[clang] [llvm] [opt][AMDGPU] Add pass to handle AMDGCN pseudo-intrinsics target specific info), start with `llvm.amdgcn.wavefrontsize` (PR #114481)

[Matt Arsenault via cfe-commits]

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@@ -0,0 +1,49 @@
+//===- AMDGPUExpandPseudoIntrinsics.cpp - Pseudo Intrinsic Expander Pass --===//
+//
+// 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 file implements a pass that deals with expanding AMDGCN generic pseudo-
+// intrinsics into target specific quantities / sequences. In this context, a
+// pseudo-intrinsic is an AMDGCN intrinsic that does not directly map to a
+// specific instruction, but rather is intended as a mechanism for abstractly
+// conveying target specific info to a HLL / the FE, without concretely
+// impacting the AST. An example of such an intrinsic is amdgcn.wavefrontsize.
+// This pass should run as early as possible / immediately after Clang CodeGen,
+// so that the optimisation pipeline and the BE operate with concrete target
+// data.
+//===----------------------------------------------------------------------===//
+
+#include "AMDGPU.h"
+#include "AMDGPUTargetMachine.h"
+#include "GCNSubtarget.h"
+
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+
+using namespace llvm;
+
+static inline PreservedAnalyses expandWaveSizeIntrinsic(const GCNSubtarget &ST,
+                                                        Function *WaveSize) {
+  if (WaveSize->hasZeroLiveUses())
+    return PreservedAnalyses::all();
+
+  for (auto &&U : WaveSize->users())
+    U->replaceAllUsesWith(ConstantInt::get(WaveSize->getReturnType(),
+                                           ST.getWavefrontSize()));
+
+  return PreservedAnalyses::none();
+}
+
+PreservedAnalyses
+  AMDGPUExpandPseudoIntrinsicsPass::run(Module &M, ModuleAnalysisManager &) {
+
+  if (auto WS = M.getFunction("llvm.amdgcn.wavefrontsize"))
+    return expandWaveSizeIntrinsic(TM.getSubtarget<GCNSubtarget>(*WS), WS);
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arsenm wrote:

Subtarget is per function, cannot rely on getting the subtarget from an intrinsic declaration 

https://github.com/llvm/llvm-project/pull/114481