[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,170 @@
+//===---- RemoveLoadsIntoFakeUses.cpp - Remove loads with no real uses ----===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// The FAKE_USE instruction is used to preserve certain values through
+/// optimizations for the sake of debugging. This may result in spilled values
+/// being loaded into registers that are only used by FAKE_USEs; this is not
+/// necessary for debugging purposes, because at that point the value must be on
+/// the stack and hence available for debugging. Therefore, this pass removes
+/// loads that are only used by FAKE_USEs.
+///
+/// This pass should run as late as possible, to ensure that we don't
+/// inadvertently shorten stack lifetimes by removing these loads, since the
+/// FAKE_USEs will also no longer be in effect.
+///
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/PostOrderIterator.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/LiveRegUnits.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/IR/Function.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "remove-loads-into-fake-uses"
+
+STATISTIC(NumLoadsDeleted, "Number of dead load instructions deleted");
+STATISTIC(NumFakeUsesDeleted, "Number of FAKE_USE instructions deleted");
+
+class RemoveLoadsIntoFakeUses : public MachineFunctionPass {
+public:
+  static char ID;
+
+  RemoveLoadsIntoFakeUses() : MachineFunctionPass(ID) {
+    initializeRemoveLoadsIntoFakeUsesPass(*PassRegistry::getPassRegistry());
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  StringRef getPassName() const override {
+    return "Remove Loads Into Fake Uses";
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+};
+
+char RemoveLoadsIntoFakeUses::ID = 0;
+char &llvm::RemoveLoadsIntoFakeUsesID = RemoveLoadsIntoFakeUses::ID;
+
+INITIALIZE_PASS_BEGIN(RemoveLoadsIntoFakeUses, DEBUG_TYPE,
+                      "Remove Loads Into Fake Uses", false, false)
+INITIALIZE_PASS_END(RemoveLoadsIntoFakeUses, DEBUG_TYPE,
+                    "Remove Loads Into Fake Uses", false, false)
+
+bool RemoveLoadsIntoFakeUses::runOnMachineFunction(MachineFunction &MF) {
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arsenm wrote:

I wonder if we can just do this in ExpandPostRAPseudos and save having yet another pass.

This would also bring the side effect where I think we should have ExpandPostRAPseudos run in reverse and lazily maintain LiveRegUnits  

https://github.com/llvm/llvm-project/pull/86149