[llvm] [AMDGPU] Add TDM Descriptor Optimization Pass (PR #173324)

[Quentin Colombet via llvm-commits]

================
@@ -0,0 +1,495 @@
+//===-- AMDGPUTDMOptimization.cpp - TDM Descriptor Optimization ----------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass optimizes Tensor Data Movement (TDM) descriptor creation patterns.
+// It identifies insertelement chains that create descriptors and transforms them
+// to use alloca+field updates, which SROA later optimizes to INSERT_SUBREG.
+//
+//===----------------------------------------------------------------------===//
+
+#include "AMDGPU.h"
+#include "AMDGPUSubtarget.h"
+#include "llvm/ADT/SmallBitVector.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Type.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "amdgpu-tdm-optimization"
+
+static cl::opt<unsigned>
+TDMOptBenefitThreshold("amdgpu-tdm-opt-threshold", cl::Hidden, cl::init(10),
+                       cl::desc("Minimum optimization benefit threshold for TDM descriptor optimization"));
+
+namespace llvm {
+  void initializeAMDGPUTDMOptimizationPass(PassRegistry &);
+}
+
+namespace {
+
+//===----------------------------------------------------------------------===//
+// Pattern Detection Data Structures
+//===----------------------------------------------------------------------===//
+
+/// Represents a single descriptor creation pattern
+struct DescriptorPattern {
+  Type *DescType;                           ///< <4 x i32> or <8 x i32>
+  Value *BaseValue;                         ///< Base template (constant or computed)
+  SmallVector<InsertElementInst *, 8> Chain; ///< Chain of insertelement instructions
+  SmallVector<unsigned, 8> VariableFields;  ///< Fields that change
+  SmallVector<unsigned, 8> ConstantFields;  ///< Fields that stay constant
+  BasicBlock *Location;                      ///< Where the pattern is located
+  Loop *ContainingLoop;                      ///< Loop containing this pattern (if any)
+
+  /// Calculate field reuse ratio (constant fields / total fields)
+  float getFieldReuseRatio() const {
+    unsigned totalFields = cast<FixedVectorType>(DescType)->getNumElements();
+    return (float)ConstantFields.size() / totalFields;
+  }
+
+  /// Check if this pattern is worth optimizing
+  bool isWorthOptimizing() const {
+    // Always optimize if in loop with reuse potential
+    if (ContainingLoop && getFieldReuseRatio() >= 0.5f)
+      return true;
+
+    // Optimize if significant field reuse
+    if (getFieldReuseRatio() >= 0.75f)
+      return true;
+
+    // Optimize address descriptors (common case)
+    if (isAddressDescriptor() && ConstantFields.size() >= 1)
----------------
qcolombet wrote:

```suggestion
    if (isAddressDescriptor() && !ConstantFields.empty())
```

https://github.com/llvm/llvm-project/pull/173324