[llvm] [BOLT] Support restartable sequences in tcmalloc (PR #167195)

[Maksim Panchenko via llvm-commits]

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@@ -0,0 +1,72 @@
+//===- bolt/Rewrite/RSeqRewriter.cpp --------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Basic support for restartable sequences used by tcmalloc. Prevent critical
+// section overrides by ignoring optimizations in containing functions.
+//
+// References:
+//   * https://google.github.io/tcmalloc/rseq.html
+//   * tcmalloc/internal/percpu_rseq_x86_64.S
+//
+//===----------------------------------------------------------------------===//
+
+#include "bolt/Core/BinaryFunction.h"
+#include "bolt/Rewrite/MetadataRewriter.h"
+#include "bolt/Rewrite/MetadataRewriters.h"
+#include "llvm/Support/Errc.h"
+
+using namespace llvm;
+using namespace bolt;
+
+namespace {
+
+class RSeqRewriter final : public MetadataRewriter {
+public:
+  RSeqRewriter(StringRef Name, BinaryContext &BC)
+      : MetadataRewriter(Name, BC) {}
+
+  Error preCFGInitializer() override {
+    for (const BinarySection &Section : BC.allocatableSections()) {
+      if (Section.getName() != "__rseq_cs")
+        continue;
+
+      auto handleRelocation = [&](const Relocation &Rel, bool IsDynamic) {
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maksfb wrote:

Thanks for the review.

Doesn't have to be, but I selected the minimal scope for `handleRelocation()`.

https://github.com/llvm/llvm-project/pull/167195