[Bolt] Teach bolt about no-return functions #115616

elhewaty · 2024-11-09T18:45:55Z

First step to fix #115154

llvmbot · 2024-11-09T18:46:27Z

@llvm/pr-subscribers-bolt

Author: None (elhewaty)

Changes

Full diff: https://github.com/llvm/llvm-project/pull/115616.diff

6 Files Affected:

(modified) bolt/include/bolt/Core/BinaryContext.h (+14)
(added) bolt/include/bolt/Passes/DiscoverNoReturnPass.h (+36)
(modified) bolt/lib/Core/BinaryContext.cpp (+14)
(modified) bolt/lib/Passes/CMakeLists.txt (+1)
(added) bolt/lib/Passes/DiscoverNoReturnPass.cpp (+84)
(modified) bolt/lib/Rewrite/BinaryPassManager.cpp (+2)

diff --git a/bolt/include/bolt/Core/BinaryContext.h b/bolt/include/bolt/Core/BinaryContext.h
index 08ce892054874c..749a82f0f453b0 100644
--- a/bolt/include/bolt/Core/BinaryContext.h
+++ b/bolt/include/bolt/Core/BinaryContext.h
@@ -272,6 +272,10 @@ class BinaryContext {
   /// DWARF line info for CUs.
   std::map<unsigned, DwarfLineTable> DwarfLineTablesCUMap;
 
+  /// Container to cache results about functions that their paths lead to
+  /// a no-return function.
+  std::unordered_map<BinaryFunction *, bool> CallsNoReturnFunction;
+
   /// Internal helper for removing section name from a lookup table.
   void deregisterSectionName(const BinarySection &Section);
 
@@ -282,6 +286,16 @@ class BinaryContext {
                       std::unique_ptr<DWARFContext> DwCtx,
                       JournalingStreams Logger);
 
+  void setHasPathToNoReturn(BinaryFunction *Func, bool value = true) {
+    CallsNoReturnFunction[Func] = value;
+  }
+
+  bool cachedInNoReturnMap(BinaryFunction *Func) {
+    return CallsNoReturnFunction.find(Func) != CallsNoReturnFunction.end();
+  }
+
+  bool hasPathToNoReturn(BinaryFunction *Func);
+
   /// Superset of compiler units that will contain overwritten code that needs
   /// new debug info. In a few cases, functions may end up not being
   /// overwritten, but it is okay to re-generate debug info for them.
diff --git a/bolt/include/bolt/Passes/DiscoverNoReturnPass.h b/bolt/include/bolt/Passes/DiscoverNoReturnPass.h
new file mode 100644
index 00000000000000..3a34e1344f3bb4
--- /dev/null
+++ b/bolt/include/bolt/Passes/DiscoverNoReturnPass.h
@@ -0,0 +1,36 @@
+//===- bolt/Passes/Discover.h -----------------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef BOLT_PASSES_DISCOVERNORETURN_H
+#define BOLT_PASSES_DISCOVERNORETURN_H
+
+#include "bolt/Core/BinaryContext.h"
+#include "bolt/Core/BinaryFunction.h"
+#include "bolt/Passes/BinaryPasses.h"
+#include <unordered_map>
+
+namespace llvm {
+
+namespace bolt {
+
+class DiscoverNoReturnPass : public BinaryFunctionPass {
+public:
+  explicit DiscoverNoReturnPass() : BinaryFunctionPass(false) {}
+
+  const char *getName() const override { return "discover-no-return"; }
+
+  Error runOnFunctions(BinaryContext &BC) override;
+
+private:
+  std::unordered_map<BinaryFunction *, bool> Visited;
+  bool traverseFromFunction(BinaryFunction *Func, BinaryContext &BC);
+};
+} // namespace bolt
+} // namespace llvm
+
+#endif
\ No newline at end of file
diff --git a/bolt/lib/Core/BinaryContext.cpp b/bolt/lib/Core/BinaryContext.cpp
index f246750209d6c4..c8eee1786b794a 100644
--- a/bolt/lib/Core/BinaryContext.cpp
+++ b/bolt/lib/Core/BinaryContext.cpp
@@ -308,6 +308,20 @@ Expected<std::unique_ptr<BinaryContext>> BinaryContext::createBinaryContext(
   return std::move(BC);
 }
 
