent pointers +are never live across safe point boundaries. But the loads cannot be copied +like this if there was an intervening store, so may be hard to get right. + +Only a concurrent mutator can trigger a collection at the libcall safe point. +So single-threaded programs do not have this requirement, even with a copying +collector. Still, LLVM optimizations would probably undo a front-end's careful +work. + +//===---------------------------------------------------------------------===// + +The ocaml frametable structure supports liveness information. It would be good +to support it. -- cgit v1.2.3