+// Lower memmove to IR. memmove is required to correctly copy overlapping memory
+// regions; therefore, it has to check the relative positions of the source and
+// destination pointers and choose the copy direction accordingly.
+//
+// The code below is an IR rendition of this C function:
+//
+// void* memmove(void* dst, const void* src, size_t n) {
+// unsigned char* d = dst;
+// const unsigned char* s = src;
+// if (s < d) {
+// // copy backwards
+// while (n--) {
+// d[n] = s[n];
+// }
+// } else {
+// // copy forward
+// for (size_t i = 0; i < n; ++i) {
+// d[i] = s[i];
+// }
+// }
+// return dst;
+// }
+void convertMemMoveToLoop(Instruction *ConvertedInst, Value *SrcAddr,
+ Value *DstAddr, Value *CopyLen, bool SrcIsVolatile,
+ bool DstIsVolatile, LLVMContext &Context,
+ Function &F) {
+ Type *TypeOfCopyLen = CopyLen->getType();
+ BasicBlock *OrigBB = ConvertedInst->getParent();
+
+ // Create the a comparison of src and dst, based on which we jump to either
+ // the forward-copy part of the function (if src >= dst) or the backwards-copy
+ // part (if src < dst).
+ // SplitBlockAndInsertIfThenElse conveniently creates the basic if-then-else
+ // structure. Its block terminators (unconditional branches) are replaced by
+ // the appropriate conditional branches when the loop is built.
+ ICmpInst *PtrCompare = new ICmpInst(ConvertedInst, ICmpInst::ICMP_ULT,
+ SrcAddr, DstAddr, "compare_src_dst");
+ TerminatorInst *ThenTerm, *ElseTerm;
+ SplitBlockAndInsertIfThenElse(PtrCompare, ConvertedInst, &ThenTerm,
+ &ElseTerm);
+
+ // Each part of the function consists of two blocks:
+ // copy_backwards: used to skip the loop when n == 0
+ // copy_backwards_loop: the actual backwards loop BB
+ // copy_forward: used to skip the loop when n == 0
+ // copy_forward_loop: the actual forward loop BB
+ BasicBlock *CopyBackwardsBB = ThenTerm->getParent();
+ CopyBackwardsBB->setName("copy_backwards");
+ BasicBlock *CopyForwardBB = ElseTerm->getParent();
+ CopyForwardBB->setName("copy_forward");
+ BasicBlock *ExitBB = ConvertedInst->getParent();
+ ExitBB->setName("memmove_done");
+
+ // Initial comparison of n == 0 that lets us skip the loops altogether. Shared
+ // between both backwards and forward copy clauses.
+ ICmpInst *CompareN =
+ new ICmpInst(OrigBB->getTerminator(), ICmpInst::ICMP_EQ, CopyLen,
+ ConstantInt::get(TypeOfCopyLen, 0), "compare_n_to_0");
+
+ // Copying backwards.
+ BasicBlock *LoopBB =
+ BasicBlock::Create(Context, "copy_backwards_loop", &F, CopyForwardBB);
+ IRBuilder<> LoopBuilder(LoopBB);
+ PHINode *LoopPhi = LoopBuilder.CreatePHI(TypeOfCopyLen, 0);
+ Value *IndexPtr = LoopBuilder.CreateSub(
+ LoopPhi, ConstantInt::get(TypeOfCopyLen, 1), "index_ptr");
+ Value *Element = LoopBuilder.CreateLoad(
+ LoopBuilder.CreateInBoundsGEP(SrcAddr, IndexPtr), "element");
+ LoopBuilder.CreateStore(Element,
+ LoopBuilder.CreateInBoundsGEP(DstAddr, IndexPtr));
+ LoopBuilder.CreateCondBr(
+ LoopBuilder.CreateICmpEQ(IndexPtr, ConstantInt::get(TypeOfCopyLen, 0)),
+ ExitBB, LoopBB);
+ LoopPhi->addIncoming(IndexPtr, LoopBB);
+ LoopPhi->addIncoming(CopyLen, CopyBackwardsBB);
+ BranchInst::Create(ExitBB, LoopBB, CompareN, ThenTerm);
+ ThenTerm->eraseFromParent();
+
+ // Copying forward.
+ BasicBlock *FwdLoopBB =
+ BasicBlock::Create(Context, "copy_forward_loop", &F, ExitBB);
+ IRBuilder<> FwdLoopBuilder(FwdLoopBB);
+ PHINode *FwdCopyPhi = FwdLoopBuilder.CreatePHI(TypeOfCopyLen, 0, "index_ptr");
+ Value *FwdElement = FwdLoopBuilder.CreateLoad(
+ FwdLoopBuilder.CreateInBoundsGEP(SrcAddr, FwdCopyPhi), "element");
+ FwdLoopBuilder.CreateStore(
+ FwdElement, FwdLoopBuilder.CreateInBoundsGEP(DstAddr, FwdCopyPhi));
+ Value *FwdIndexPtr = FwdLoopBuilder.CreateAdd(
+ FwdCopyPhi, ConstantInt::get(TypeOfCopyLen, 1), "index_increment");
+ FwdLoopBuilder.CreateCondBr(FwdLoopBuilder.CreateICmpEQ(FwdIndexPtr, CopyLen),
+ ExitBB, FwdLoopBB);
+ FwdCopyPhi->addIncoming(FwdIndexPtr, FwdLoopBB);
+ FwdCopyPhi->addIncoming(ConstantInt::get(TypeOfCopyLen, 0), CopyForwardBB);
+
+ BranchInst::Create(ExitBB, FwdLoopBB, CompareN, ElseTerm);
+ ElseTerm->eraseFromParent();
+}