+bool BinaryContext::hasPathToNoReturn(BinaryFunction *Func) {
+  // Dummy way to mark no-return functions.
+  // FIXME: Find a better way.
+  if (std::string FuncName = Func->getPrintName();
+      FuncName == "__cxa_throw@PLT" || FuncName != "_Unwind_Resume@PLT" ||
+      FuncName == "__cxa_rethrow@PLT" || FuncName != "exit@PLT" ||
+      FuncName == "abort@PLT" || FuncName == "setjmp@PLT" ||
+      FuncName == "longjmp@PLT")
+    return true;
+
+  auto itr = CallsNoReturnFunction.find(Func);
+  return itr != CallsNoReturnFunction.end() && itr->second;
+}
+
 bool BinaryContext::forceSymbolRelocations(StringRef SymbolName) const {
   if (opts::HotText &&
       (SymbolName == "__hot_start" || SymbolName == "__hot_end"))
diff --git a/bolt/lib/Passes/CMakeLists.txt b/bolt/lib/Passes/CMakeLists.txt
index 1c1273b3d2420d..7c950ec1decf59 100644
--- a/bolt/lib/Passes/CMakeLists.txt
+++ b/bolt/lib/Passes/CMakeLists.txt
@@ -8,6 +8,7 @@ add_llvm_library(LLVMBOLTPasses
   CacheMetrics.cpp
   DataflowAnalysis.cpp
   DataflowInfoManager.cpp
+  DiscoverNoReturnPass.cpp
   FrameAnalysis.cpp
   FrameOptimizer.cpp
   FixRelaxationPass.cpp
diff --git a/bolt/lib/Passes/DiscoverNoReturnPass.cpp b/bolt/lib/Passes/DiscoverNoReturnPass.cpp
new file mode 100644
index 00000000000000..5b64dc1da2f01f
--- /dev/null
+++ b/bolt/lib/Passes/DiscoverNoReturnPass.cpp
@@ -0,0 +1,84 @@
+//===- bolt/Passes/ReorderSection.cpp - Reordering of section data --------===//
+//
+// 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 DiscoverNoReturnPass class.
+//
+//===----------------------------------------------------------------------===//
+
+#include "bolt/Passes/DiscoverNoReturnPass.h"
+
+using namespace llvm;
+
+namespace opts {
+extern cl::OptionCategory BoltCategory;
+
+static cl::opt<bool> DiscoverNoReturnAnalysis(
+    "discover-no-return",
+    cl::desc("analyze the binary and mark no-return functions"), cl::init(true),
+    cl::cat(BoltCategory), cl::ReallyHidden);
+} // namespace opts
+
+namespace llvm {
+namespace bolt {
+
+Error DiscoverNoReturnPass::runOnFunctions(BinaryContext &BC) {
+  bool Changed;
+  do {
+    Changed = false;
+    for (auto &BFI : BC.getBinaryFunctions()) {
+      auto &Func = BFI.second;
+      bool PrevStat = BC.hasPathToNoReturn(&Func);
+      bool CurStat = traverseFromFunction(&Func, BC);
+      Changed |= (PrevStat != CurStat);
+    }
+  } while (Changed);
+
+  return Error::success();
+}
+
+bool DiscoverNoReturnPass::traverseFromFunction(BinaryFunction *Func,
+                                                BinaryContext &BC) {
+  // The Function cached before, so return its value
+  if (BC.cachedInNoReturnMap(Func))
+    return BC.hasPathToNoReturn(Func);
+
+  Visited[Func] = true;
+  bool Result = true;
+  bool hasCalls = 0;
+  bool hasReturns = 0;
+  for (auto &BB : *Func) {
+    if (!BB.getNumCalls())
+      continue;
+    for (auto &Inst : BB) {
+      if (BC.MIB->isCall(Inst)) {
+        hasCalls = true;
+        const MCSymbol *TargetSymbol = BC.MIB->getTargetSymbol(Inst);
+        BinaryFunction *TargetFunction = BC.SymbolToFunctionMap[TargetSymbol];
+        if (!Visited.count(TargetFunction))
+          Result &= traverseFromFunction(TargetFunction, BC);
+      }
+      hasReturns |= BC.MIB->isReturn(Inst);
+    }
+  }
+
+  // This functions is represented as a leaf in the call graph and doesn't
+  // have a no-return attribute.
+  if (!hasCalls && hasReturns)
+    Result = false;
+
+  // If the function doens't have a return instruction then it's a
+  // no-return function.
+  if (!hasReturns)
+    Result = true;
+
+  BC.setHasPathToNoReturn(Func, Result);
+  return Result;
+}
+
+} // end namespace bolt
+} // end namespace llvm
diff --git a/bolt/lib/Rewrite/BinaryPassManager.cpp b/bolt/lib/Rewrite/BinaryPassManager.cpp
index b0906041833484..f705e925dd9be9 100644
--- a/bolt/lib/Rewrite/BinaryPassManager.cpp
+++ b/bolt/lib/Rewrite/BinaryPassManager.cpp
@@ -13,6 +13,7 @@
 #include "bolt/Passes/AsmDump.h"
 #include "bolt/Passes/CMOVConversion.h"
 #include "bolt/Passes/ContinuityStats.h"
+#include "bolt/Passes/DiscoverNoReturnPass.h"
 #include "bolt/Passes/FixRISCVCallsPass.h"
 #include "bolt/Passes/FixRelaxationPass.h"
 #include "bolt/Passes/FrameOptimizer.h"
@@ -443,6 +444,7 @@ Error BinaryFunctionPassManager::runAllPasses(BinaryContext &BC) {
 
