From c0441c29df6498257eca59d41d9ccaf6d5a9adf3 Mon Sep 17 00:00:00 2001
From: Sanjoy Das <sanjoy@playingwithpointers.com>
Date: Tue, 19 Apr 2016 05:24:47 +0000
Subject: Introduce a "patchable-function" function attribute

Summary:
The `"patchable-function"` attribute can be used by an LLVM client to
influence LLVM's code generation in ways that makes the generated code
easily patchable at runtime (for instance, to redirect control).
Right now only one patchability scheme is supported,
`"prologue-short-redirect"`, but this can be expanded in the future.

Reviewers: joker.eph, rnk, echristo, dberris

Subscribers: joker.eph, echristo, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19046

llvm-svn: 266715
---
 llvm/lib/CodeGen/PatchableFunction.cpp | 70 ++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 llvm/lib/CodeGen/PatchableFunction.cpp

(limited to 'llvm/lib/CodeGen/PatchableFunction.cpp')

diff --git a/llvm/lib/CodeGen/PatchableFunction.cpp b/llvm/lib/CodeGen/PatchableFunction.cpp
new file mode 100644
index 00000000000..ca345dfaf2b
--- /dev/null
+++ b/llvm/lib/CodeGen/PatchableFunction.cpp
@@ -0,0 +1,70 @@
+//===-- PatchableFunction.cpp - Patchable prologues for LLVM -------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements edits function bodies in place to support the
+// "patchable-function" attribute.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/Analysis.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/Target/TargetFrameLowering.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetSubtargetInfo.h"
+
+using namespace llvm;
+
+namespace {
+struct PatchableFunction : public MachineFunctionPass {
+  static char ID; // Pass identification, replacement for typeid
+  PatchableFunction() : MachineFunctionPass(ID) {
+    initializePatchableFunctionPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnMachineFunction(MachineFunction &F) override;
+   MachineFunctionProperties getRequiredProperties() const override {
+    return MachineFunctionProperties().set(
+        MachineFunctionProperties::Property::AllVRegsAllocated);
+  }
+};
+}
+
+bool PatchableFunction::runOnMachineFunction(MachineFunction &MF) {
+  if (!MF.getFunction()->hasFnAttribute("patchable-function"))
+    return false;
+
+#ifndef NDEBUG
+  Attribute PatchAttr = MF.getFunction()->getFnAttribute("patchable-function");
+  StringRef PatchType = PatchAttr.getValueAsString();
+  assert(PatchType == "prologue-short-redirect" && "Only possibility today!");
+#endif
+
+  auto &FirstMBB = *MF.begin();
+  auto &FirstMI = *FirstMBB.begin();
+
+  auto *TII = MF.getSubtarget().getInstrInfo();
+  auto MIB = BuildMI(FirstMBB, FirstMBB.begin(), FirstMI.getDebugLoc(),
+                     TII->get(TargetOpcode::PATCHABLE_OP))
+                 .addImm(2)
+                 .addImm(FirstMI.getOpcode());
+
+  for (auto &MO : FirstMI.operands())
+    MIB.addOperand(MO);
+
+  FirstMI.eraseFromParent();
+  MF.ensureAlignment(4);
+  return true;
+}
+
+char PatchableFunction::ID = 0;
+char &llvm::PatchableFunctionID = PatchableFunction::ID;
+INITIALIZE_PASS(PatchableFunction, "patchable-function", "", false, false)
-- 
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