Strip off invariant.start because memory locations arent invariant

The original change was reverted in rL317217 because of the failure in
the RS4GC testcase. I couldn't reproduce the failure on my local machine
(macbook) but could reproduce it on a linux box.

The failure was around removing the uses of invariant.start. The fix
here is to just RAUW undef (which was the first implementation in D39388).
This is perfectly valid IR as discussed in the review.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317225 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

@@ -125,10 +125,10 @@ struct RewriteStatepointsForGC : public ModulePass {
       Changed |= runOnFunction(F);
 
     if (Changed) {
-      // stripNonValidAttributesAndMetadata asserts that shouldRewriteStatepointsIn
+      // stripNonValidData 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.
-      stripNonValidAttributesAndMetadata(M);
+      stripNonValidData(M);
     }
 
     return Changed;
@@ -146,15 +146,17 @@ struct RewriteStatepointsForGC : public ModulePass {
   /// metadata implying dereferenceability that are no longer valid/correct after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. stripNonValidAttributesAndMetadata (conservatively) restores
+  /// heap. stripNonValidData (conservatively) restores
   /// correctness by erasing all attributes in the module that externally imply
   /// dereferenceability. Similar reasoning also applies to the noalias
   /// attributes and metadata. gc.statepoint can touch the entire heap including
   /// noalias objects.
-  void stripNonValidAttributesAndMetadata(Module &M);
+  /// Apart from attributes and metadata, we also remove instructions that imply
+  /// constant physical memory: llvm.invariant.start.
+  void stripNonValidData(Module &M);
 
-  // Helpers for stripNonValidAttributesAndMetadata
-  void stripNonValidAttributesAndMetadataFromBody(Function &F);
+  // Helpers for stripNonValidData
+  void stripNonValidDataFromBody(Function &F);
   void stripNonValidAttributesFromPrototype(Function &F);
 
   // Certain metadata on instructions are invalid after running RS4GC.
@@ -2385,14 +2387,30 @@ void RewriteStatepointsForGC::stripInvalidMetadataFromInstruction(Instruction &I
   I.dropUnknownNonDebugMetadata(ValidMetadataAfterRS4GC);
 }
 
-void RewriteStatepointsForGC::stripNonValidAttributesAndMetadataFromBody(Function &F) {
+void RewriteStatepointsForGC::stripNonValidDataFromBody(Function &F) {
   if (F.empty())
     return;
 
   LLVMContext &Ctx = F.getContext();
   MDBuilder Builder(Ctx);
 
+  // Set of invariantstart instructions that we need to remove.
+  // Use this to avoid invalidating the instruction iterator.
+  SmallVector<IntrinsicInst*, 12> InvariantStartInstructions;
+
   for (Instruction &I : instructions(F)) {
+    // invariant.start on memory location implies that the referenced memory
+    // location is constant and unchanging. This is no longer true after
+    // RewriteStatepointsForGC runs because there can be calls to gc.statepoint
+    // which frees the entire heap and the presence of invariant.start allows
+    // the optimizer to sink the load of a memory location past a statepoint,
+    // which is incorrect.
+    if (auto *II = dyn_cast<IntrinsicInst>(&I))
+      if (II->getIntrinsicID() == Intrinsic::invariant_start) {
+        InvariantStartInstructions.push_back(II);
+        continue;
+      }
+
     if (const MDNode *MD = I.getMetadata(LLVMContext::MD_tbaa)) {
       assert(MD->getNumOperands() < 5 && "unrecognized metadata shape!");
       bool IsImmutableTBAA =
@@ -2422,6 +2440,12 @@ void RewriteStatepointsForGC::stripNonValidAttributesAndMetadataFromBody(Functio
         RemoveNonValidAttrAtIndex(Ctx, CS, AttributeList::ReturnIndex);
     }
   }
+
+  // Delete the invariant.start instructions and RAUW undef.
+  for (auto *II : InvariantStartInstructions) {
+    II->replaceAllUsesWith(UndefValue::get(II->getType()));
+    II->eraseFromParent();
+  }
 }
 
