[rs4gc] Optionally directly relocated vector of pointers
This patch teaches rewrite-statepoints-for-gc to relocate vector-of-pointers directly rather than trying to split them. This builds on the recent lowering/IR changes to allow vector typed gc.relocates.
The motivation for this is that we recently found a bug in the vector splitting code where depending on visit order, a vector might not be relocated at some safepoint. Specifically, the bug is that the splitting code wasn't updating the side tables (live vector) of other safepoints. As a result, a vector which was live at two safepoints might not be updated at one of them. However, if you happened to visit safepoints in post order over the dominator tree, everything worked correctly. Weirdly, it turns out that post order is actually an incredibly common order to visit instructions in in practice. Frustratingly, I have not managed to write a test case which actually hits this. I can only reproduce it in large IR files produced by actual applications.
Rather than continue to make this code more complicated, we can remove all of the complexity by just representing the relocation of the entire vector natively in the IR.
At the moment, the new functionality is hidden behind a flag. To use this code, you need to pass "-rs4gc-split-vector-values=0". Once I have a chance to stress test with this option and get feedback from other users, my plan is to flip the default and remove the original splitting code. I would just remove it now, but given the rareness of the bug, I figured it was better to leave it in place until the new approach has been stress tested.
Differential Revision: http://reviews.llvm.org/D15982
llvm-svn: 257244
diff --git a/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp b/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
index 94768a2..19d50db 100644
--- a/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
+++ b/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
@@ -1317,18 +1317,29 @@
assert(Index < LiveVec.size() && "Bug in std::find?");
return Index;
};
-
- // All gc_relocate are set to i8 addrspace(1)* type. We originally generated
- // unique declarations for each pointer type, but this proved problematic
- // because the intrinsic mangling code is incomplete and fragile. Since
- // we're moving towards a single unified pointer type anyways, we can just
- // cast everything to an i8* of the right address space. A bitcast is added
- // later to convert gc_relocate to the actual value's type.
Module *M = StatepointToken->getModule();
- auto AS = cast<PointerType>(LiveVariables[0]->getType())->getAddressSpace();
- Type *Types[] = {Type::getInt8PtrTy(M->getContext(), AS)};
- Value *GCRelocateDecl =
- Intrinsic::getDeclaration(M, Intrinsic::experimental_gc_relocate, Types);
+
+ // All gc_relocate are generated as i8 addrspace(1)* (or a vector type whose
+ // element type is i8 addrspace(1)*). We originally generated unique
+ // declarations for each pointer type, but this proved problematic because
+ // the intrinsic mangling code is incomplete and fragile. Since we're moving
+ // towards a single unified pointer type anyways, we can just cast everything
+ // to an i8* of the right address space. A bitcast is added later to convert
+ // gc_relocate to the actual value's type.
+ auto getGCRelocateDecl = [&] (Type *Ty) {
+ assert(isHandledGCPointerType(Ty));
+ auto AS = Ty->getScalarType()->getPointerAddressSpace();
+ Type *NewTy = Type::getInt8PtrTy(M->getContext(), AS);
+ if (auto *VT = dyn_cast<VectorType>(Ty))
+ NewTy = VectorType::get(NewTy, VT->getNumElements());
+ return Intrinsic::getDeclaration(M, Intrinsic::experimental_gc_relocate,
+ {NewTy});
+ };
+
+ // Lazily populated map from input types to the canonicalized form mentioned
+ // in the comment above. This should probably be cached somewhere more
+ // broadly.
+ DenseMap<Type*, Value*> TypeToDeclMap;
for (unsigned i = 0; i < LiveVariables.size(); i++) {
// Generate the gc.relocate call and save the result
@@ -1336,6 +1347,11 @@
Builder.getInt32(LiveStart + FindIndex(LiveVariables, BasePtrs[i]));
Value *LiveIdx = Builder.getInt32(LiveStart + i);
+ Type *Ty = LiveVariables[i]->getType();
+ if (!TypeToDeclMap.count(Ty))
+ TypeToDeclMap[Ty] = getGCRelocateDecl(Ty);
+ Value *GCRelocateDecl = TypeToDeclMap[Ty];
+
// only specify a debug name if we can give a useful one
CallInst *Reloc = Builder.CreateCall(
GCRelocateDecl, {StatepointToken, BaseIdx, LiveIdx},
@@ -2478,7 +2494,8 @@
#ifndef NDEBUG
// sanity check
for (auto *Ptr : Live)
- assert(isGCPointerType(Ptr->getType()) && "must be a gc pointer type");
+ assert(isHandledGCPointerType(Ptr->getType()) &&
+ "must be a gc pointer type");
#endif
relocationViaAlloca(F, DT, Live, Records);