[RewriteStatepointsForGC] Strip deref info after rewriting.

[sanjoy]

Summary

Once a gc.statepoint has been rewritten to relocate live references, the
SSA values represent physical pointers instead of logical references.
Logical dereferencability does not imply physical dereferencability and
after RewriteStatepointsForGC has run any attributes that imply
dereferencability of the logical references need to be stripped.

This current approach is conservative, and can be made more precise
later if needed. For starters, we need to strip dereferencable
attributes only from pointers that live in the GC address space.

[reames]

Lots of minor code comments.

As we discussed offline, I'm not particular happy with the idea of this patch. I can't really see a better solution, but the fact we need to do this for entire module is problematic. In particular, it breaks the gc per function abstraction...

(Oh, you need to make this conditional on the presence of the GC attribute on at least one function. Arguably, the changed check handles this, maybe just a clear comment?)

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

2230

The IfNeeded doesn't add anything in terms of meaning.

2231

Comment should be outside lambda. I'd be tempted to pull the lambda out as a static helper function since it's fairly large.

2260

Code duplication ahoy. Could/should this be commoned with a templated helper function?

2279

Is there a TBAA helper class we can use here? Not sure there is, but please check.

2281

It looks like you're only handling one of two forms of TBAA metadata here...

2301

Use Atributes::ReturnIndex both here and above. No more magic constants!

2370

Whitespace?

[sanjoy]

• address review

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

2230

Will fix.

2231

Moved out the lambda as a templated static helper.

2260

Done.

2279

Looked, could not find anything.

2281

AFAICT, LLVM's autoupgrade rewrites all TBAA metadata to the struct-path format (in llvm::UpgradeInstWithTBAATag).

2301

Will fix.

2370

Will fix.

[sanjoy]

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

2279

To clarify -- I could not find a public interface for the TBAA metadata. There is a helper class inside TypeBasedAliasAnalysis.cpp that's not exported.

[reames]

LGTM. Still not crazy about the fact we have to do this, but the actual implementation seems reasonable and I can't come up with a better approach.

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstIterator.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/Statepoint.h"
 #include "llvm/IR/Value.h"
 #include "llvm/IR/Verifier.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Cloning.h"
struct RewriteStatepointsForGC : public ModulePass {
   RewriteStatepointsForGC() : ModulePass(ID) {
     initializeRewriteStatepointsForGCPass(*PassRegistry::getPassRegistry());
   }
   bool runOnFunction(Function &F);
   bool runOnModule(Module &M) override {
     bool Changed = false;
     for (Function &F : M)
       Changed |= runOnFunction(F);
+
+    if (Changed) {
+      // stripDereferenceabilityInfo asserts that shouldRewriteStatepointsIn
+      // returns true for at least one function in the module.  Since at least
+      // one function changed, we know that the precondition is satisfied.
+      stripDereferenceabilityInfo(M);
+    }
+
     return Changed;
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     // We add and rewrite a bunch of instructions, but don't really do much
     // else.  We could in theory preserve a lot more analyses here.
     AU.addRequired<DominatorTreeWrapperPass>();
     AU.addRequired<TargetTransformInfoWrapperPass>();
   }
+
+  /// The IR fed into RewriteStatepointsForGC may have had attributes implying
+  /// dereferenceability that are no longer valid/correct after
+  /// RewriteStatepointsForGC has run.  This is because semantically, after
+  /// RewriteStatepointsForGC runs, all calls to gc.statepoint "free" the entire
+  /// heap.  stripDereferenceabilityInfo (conservatively) restores correctness
+  /// by erasing all attributes in the module that externally imply
+  /// dereferenceability.
+  ///
+  void stripDereferenceabilityInfo(Module &M);
+
+  // Helpers for stripDereferenceabilityInfo
+  void stripDereferenceabilityInfoFromBody(Function &F);
+  void stripDereferenceabilityInfoFromPrototype(Function &F);
 };
 } // namespace
 
 char RewriteStatepointsForGC::ID = 0;
 
