Index: include/llvm/Analysis/ValueTracking.h
===================================================================
--- include/llvm/Analysis/ValueTracking.h
+++ include/llvm/Analysis/ValueTracking.h
@@ -258,6 +258,14 @@
                                     const DominatorTree *DT = nullptr,
                                     const TargetLibraryInfo *TLI = nullptr);
 
+  /// Returns true if the result or effects of the given instructions \p I
+  /// depend on the position of the instruction.
+  /// Control dependence arises for example if the the instruction reads from
+  /// memory or may produce effects or undefined behaviour which must not be
+  /// reordered. Instructions which just compute a value based on the values of
+  /// their operands are not control dependent.
+  bool mayBeControlDependent(const Instruction &I);
+
   /// isKnownNonNull - Return true if this pointer couldn't possibly be null by
   /// its definition.  This returns true for allocas, non-extern-weak globals
   /// and byval arguments.
Index: lib/Analysis/ValueTracking.cpp
===================================================================
--- lib/Analysis/ValueTracking.cpp
+++ lib/Analysis/ValueTracking.cpp
@@ -3155,6 +3155,10 @@
   }
 }
 
+bool llvm::mayBeControlDependent(const Instruction &I) {
+  return I.mayReadOrWriteMemory() || isSafeToSpeculativelyExecute(&I);
+}
+
 /// Return true if we know that the specified value is never null.
 bool llvm::isKnownNonNull(const Value *V, const TargetLibraryInfo *TLI) {
   // Alloca never returns null, malloc might.
Index: lib/Transforms/Scalar/Reassociate.cpp
===================================================================
--- lib/Transforms/Scalar/Reassociate.cpp
+++ lib/Transforms/Scalar/Reassociate.cpp
@@ -26,6 +26,7 @@
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/CFG.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -255,27 +256,6 @@
   return nullptr;
 }
 
-static bool isUnmovableInstruction(Instruction *I) {
-  switch (I->getOpcode()) {
-  case Instruction::PHI:
-  case Instruction::LandingPad:
-  case Instruction::Alloca:
-  case Instruction::Load:
-  case Instruction::Invoke:
-  case Instruction::UDiv:
-  case Instruction::SDiv:
-  case Instruction::FDiv:
-  case Instruction::URem:
-  case Instruction::SRem:
-  case Instruction::FRem:
-    return true;
-  case Instruction::Call:
-    return !isa<DbgInfoIntrinsic>(I);
-  default:
-    return false;
-  }
-}
-
 void Reassociate::BuildRankMap(Function &F) {
   unsigned i = 2;
 
@@ -295,7 +275,7 @@
     // we cannot move.  This ensures that the ranks for these instructions are
     // all different in the block.
     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
-      if (isUnmovableInstruction(I))
+      if (mayBeControlDependent(*I))
         ValueRankMap[&*I] = ++BBRank;
   }
 }
Index: test/Transforms/Reassociate/vaarg_movable.ll
===================================================================
--- /dev/null
+++ test/Transforms/Reassociate/vaarg_movable.ll
@@ -0,0 +1,28 @@
+; RUN: opt -S -reassociate -die < %s | FileCheck %s
+
+; The two va_arg instructions depend on the memory/context, are therfore not
+; identical and the sub should not be optimized to 0 by reassociate.
+;
+; CHECK-LABEL @func(
+; ...
+; CHECK: %v0 = va_arg i8** %varargs, i32
+; CHECK: %v1 = va_arg i8** %varargs, i32
+; CHECK: %v0.neg = sub i32 0, %v0
+; CHECK: %sub = add i32 %v0.neg, 1
+; CHECK: %add = add i32 %sub, %v1
+; ...
+; CHECK: ret i32 %add
+define i32 @func(i32 %dummy, ...) {
+  %varargs = alloca i8*, align 8
+  %varargs1 = bitcast i8** %varargs to i8*
+  call void @llvm.va_start(i8* %varargs1)
+  %v0 = va_arg i8** %varargs, i32
+  %v1 = va_arg i8** %varargs, i32
+  %sub = sub nsw i32 %v1, %v0
+  %add = add nsw i32 %sub, 1
+  call void @llvm.va_end(i8* %varargs1)
+  ret i32 %add
+}
+
+declare void @llvm.va_start(i8*)
+declare void @llvm.va_end(i8*)