[CodeGen] Simplify codegen for array initialization (#93956)

This makes codegen for array initialization simpler in two ways:
1. Drop the zero-index GEP at the start, which is no longer needed with
opaque pointers.
2. Emit GEPs directly to the correct element, instead of having a long
chain of +1 GEPs. This is more canonical, and also avoids regressions in
unoptimized builds from #93823.

1 files changed, 9 insertions, 20 deletions

diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index 5b2039a..b2a5cee 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -513,15 +513,6 @@ void AggExprEmitter::EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
 
   QualType elementType =
       CGF.getContext().getAsArrayType(ArrayQTy)->getElementType();
-
-  // DestPtr is an array*.  Construct an elementType* by drilling
-  // down a level.
-  llvm::Value *zero = llvm::ConstantInt::get(CGF.SizeTy, 0);
-  llvm::Value *indices[] = { zero, zero };
-  llvm::Value *begin = Builder.CreateInBoundsGEP(DestPtr.getElementType(),
-                                                 DestPtr.emitRawPointer(CGF),
-                                                 indices, "arrayinit.begin");
-
   CharUnits elementSize = CGF.getContext().getTypeSizeInChars(elementType);
   CharUnits elementAlign =
     DestPtr.getAlignment().alignmentOfArrayElement(elementSize);
@@ -562,6 +553,7 @@ void AggExprEmitter::EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
   Address endOfInit = Address::invalid();
   CodeGenFunction::CleanupDeactivationScope deactivation(CGF);
 
+  llvm::Value *begin = DestPtr.emitRawPointer(CGF);
   if (dtorKind) {
     CodeGenFunction::AllocaTrackerRAII allocaTracker(CGF);
     // In principle we could tell the cleanup where we are more
@@ -585,19 +577,13 @@ void AggExprEmitter::EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
 
   llvm::Value *one = llvm::ConstantInt::get(CGF.SizeTy, 1);
 
-  // The 'current element to initialize'.  The invariants on this
-  // variable are complicated.  Essentially, after each iteration of
-  // the loop, it points to the last initialized element, except
-  // that it points to the beginning of the array before any
-  // elements have been initialized.
-  llvm::Value *element = begin;
-
   // Emit the explicit initializers.
   for (uint64_t i = 0; i != NumInitElements; ++i) {
-    // Advance to the next element.
+    llvm::Value *element = begin;
     if (i > 0) {
-      element = Builder.CreateInBoundsGEP(
-          llvmElementType, element, one, "arrayinit.element");
+      element = Builder.CreateInBoundsGEP(llvmElementType, begin,
+                                          llvm::ConstantInt::get(CGF.SizeTy, i),
+                                          "arrayinit.element");
 
       // Tell the cleanup that it needs to destroy up to this
       // element.  TODO: some of these stores can be trivially
@@ -624,9 +610,12 @@ void AggExprEmitter::EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
     //   do { *array++ = filler; } while (array != end);
 
     // Advance to the start of the rest of the array.
+    llvm::Value *element = begin;
     if (NumInitElements) {
       element = Builder.CreateInBoundsGEP(
-          llvmElementType, element, one, "arrayinit.start");
+          llvmElementType, element,
+          llvm::ConstantInt::get(CGF.SizeTy, NumInitElements),
+          "arrayinit.start");
       if (endOfInit.isValid()) Builder.CreateStore(element, endOfInit);
     }