 /// Returns true if this function should be rewritten by this pass.  The main
@@ -2438,7 +2462,7 @@ static bool shouldRewriteStatepointsIn(Function &F) {
     return false;
 }
 
-void RewriteStatepointsForGC::stripNonValidAttributesAndMetadata(Module &M) {
+void RewriteStatepointsForGC::stripNonValidData(Module &M) {
 #ifndef NDEBUG
   assert(llvm::any_of(M, shouldRewriteStatepointsIn) && "precondition!");
 #endif
@@ -2447,7 +2471,7 @@ void RewriteStatepointsForGC::stripNonValidAttributesAndMetadata(Module &M) {
     stripNonValidAttributesFromPrototype(F);
 
   for (Function &F : M)
-    stripNonValidAttributesAndMetadataFromBody(F);
+    stripNonValidDataFromBody(F);
 }
 
 bool RewriteStatepointsForGC::runOnFunction(Function &F) {

test/Transforms/RewriteStatepointsForGC/drop-invalid-metadata.ll

@@ -75,6 +75,54 @@ define void @test_dereferenceable(i32 addrspace(1)* addrspace(1)* %p, i32 %x, i3
   ret void
 }
 
+; invariant.start allows us to sink the load past the baz statepoint call into taken block, which is
+; incorrect. remove the invariant.start and RAUW undef.
+define void @test_inv_start(i1 %cond, i32 addrspace(1)* addrspace(1)* %p, i32 %x, i32 addrspace(1)* %q) gc "statepoint-example" {
+; CHECK-LABEL: test_inv_start
+; CHECK-NOT: invariant.start
+; CHECK: gc.statepoint
+  %v1 = load i32 addrspace(1)*, i32 addrspace(1)* addrspace(1)* %p
+  %invst = call {}* @llvm.invariant.start.p1i32(i64 1, i32 addrspace(1)* %v1)
+  %v2 = load i32, i32 addrspace(1)* %v1
+  call void @baz(i32 %x)
+  br i1 %cond, label %taken, label %untaken
+
+taken:
+  store i32 %v2, i32 addrspace(1)* %q, align 16
+  call void @llvm.invariant.end.p1i32({}* %invst, i64 4, i32 addrspace(1)* %v1)
+  ret void
+
+; CHECK-LABEL: untaken:
+; CHECK: gc.statepoint
+untaken:
+  %foo = call i32 @escaping.invariant.start({}* %invst)
+  call void @dummy(i32 %foo)
+  ret void
+}
+
+; invariant.start is removed and the uses are undef'ed.
+define void @test_inv_start2(i1 %cond, i32 addrspace(1)* addrspace(1)* %p, i32 %x, i32 addrspace(1)* %q) gc "statepoint-example" {
+; CHECK-LABEL: test_inv_start2
+; CHECK-NOT: invariant.start
+; CHECK: gc.statepoint
+  %v1 = load i32 addrspace(1)*, i32 addrspace(1)* addrspace(1)* %p
+  %invst = call {}* @llvm.invariant.start.p1i32(i64 1, i32 addrspace(1)* %v1)
+  %v2 = load i32, i32 addrspace(1)* %v1
+  call void @baz(i32 %x)
+  br i1 %cond, label %taken, label %untaken
+
+taken:
+  store i32 %v2, i32 addrspace(1)* %q, align 16
+  call void @llvm.invariant.end.p1i32({}* %invst, i64 4, i32 addrspace(1)* %v1)
+  ret void
+
+untaken:
+  ret void
+}
+declare {}* @llvm.invariant.start.p1i32(i64, i32 addrspace(1)*  nocapture) nounwind readonly
+declare void @llvm.invariant.end.p1i32({}*, i64, i32 addrspace(1)* nocapture) nounwind
+declare i32 @escaping.invariant.start({}*) nounwind
+declare void @dummy(i32)
 declare token @llvm.experimental.gc.statepoint.p0f_isVoidi32f(i64, i32, void (i32)*, i32, i32, ...)
 
 ; Function Attrs: nounwind readonly