 ModulePass *llvm::createRewriteStatepointsForGCPass() {
   return new RewriteStatepointsForGC();
 }
for (auto ptr : live) {
     assert(isGCPointerType(ptr->getType()) && "must be a gc pointer type");
   }
 #endif
 
   relocationViaAlloca(F, DT, live, records);
   return !records.empty();
 }
 
+// Handles both return values and arguments for Functions and CallSites.
+template <typename AttrHolder>
+static void RemoveDerefAttrAtIndex(LLVMContext &Ctx, AttrHolder &AH,
+                                   unsigned Index) {
+  AttrBuilder R;
+  if (AH.getDereferenceableBytes(Index))
+    R.addAttribute(Attribute::get(Ctx, Attribute::Dereferenceable,
+                                  AH.getDereferenceableBytes(Index)));
+  if (AH.getDereferenceableOrNullBytes(Index))
+    R.addAttribute(Attribute::get(Ctx, Attribute::DereferenceableOrNull,
+                                  AH.getDereferenceableOrNullBytes(Index)));
+
+  if (!R.empty())
+    AH.setAttributes(AH.getAttributes().removeAttributes(
+        Ctx, Index, AttributeSet::get(Ctx, Index, R)));
+};
+
+void
+RewriteStatepointsForGC::stripDereferenceabilityInfoFromPrototype(Function &F) {
+  LLVMContext &Ctx = F.getContext();
+
+  for (Argument &A : F.args())
+    if (isa<PointerType>(A.getType()))
+      RemoveDerefAttrAtIndex(Ctx, F, A.getArgNo() + 1);
+
+  if (isa<PointerType>(F.getReturnType()))
+    RemoveDerefAttrAtIndex(Ctx, F, AttributeSet::ReturnIndex);
+}
+
+void RewriteStatepointsForGC::stripDereferenceabilityInfoFromBody(Function &F) {
+  if (F.empty())
+    return;
+
+  LLVMContext &Ctx = F.getContext();
+  MDBuilder Builder(Ctx);
+
+  for (Instruction &I : inst_range(F)) {
+    if (const MDNode *MD = I.getMetadata(LLVMContext::MD_tbaa)) {
+      assert(MD->getNumOperands() < 5 && "unrecognized metadata shape!");
+      bool IsImmutableTBAA =
+          MD->getNumOperands() == 4 &&
+          mdconst::extract<ConstantInt>(MD->getOperand(3))->getValue() == 1;
+
+      if (!IsImmutableTBAA)
+        continue; // no work to do, MD_tbaa is already marked mutable
+
+      MDNode *Base = cast<MDNode>(MD->getOperand(0));
+      MDNode *Access = cast<MDNode>(MD->getOperand(1));
+      uint64_t Offset =
+          mdconst::extract<ConstantInt>(MD->getOperand(2))->getZExtValue();
+
+      MDNode *MutableTBAA =
+          Builder.createTBAAStructTagNode(Base, Access, Offset);
+      I.setMetadata(LLVMContext::MD_tbaa, MutableTBAA);
+    }
+
+    if (CallSite CS = CallSite(&I)) {
+      for (int i = 0, e = CS.arg_size(); i != e; i++)
+        if (isa<PointerType>(CS.getArgument(i)->getType()))
+          RemoveDerefAttrAtIndex(Ctx, CS, i + 1);
+      if (isa<PointerType>(CS.getType()))
+        RemoveDerefAttrAtIndex(Ctx, CS, AttributeSet::ReturnIndex);
+    }
+  }
+}
+
 /// Returns true if this function should be rewritten by this pass.  The main
 /// point of this function is as an extension point for custom logic.
 static bool shouldRewriteStatepointsIn(Function &F) {
   // TODO: This should check the GCStrategy
   if (F.hasGC()) {
     const char *FunctionGCName = F.getGC();
     const StringRef StatepointExampleName("statepoint-example");
     const StringRef CoreCLRName("coreclr");
     return (StatepointExampleName == FunctionGCName) ||
            (CoreCLRName == FunctionGCName);
   } else
     return false;
 }
 
+void RewriteStatepointsForGC::stripDereferenceabilityInfo(Module &M) {
+#ifndef NDEBUG
+  assert(std::any_of(M.begin(), M.end(), shouldRewriteStatepointsIn) &&
+         "precondition!");
+#endif
+
+  for (Function &F : M)
+    stripDereferenceabilityInfoFromPrototype(F);
+
+  for (Function &F : M)
+    stripDereferenceabilityInfoFromBody(F);
+}
+
 bool RewriteStatepointsForGC::runOnFunction(Function &F) {
   // Nothing to do for declarations.
   if (F.isDeclaration() || F.empty())
     return false;
 
