[HLSL] Implicit resource binding for cbuffers (#139022)

Constant buffers defined with the `cbuffer` keyword do not have a
constructor. Instead, the call to initialize the resource handle based
on its binding is generated in codegen. This change adds initialization
of `cbuffer` handles that have implicit binding.

Closes  #139617

1 files changed, 29 insertions, 21 deletions

diff --git a/clang/lib/CodeGen/CGHLSLRuntime.cpp b/clang/lib/CodeGen/CGHLSLRuntime.cpp
index 5bc71a9..a708b3a 100644
--- a/clang/lib/CodeGen/CGHLSLRuntime.cpp
+++ b/clang/lib/CodeGen/CGHLSLRuntime.cpp
@@ -23,6 +23,7 @@
 #include "clang/AST/Type.h"
 #include "clang/Basic/TargetOptions.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/LLVMContext.h"
@@ -42,8 +43,8 @@ using namespace llvm;
 using llvm::hlsl::CBufferRowSizeInBytes;
 
 static void initializeBufferFromBinding(CodeGenModule &CGM,
-                                        llvm::GlobalVariable *GV, unsigned Slot,
-                                        unsigned Space);
+                                        llvm::GlobalVariable *GV,
+                                        HLSLResourceBindingAttr *RBA);
 
 namespace {
 
@@ -271,13 +272,10 @@ void CGHLSLRuntime::addBuffer(const HLSLBufferDecl *BufDecl) {
   emitBufferGlobalsAndMetadata(BufDecl, BufGV);
 
   // Initialize cbuffer from binding (implicit or explicit)
-  const HLSLResourceBindingAttr *RBA =
-      BufDecl->getAttr<HLSLResourceBindingAttr>();
-  // FIXME: handle implicit binding if no binding attribute is found
-  // (llvm/llvm-project#110722)
-  if (RBA && RBA->hasRegisterSlot())
-    initializeBufferFromBinding(CGM, BufGV, RBA->getSlotNumber(),
-                                RBA->getSpaceNumber());
+  HLSLResourceBindingAttr *RBA = BufDecl->getAttr<HLSLResourceBindingAttr>();
+  assert(RBA &&
+         "cbuffer/tbuffer should always have resource binding attribute");
+  initializeBufferFromBinding(CGM, BufGV, RBA);
 }
 
 llvm::TargetExtType *
@@ -560,19 +558,29 @@ static void initializeBuffer(CodeGenModule &CGM, llvm::GlobalVariable *GV,
 }
 
 static void initializeBufferFromBinding(CodeGenModule &CGM,
-                                        llvm::GlobalVariable *GV, unsigned Slot,
-                                        unsigned Space) {
+                                        llvm::GlobalVariable *GV,
+                                        HLSLResourceBindingAttr *RBA) {
   llvm::Type *Int1Ty = llvm::Type::getInt1Ty(CGM.getLLVMContext());
-  llvm::Value *Args[] = {
-      llvm::ConstantInt::get(CGM.IntTy, Space), /* reg_space */
-      llvm::ConstantInt::get(CGM.IntTy, Slot),  /* lower_bound */
-      llvm::ConstantInt::get(CGM.IntTy, 1),     /* range_size */
-      llvm::ConstantInt::get(CGM.IntTy, 0),     /* index */
-      llvm::ConstantInt::get(Int1Ty, false)     /* non-uniform */
-  };
-  initializeBuffer(CGM, GV,
-                   CGM.getHLSLRuntime().getCreateHandleFromBindingIntrinsic(),
-                   Args);
+  auto *NonUniform = llvm::ConstantInt::get(Int1Ty, false);
+  auto *Index = llvm::ConstantInt::get(CGM.IntTy, 0);
+  auto *RangeSize = llvm::ConstantInt::get(CGM.IntTy, 1);
+  auto *Space =
+      llvm::ConstantInt::get(CGM.IntTy, RBA ? RBA->getSpaceNumber() : 0);
+
+  if (RBA->hasRegisterSlot()) {
+    auto *RegSlot = llvm::ConstantInt::get(CGM.IntTy, RBA->getSlotNumber());
+    Intrinsic::ID Intr =
+        CGM.getHLSLRuntime().getCreateHandleFromBindingIntrinsic();
+    initializeBuffer(CGM, GV, Intr,
+                     {Space, RegSlot, RangeSize, Index, NonUniform});
+  } else {
+    auto *OrderID =
+        llvm::ConstantInt::get(CGM.IntTy, RBA->getImplicitBindingOrderID());
+    Intrinsic::ID Intr =
+        CGM.getHLSLRuntime().getCreateHandleFromImplicitBindingIntrinsic();
+    initializeBuffer(CGM, GV, Intr,
+                     {OrderID, Space, RangeSize, Index, NonUniform});
+  }
 }
 
 llvm::Instruction *CGHLSLRuntime::getConvergenceToken(BasicBlock &BB) {