isDereferenceablePointer: look through gc.relocate calls

[artagnon]

Summary

A pointer dereferenceable before a gc.relocate should be dereferenceable
after a gc.relocate, because the gc might not kick in, making the
gc.relocate a noop in some cases (and hence the invariant should hold in
all cases).

[reames]

While your reasoning in the opening comment is faulty, the actual code LGTM.

You can't reason only about what happens when GC doesn't trigger to infer a property of the potentially relocated value. You need to reason about what happens in both cases: relocation and non-relocation.

Side note (mostly for my own later reference): I could see a theoretically GC like thing which changed dereferenceability on collection, but no practical collector I know of does so. As such, I don't see a point in controlling this via a flag at this time.

lib/IR/Value.cpp

 #include "llvm/IR/CallSite.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/GetElementPtrTypeIterator.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
+#include "llvm/IR/Statepoint.h"
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/IR/ValueSymbolTable.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/ManagedStatic.h"
 #include <algorithm>
 using namespace llvm;
 
for (User::const_op_iterator I = GEP->op_begin()+1,
         return false;
       if (CI->getZExtValue() >= ATy->getNumElements())
         return false;
     }
     // Indices check out; this is dereferenceable.
     return true;
   }
 
+  // For gc.relocate, look through relocations
+  if (const IntrinsicInst *I = dyn_cast<IntrinsicInst>(V))
+    if (I->getIntrinsicID() == Intrinsic::experimental_gc_relocate) {
+      GCRelocateOperands RelocateInst(I);
+      return isDereferenceablePointer(RelocateInst.derivedPtr(), DL, Visited);
+    }
+
   if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V))
     return isDereferenceablePointer(ASC->getOperand(0), DL, Visited);
 
   // If we don't know, assume the worst.
   return false;
 }
 
 bool Value::isDereferenceablePointer(const DataLayout *DL) const {

test/Analysis/ValueTracking/memory-dereferenceable.ll

 ; RUN: opt -print-memderefs -analyze -S <%s | FileCheck %s
 
 ; Uses the print-deref (+ analyze to print) pass to run
 ; isDereferenceablePointer() on many load instruction operands
 
 declare zeroext i1 @return_i1()
 
 @globalstr = global [6 x i8] c"hello\00"
 
 define void @test(i32 addrspace(1)* byval %dparam) {
 ; CHECK: The following are dereferenceable:
 ; CHECK: %globalptr
 ; CHECK: %alloca
 ; CHECK: %dparam
-; We haven't yet taught it to look through relocations
-; CHECK-NOT: %relocate
+; CHECK: %relocate
 ; CHECK-NOT: %nparam
 entry:
     %globalptr = getelementptr inbounds [6 x i8]* @globalstr, i32 0, i32 0
     %load1 = load i8* %globalptr
     %alloca = alloca i1
     %load2 = load i1* %alloca
     %load3 = load i32 addrspace(1)* %dparam
     %tok = tail call i32 (i1 ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_i1f(i1 ()* @return_i1, i32 0, i32 0, i32 0, i32 addrspace(1)* %dparam)