Refactoring gc_relocate related code in CodeGenPrepare.cpp

[• chenli]

Summary

The original code insert new instructions by following a Create->Remove->ReInsert flow. This patch removes the unnecessary Remove->ReInsert part by setting up the InsertPoint correctly at the very beginning. This change does not introduce any functionality change.

[sanjoy]

LGTM under the assumption that this is NFC.

I assume it is okay if I check this in for you?

[• chenli]

Thanks for checking it in!

llvm/trunk/lib/CodeGen/CodeGenPrepare.cpp

for (auto &ToReplace : Targets) {
     if (!Derived || Derived->getPointerOperand() != Base)
       continue;
 
     SmallVector<Value *, 2> OffsetV;
     if (!getGEPSmallConstantIntOffsetV(Derived, OffsetV))
       continue;
 
     // Create a Builder and replace the target callsite with a gep
-    IRBuilder<> Builder(ToReplace);
+    assert(RelocatedBase->getNextNode() && "Should always have one since it's not a terminator");
+
+    // Insert after RelocatedBase
+    IRBuilder<> Builder(RelocatedBase->getNextNode());
     Builder.SetCurrentDebugLocation(ToReplace->getDebugLoc());
 
     // If gc_relocate does not match the actual type, cast it to the right type.
     // In theory, there must be a bitcast after gc_relocate if the type does not
     // match, and we should reuse it to get the derived pointer. But it could be
     // cases like this:
     // bb1:
     //  ...
for (auto &ToReplace : Targets) {
     //
     // In this case, we can not find the bitcast any more. So we insert a new bitcast
     // no matter there is already one or not. In this way, we can handle all cases, and
     // the extra bitcast should be optimized away in later passes.
     Instruction *ActualRelocatedBase = RelocatedBase;
     if (RelocatedBase->getType() != Base->getType()) {
       ActualRelocatedBase =
           cast<Instruction>(Builder.CreateBitCast(RelocatedBase, Base->getType()));
-      ActualRelocatedBase->removeFromParent();
-      ActualRelocatedBase->insertAfter(cast<Instruction>(RelocatedBase));
     }
     Value *Replacement = Builder.CreateGEP(
         Derived->getSourceElementType(), ActualRelocatedBase, makeArrayRef(OffsetV));
     Instruction *ReplacementInst = cast<Instruction>(Replacement);
-    ReplacementInst->removeFromParent();
-    ReplacementInst->insertAfter(ActualRelocatedBase);
     Replacement->takeName(ToReplace);
     // If the newly generated derived pointer's type does not match the original derived
     // pointer's type, cast the new derived pointer to match it. Same reasoning as above.
     Instruction *ActualReplacement = ReplacementInst;
     if (ReplacementInst->getType() != ToReplace->getType()) {
       ActualReplacement =
           cast<Instruction>(Builder.CreateBitCast(ReplacementInst, ToReplace->getType()));
-      ActualReplacement->removeFromParent();
-      ActualReplacement->insertAfter(ReplacementInst);
     }
     ToReplace->replaceAllUsesWith(ActualReplacement);
     ToReplace->eraseFromParent();
 
     MadeChange = true;
   }
   return MadeChange;
 }