   // Policy choice says not to rewrite - the most common reason is that we're
   // compiling code without a GCStrategy.
   if (!shouldRewriteStatepointsIn(F))
bool RewriteStatepointsForGC::runOnFunction(Function &F) {
   // insertion since relocations and base phis can confuse things.
   for (BasicBlock &BB : F)
     if (BB.getUniquePredecessor()) {
       MadeChange = true;
       FoldSingleEntryPHINodes(&BB);
     }
 
   MadeChange |= insertParsePoints(F, DT, this, ParsePointNeeded);
   return MadeChange;
 }
 
 // liveness computation via standard dataflow
 // -------------------------------------------------------------------
 
 // TODO: Consider using bitvectors for liveness, the set of potentially
 // interesting values should be small and easy to pre-compute.
 

test/Transforms/RewriteStatepointsForGC/deref-pointers.ll

+; RUN: opt -S -rewrite-statepoints-for-gc < %s | FileCheck %s
+
+declare void @foo()
+declare i8 addrspace(1)* @some_function()
+declare void @some_function_consumer(i8 addrspace(1)*)
+declare dereferenceable(4) i8 addrspace(1)* @some_function_ret_deref()
+; CHECK: declare i8 addrspace(1)* @some_function_ret_deref()
+
+define i8 addrspace(1)* @test_deref_arg(i8 addrspace(1)* dereferenceable(4) %a) gc "statepoint-example" {
+; CHECK: define i8 addrspace(1)* @test_deref_arg(i8 addrspace(1)* %a)
+entry:
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0)
+  ret i8 addrspace(1)* %a
+}
+
+define i8 addrspace(1)* @test_deref_or_null_arg(i8 addrspace(1)* dereferenceable_or_null(4) %a) gc "statepoint-example" {
+; CHECK: define i8 addrspace(1)* @test_deref_or_null_arg(i8 addrspace(1)* %a)
+entry:
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0)
+  ret i8 addrspace(1)* %a
+}
+
+define i8 addrspace(1)* @test_deref_retval() gc "statepoint-example" {
+; CHECK-LABEL: @test_deref_retval(
+entry:
+  %a = call dereferenceable(4) i8 addrspace(1)* @some_function()
+; CHECK: %a = call i8 addrspace(1)* @some_function()
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0)
+  ret i8 addrspace(1)* %a
+}
+
+define i8 addrspace(1)* @test_deref_or_null_retval() gc "statepoint-example" {
+; CHECK-LABEL: @test_deref_or_null_retval(
+entry:
+  %a = call dereferenceable_or_null(4) i8 addrspace(1)* @some_function()
+; CHECK: %a = call i8 addrspace(1)* @some_function()
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0)
+  ret i8 addrspace(1)* %a
+}
+
+define i8 @test_md(i8 addrspace(1)* %ptr) gc "statepoint-example" {
+; CHECK-LABEL: @test_md(
+ entry:
+; CHECK: %tmp = load i8, i8 addrspace(1)* %ptr, !tbaa !0
+  %tmp = load i8, i8 addrspace(1)* %ptr, !tbaa !0
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0)
+  ret i8 %tmp
+}
+
+define i8 addrspace(1)* @test_decl_only_attribute(i8 addrspace(1)* %ptr) gc "statepoint-example" {
+; CHECK-LABEL: @test_decl_only_attribute(
+entry:
+; No change here, but the prototype of some_function_ret_deref should have changed.
+; CHECK: call i8 addrspace(1)* @some_function_ret_deref()
+  %a = call i8 addrspace(1)* @some_function_ret_deref()
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0)
+  ret i8 addrspace(1)* %a
+}
+
+define i8 addrspace(1)* @test_callsite_arg_attribute(i8 addrspace(1)* %ptr) gc "statepoint-example" {
+; CHECK-LABEL: @test_callsite_arg_attribute(
+entry:
+; CHECK: call void @some_function_consumer(i8 addrspace(1)* %ptr)
+  call void @some_function_consumer(i8 addrspace(1)* dereferenceable(4) %ptr)
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0)
+  ret i8 addrspace(1)* %ptr
+}
+
+declare i32 @llvm.experimental.gc.statepoint.p0f_isVoidf(i64, i32, void ()*, i32, i32, ...)
+
+!0 = !{!1, !1, i64 0, i64 1}
+!1 = !{!"red", !2}
+!2 = !{!"blue"}
+
+; CHECK: !0 = !{!1, !1, i64 0}
+; CHECK: !1 = !{!"red", !2}
+; CHECK: !2 = !{!"blue"}