[CIR][LowerToLLVM] Fold base address computation if offset is zero

Author: bcardosolopes

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -837,6 +837,12 @@ class CIRBaseClassAddrOpLowering
         adaptor.getOffset().getZExtValue()};
     mlir::Type byteType = mlir::IntegerType::get(resultType.getContext(), 8,
                                                  mlir::IntegerType::Signless);
+    if (adaptor.getOffset().getZExtValue() == 0) {
+      rewriter.replaceOpWithNewOp<mlir::LLVM::BitcastOp>(
+          baseClassOp, resultType, adaptor.getDerivedAddr());
+      return mlir::success();
+    }
+
     if (baseClassOp.getAssumeNotNull()) {
       rewriter.replaceOpWithNewOp<mlir::LLVM::GEPOp>(
           baseClassOp, resultType, byteType, derivedAddr, offset);

clang/test/CIR/CodeGen/derived-cast.cpp

@@ -36,6 +36,5 @@ A *B::getAsA() {
 // LLVM:  store ptr %[[VAL_2:.*]], ptr %[[VAL_0:.*]], align 8
 // LLVM:  %[[VAL_3:.*]] = load ptr, ptr %[[VAL_0]], align 8
 // LLVM:  %[[VAL_4:.*]] = getelementptr i8, ptr %[[VAL_3]], i32 -4
-// LLVM:  %[[VAL_5:.*]] = icmp eq ptr %[[VAL_4]], null,
-// LLVM:  %[[VAL_6:.*]] = getelementptr i8, ptr %[[VAL_4]], i32 0,
-// LLVM:  %[[VAL_7:.*]] = select i1 %[[VAL_5]], ptr %[[VAL_4]], ptr %[[VAL_6]],
+// LLVM-NOT: select i1
+// LLVM:  ret ptr

clang/test/CIR/CodeGen/vtt.cpp

@@ -143,8 +143,7 @@ int f() {
 // LLVM:   %[[THIS:.*]] = load ptr, ptr %2, align 8
 // LLVM:   %[[BASE_A:.*]] = getelementptr i8, ptr %[[THIS]], i32 40
 // LLVM:   call void @_ZN1AC2Ev(ptr %[[BASE_A]])
-// LLVM:   %[[BASE_B:.*]] = getelementptr i8, ptr %[[THIS]], i32 0
-// LLVM:   call void @_ZN1BC2Ev(ptr %[[BASE_B]], ptr getelementptr inbounds ([7 x ptr], ptr @_ZTT1D, i32 0, i32 1))
+// LLVM:   call void @_ZN1BC2Ev(ptr %[[THIS]], ptr getelementptr inbounds ([7 x ptr], ptr @_ZTT1D, i32 0, i32 1))
 // LLVM:   %[[BASE_C:.*]] = getelementptr i8, ptr %[[THIS]], i32 16
 // LLVM:   call void @_ZN1CC2Ev(ptr %[[BASE_C]], ptr getelementptr inbounds ([7 x ptr], ptr @_ZTT1D, i32 0, i32 3))
 // LLVM:   ret void

clang/test/CIR/Lowering/derived-to-base.cpp

@@ -8,21 +8,20 @@ void test_multi_base() {
   Derived d;
 
   Base2& bref = d; // no null check needed
-  // LLVM: %7 = getelementptr i8, ptr %1, i32 4
+  // LLVM: getelementptr i8, ptr %[[D:.*]], i32 4
 
   Base2* bptr = &d; // has null pointer check
-  // LLVM: %8 = icmp eq ptr %1, null
-  // LLVM: %9 = getelementptr i8, ptr %1, i32 4
-  // LLVM: %10 = select i1 %8, ptr %1, ptr %9
+  // LLVM: %[[CHECK:.*]] = icmp eq ptr %[[D]], null
+  // LLVM: %[[BPTR:.*]] = getelementptr i8, ptr %[[D]], i32 4
+  // LLVM: select i1 %[[CHECK]], ptr %[[D]], ptr %[[BPTR]]
 
   int a = d.a;
-  // LLVM: %11 = getelementptr i8, ptr %1, i32 0
-  // LLVM: %12 = getelementptr %struct.Base1, ptr %11, i32 0, i32 0
+  // LLVM: getelementptr %struct.Base1, ptr %[[D]], i32 0, i32 0
 
   int b = d.b;
-  // LLVM: %14 = getelementptr i8, ptr %1, i32 4
-  // LLVM: %15 = getelementptr %struct.Base2, ptr %14, i32 0, i32 0
+  // LLVM: %[[BASE2_OFFSET:.*]] = getelementptr i8, ptr %[[D]], i32 4
+  // LLVM: %[[BASE2:.*]] = getelementptr %struct.Base2, ptr %[[BASE2_OFFSET]], i32 0, i32 0
 
   int c = d.c;
-  // LLVM: %17 = getelementptr %struct.Derived, ptr %1, i32 0, i32 2
+  // LLVM: getelementptr %struct.Derived, ptr %[[D]], i32 0, i32 2
 }