   Manager.registerPass(std::make_unique<CMOVConversion>(),
                        opts::CMOVConversionFlag);
+  Manager.registerPass(std::make_unique<DiscoverNoReturnPass>());
 
   // This pass syncs local branches with CFG. If any of the following
   // passes breaks the sync - they either need to re-run the pass or

elhewaty · 2024-11-09T19:01:52Z

@bgergely0 Can you share your thoughts.

bgergely0 · 2024-11-11T15:07:21Z

I think currently this marks all functions noreturn. Can you please share an example code, and which functions this marks as noreturn and which it does not? (Or we can test using the example I had in the original issue)

elhewaty · 2024-11-11T15:11:57Z

@bgergely0, I am not familiar with bolt testing flow, I am looking at it. But why do you think it will mark them all?

bgergely0 · 2024-11-11T15:26:50Z

@elhewaty I am not referring to the unittests or similar. I mean you should check the example code in from the issue, compile it, and run BOLT on it, and after the Pass is done, check which functions it marked as noreturn.

You can run an instrumentation, and BOLT will run this Pass during it.

elhewaty · 2024-11-11T22:03:36Z

@bgergely0, I ran the pass, and it marks all of them as no-return, as you said. I modified the pass a little, but why is the size of the my_error function in the issue zero? I think LLVM optimized it away. Does it work as intended in X86(which I am using)?

It marks abort as a no-return but not my_error as it doesn't see anything in it to traverse. any hint?

bgergely0 · 2024-11-12T08:45:24Z

@elhewaty Hi! I would check with objdump, to see if my_error is indeed in the binary you built. But if BOLT finds it, I suspect it is in there.

What I did to test the pass was commenting out most other passes in the BinaryPassManager, to only have the effect of this pass. I could not comment out all (BOLT stops working if we remove too many Passes), so I kept these:

FinalizeFunctions
AssignSections
PatchEntries
InstructionLowering
CheckLargeFunctions
LowerAnnotations
CleanMCState

I will check with the x86 target as well, but I think it could be another BOLT pass inlining something.

bgergely0 · 2024-11-11T14:26:37Z

bolt/lib/Passes/DiscoverNoReturnPass.cpp

+        const MCSymbol *TargetSymbol = BC.MIB->getTargetSymbol(Inst);
+        BinaryFunction *TargetFunction = BC.SymbolToFunctionMap[TargetSymbol];
+        if (!Visited.count(TargetFunction))


TargetFuntion needs to be checked for nullptr

bgergely0 · 2024-11-11T15:09:10Z

bolt/lib/Passes/DiscoverNoReturnPass.cpp

+  if (!hasCalls && hasReturns)
+    Result = false;
+
+  // If the function doens't have a return instruction then it's a
+  // no-return function.
+  if (!hasReturns)
+    Result = true;


Tailcalls should also be taken into account (BC.MIB->isTailCall(Inst)). If a function ends in a tailcall instruction, it depends on the Called function if it is NoReturn.

bgergely0 · 2024-11-12T14:43:05Z

bolt/lib/Core/BinaryContext.cpp

+  if (std::string FuncName = Func->getPrintName();
+      FuncName == "__cxa_throw@PLT" || FuncName != "_Unwind_Resume@PLT" ||
+      FuncName == "__cxa_rethrow@PLT" || FuncName != "exit@PLT" ||
+      FuncName == "abort@PLT" || FuncName == "setjmp@PLT" ||
+      FuncName == "longjmp@PLT")
+    return true;


FuncName != "A" || FuncName != "B" is always true.

bgergely0 · 2024-11-28T13:27:57Z

Hey @elhewaty !

I left a few comments some time ago, but forgot to hit finalize on the review, sorry!

