+/// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
+/// memory region into an identical pointer) then it doesn't actually make its
+/// input dead in the traditional sense. Consider this case:
+///
+/// memcpy(A <- B)
+/// memcpy(A <- A)
+///
+/// In this case, the second store to A does not make the first store to A dead.
+/// The usual situation isn't an explicit A<-A store like this (which can be
+/// trivially removed) but a case where two pointers may alias.
+///
+/// This function detects when it is unsafe to remove a dependent instruction
+/// because the DSE inducing instruction may be a self-read.
+static bool isPossibleSelfRead(Instruction *Inst,
+ const AliasAnalysis::Location &InstStoreLoc,
+ Instruction *DepWrite, AliasAnalysis &AA) {
+ // Self reads can only happen for instructions that read memory. Get the
+ // location read.
+ AliasAnalysis::Location InstReadLoc = getLocForRead(Inst, AA);
+ if (InstReadLoc.Ptr == 0) return false; // Not a reading instruction.
+
+ // If the read and written loc obviously don't alias, it isn't a read.
+ if (AA.isNoAlias(InstReadLoc, InstStoreLoc)) return false;
+
+ // Okay, 'Inst' may copy over itself. However, we can still remove a the
+ // DepWrite instruction if we can prove that it reads from the same location
+ // as Inst. This handles useful cases like:
+ // memcpy(A <- B)
+ // memcpy(A <- B)
+ // Here we don't know if A/B may alias, but we do know that B/B are must
+ // aliases, so removing the first memcpy is safe (assuming it writes <= #
+ // bytes as the second one.
+ AliasAnalysis::Location DepReadLoc = getLocForRead(DepWrite, AA);
+
+ if (DepReadLoc.Ptr && AA.isMustAlias(InstReadLoc.Ptr, DepReadLoc.Ptr))
+ return false;
+
+ // If DepWrite doesn't read memory or if we can't prove it is a must alias,
+ // then it can't be considered dead.
+ return true;
+}
+