[ARM64][Windows] Share unwind codes between epilogues

There are cases where we have multiple epilogues that have the exact same unwind
code sequence.  In that case, the epilogues can share the same unwind codes in
the .xdata section.  This should get us past the assert "SEH unwind data
splitting not yet implemented" in many cases.

We still need to add support for generating multiple .pdata/.xdata sections for
those functions that need to be split into fragments.

Differential Revision: https://reviews.llvm.org/D56813

llvm-svn: 351421

1 files changed, 51 insertions, 2 deletions

diff --git a/llvm/lib/MC/MCWin64EH.cpp b/llvm/lib/MC/MCWin64EH.cpp
index 0724b109e1a..2e4eea3411f 100644
--- a/llvm/lib/MC/MCWin64EH.cpp
+++ b/llvm/lib/MC/MCWin64EH.cpp
@@ -453,6 +453,38 @@ static void ARM64EmitUnwindCode(MCStreamer &streamer, const MCSymbol *begin,
   }
 }
 
+// Returns the epilog symbol of an epilog with the exact same unwind code
+// sequence, if it exists.  Otherwise, returns nulltpr.
+// EpilogInstrs - Unwind codes for the current epilog.
+// Epilogs - Epilogs that potentialy match the current epilog.
+static MCSymbol*
+FindMatchingEpilog(const std::vector<WinEH::Instruction>& EpilogInstrs,
+                   const std::vector<MCSymbol *>& Epilogs,
+                   const WinEH::FrameInfo *info) {
+  for (auto *EpilogStart : Epilogs) {
+    auto InstrsIter = info->EpilogMap.find(EpilogStart);
+    assert(InstrsIter != info->EpilogMap.end() &&
+           "Epilog not found in EpilogMap");
+    const auto &Instrs = InstrsIter->second;
+
+    if (Instrs.size() != EpilogInstrs.size())
+      continue;
+
+    bool Match = true;
+    for (unsigned i = 0; i < Instrs.size(); ++i)
+      if (Instrs[i].Operation != EpilogInstrs[i].Operation ||
+          Instrs[i].Offset != EpilogInstrs[i].Offset ||
+          Instrs[i].Register != EpilogInstrs[i].Register) {
+         Match = false;
+         break;
+      }
+
+    if (Match)
+      return EpilogStart;
+  }
+  return nullptr;
+}
+
 // Populate the .xdata section.  The format of .xdata on ARM64 is documented at
 // https://docs.microsoft.com/en-us/cpp/build/arm64-exception-handling
 static void ARM64EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
@@ -477,12 +509,29 @@ static void ARM64EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
 
   // Process epilogs.
   MapVector<MCSymbol *, uint32_t> EpilogInfo;
+  // Epilogs processed so far.
+  std::vector<MCSymbol *> AddedEpilogs;
+
   for (auto &I : info->EpilogMap) {
     MCSymbol *EpilogStart = I.first;
     auto &EpilogInstrs = I.second;
     uint32_t CodeBytes = ARM64CountOfUnwindCodes(EpilogInstrs);
-    EpilogInfo[EpilogStart] = TotalCodeBytes;
-    TotalCodeBytes += CodeBytes;
+
+    uint32_t NumUnwindCodes = EpilogInstrs.size();
+    MCSymbol* MatchingEpilog =
+      FindMatchingEpilog(EpilogInstrs, AddedEpilogs, info);
+    if (MatchingEpilog) {
+      assert(EpilogInfo.find(MatchingEpilog) != EpilogInfo.end() &&
+             "Duplicate epilog not found");
+      EpilogInfo[EpilogStart] = EpilogInfo[MatchingEpilog];
+      // Clear the unwind codes in the EpilogMap, so that they don't get output
+      // in the logic below.
+      EpilogInstrs.clear();
+    } else {
+      EpilogInfo[EpilogStart] = TotalCodeBytes;
+      TotalCodeBytes += CodeBytes;
+      AddedEpilogs.push_back(EpilogStart);
+    }
   }
 
   // Code Words, Epilog count, E, X, Vers, Function Length