After some more looking into this, I believe there is additional problems we need to tackle. The biggest is, we have no information about PLT entries. I can see you listed some, but I don't have a solution that covers any possible libraries that can be linked dynamically to the binary BOLT is ran on.

atrosinenko · 2025-04-18T13:21:11Z

bolt/include/bolt/Core/BinaryContext.h


+  /// Container to cache results about functions that their paths lead to
+  /// a no-return function.
+  std::unordered_map<BinaryFunction *, bool> CallsNoReturnFunction;


Hmm, it looks like LLVM Programmer's Manual advises against the usage of std::unordered_map, though there are several tens of occurrences even in the existing code under llvm/lib. The same in DiscoverNoReturnPass.h below.

atrosinenko · 2025-04-18T13:22:24Z

bolt/include/bolt/Passes/DiscoverNoReturnPass.h

@@ -0,0 +1,36 @@
+//===- bolt/Passes/Discover.h -----------------------------------*- C++ -*-===//


[nit] Invalid file name in the comment.

atrosinenko · 2025-04-18T13:23:20Z

bolt/include/bolt/Passes/DiscoverNoReturnPass.h

+#ifndef BOLT_PASSES_DISCOVERNORETURN_H
+#define BOLT_PASSES_DISCOVERNORETURN_H


[nit] Defined name is out of sync with the file name.

atrosinenko · 2025-04-18T13:26:02Z

bolt/include/bolt/Core/BinaryContext.h

                      std::unique_ptr<DWARFContext> DwCtx,
                      JournalingStreams Logger);

+  void setHasPathToNoReturn(BinaryFunction *Func, bool value = true) {


[nit] value -> Value

atrosinenko · 2025-04-18T13:31:20Z

bolt/lib/Core/BinaryContext.cpp

+bool BinaryContext::hasPathToNoReturn(BinaryFunction *Func) {
+  // Dummy way to mark no-return functions.
+  // FIXME: Find a better way.
+  if (std::string FuncName = Func->getPrintName();


I wonder if getPrintName() is the right accessor to call - maybe getOneName() (or even asking hasName(...)) is more appropriate?

atrosinenko · 2025-04-18T13:31:44Z

bolt/lib/Core/BinaryContext.cpp

+      FuncName == "longjmp@PLT")
+    return true;
+
+  auto itr = CallsNoReturnFunction.find(Func);


[nit] itr -> Itr

atrosinenko · 2025-04-18T13:32:20Z

bolt/lib/Passes/DiscoverNoReturnPass.cpp

@@ -0,0 +1,84 @@
+//===- bolt/Passes/ReorderSection.cpp - Reordering of section data --------===//


[nit] Wrong file name in the comment.

atrosinenko · 2025-04-18T13:37:46Z

bolt/lib/Passes/DiscoverNoReturnPass.cpp

+  bool hasCalls = 0;
+  bool hasReturns = 0;


[nit]

Suggested change

bool hasCalls = 0;

bool hasReturns = 0;

bool HasCalls = 0;

bool HasReturns = 0;

atrosinenko · 2025-04-18T13:39:20Z

bolt/lib/Passes/DiscoverNoReturnPass.cpp

+      bool PrevStat = BC.hasPathToNoReturn(&Func);
+      bool CurStat = traverseFromFunction(&Func, BC);


[nit] Was "State" intended here instead of "Stat"?

atrosinenko · 2025-04-18T14:03:51Z

Left a few unsorted comments, mostly stylistic nit-picking.

elhewaty requested review from aaupov, ayermolo, dcci, maksfb and rafaelauler as code owners November 9, 2024 18:45

llvmbot added the BOLT label Nov 9, 2024

elhewaty requested a review from kbeyls November 9, 2024 18:48

[Bolt] Teach bolt about no-return functions

47e2fa2

elhewaty force-pushed the bolt-no-return-analysis branch from 69e8ed4 to 47e2fa2 Compare November 9, 2024 19:04

bgergely0 reviewed Nov 28, 2024

View reviewed changes

atrosinenko reviewed Apr 18, 2025

View reviewed changes

		@@ -0,0 +1,36 @@
		//===- bolt/Passes/Discover.h ------------------------------------ C++ --===//

		#ifndef BOLT_PASSES_DISCOVERNORETURN_H
		#define BOLT_PASSES_DISCOVERNORETURN_H

		@@ -0,0 +1,84 @@
		//===- bolt/Passes/ReorderSection.cpp - Reordering of section data --------===//

		bool PrevStat = BC.hasPathToNoReturn(&Func);
		bool CurStat = traverseFromFunction(&Func, BC);

[Bolt] Teach bolt about no-return functions #115616

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[Bolt] Teach bolt about no-return functions #115616

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