[clang] [llvm] [llvm][clang] Allocate a new stack instead of spawning a new thread to get more stack space (PR #133173)
Michael Spencer via llvm-commits
llvm-commits at lists.llvm.org
Thu Apr 3 17:16:59 PDT 2025
https://github.com/Bigcheese updated https://github.com/llvm/llvm-project/pull/133173
>From cc251995b3d55b3cd76b0b65753be88c9fb9d627 Mon Sep 17 00:00:00 2001
From: Michael Spencer <bigcheesegs at gmail.com>
Date: Wed, 26 Mar 2025 14:48:19 -0700
Subject: [PATCH] [llvm][clang] Allocate a new stack instead of spawning a new
thread to get more stack space
Clang spawns a new thread to avoid running out of stack space. This
can make debugging and performance analysis more difficult as how the
threads are connected is difficult to recover.
This patch introduces `runOnNewStack` and applies it in Clang. On
platforms that have good support for it this allocates a new stack and
moves to it using assembly. Doing split stacks like this actually runs
on most platforms, but many debuggers and unwinders reject the large
or backwards stack offsets that occur. Apple platforms and tools are
known to support this, so this only enables it there for now.
---
clang/docs/ReleaseNotes.rst | 4 +
clang/include/clang/Basic/Stack.h | 5 +-
clang/lib/Basic/Stack.cpp | 40 ++----
clang/lib/Frontend/CompilerInstance.cpp | 2 +-
.../llvm/Support/CrashRecoveryContext.h | 3 +
llvm/include/llvm/Support/ProgramStack.h | 45 +++++++
llvm/lib/Support/CMakeLists.txt | 1 +
llvm/lib/Support/CrashRecoveryContext.cpp | 19 +++
llvm/lib/Support/ProgramStack.cpp | 123 ++++++++++++++++++
llvm/unittests/Support/CMakeLists.txt | 1 +
llvm/unittests/Support/ProgramStackTest.cpp | 29 +++++
11 files changed, 242 insertions(+), 30 deletions(-)
create mode 100644 llvm/include/llvm/Support/ProgramStack.h
create mode 100644 llvm/lib/Support/ProgramStack.cpp
create mode 100644 llvm/unittests/Support/ProgramStackTest.cpp
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index c521b56a98606..5c76be9f8b5c9 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -168,6 +168,10 @@ Non-comprehensive list of changes in this release
- Support parsing the `cc` operand modifier and alias it to the `c` modifier (#GH127719).
- Added `__builtin_elementwise_exp10`.
- For AMDPGU targets, added `__builtin_v_cvt_off_f32_i4` that maps to the `v_cvt_off_f32_i4` instruction.
+- Clang itself now uses split stacks instead of threads for allocating more
+ stack space when running on Apple AArch64 based platforms. This means that
+ stack traces of Clang from debuggers, crashes, and profilers may look
+ different than before.
New Compiler Flags
------------------
diff --git a/clang/include/clang/Basic/Stack.h b/clang/include/clang/Basic/Stack.h
index 30ebd94aedd1f..9674b9d9b62c3 100644
--- a/clang/include/clang/Basic/Stack.h
+++ b/clang/include/clang/Basic/Stack.h
@@ -27,7 +27,10 @@ namespace clang {
/// Call this once on each thread, as soon after starting the thread as
/// feasible, to note the approximate address of the bottom of the stack.
- void noteBottomOfStack();
+ ///
+ /// \param ForceSet set to true if you know the call is near the bottom of a
+ /// new stack. Used for split stacks.
+ void noteBottomOfStack(bool ForceSet = false);
/// Determine whether the stack is nearly exhausted.
bool isStackNearlyExhausted();
diff --git a/clang/lib/Basic/Stack.cpp b/clang/lib/Basic/Stack.cpp
index aa15d8e66950f..8cbb84943f8d3 100644
--- a/clang/lib/Basic/Stack.cpp
+++ b/clang/lib/Basic/Stack.cpp
@@ -13,33 +13,13 @@
#include "clang/Basic/Stack.h"
#include "llvm/Support/CrashRecoveryContext.h"
+#include "llvm/Support/ProgramStack.h"
-#ifdef _MSC_VER
-#include <intrin.h> // for _AddressOfReturnAddress
-#endif
+static LLVM_THREAD_LOCAL uintptr_t BottomOfStack = 0;
-static LLVM_THREAD_LOCAL void *BottomOfStack = nullptr;
-
-static void *getStackPointer() {
-#if __GNUC__ || __has_builtin(__builtin_frame_address)
- return __builtin_frame_address(0);
-#elif defined(_MSC_VER)
- return _AddressOfReturnAddress();
-#else
- char CharOnStack = 0;
- // The volatile store here is intended to escape the local variable, to
- // prevent the compiler from optimizing CharOnStack into anything other
- // than a char on the stack.
- //
- // Tested on: MSVC 2015 - 2019, GCC 4.9 - 9, Clang 3.2 - 9, ICC 13 - 19.
- char *volatile Ptr = &CharOnStack;
- return Ptr;
-#endif
-}
-
-void clang::noteBottomOfStack() {
- if (!BottomOfStack)
- BottomOfStack = getStackPointer();
+void clang::noteBottomOfStack(bool ForceSet) {
+ if (!BottomOfStack || ForceSet)
+ BottomOfStack = llvm::getStackPointer();
}
bool clang::isStackNearlyExhausted() {
@@ -51,7 +31,8 @@ bool clang::isStackNearlyExhausted() {
if (!BottomOfStack)
return false;
- intptr_t StackDiff = (intptr_t)getStackPointer() - (intptr_t)BottomOfStack;
+ intptr_t StackDiff =
+ (intptr_t)llvm::getStackPointer() - (intptr_t)BottomOfStack;
size_t StackUsage = (size_t)std::abs(StackDiff);
// If the stack pointer has a surprising value, we do not understand this
@@ -66,9 +47,12 @@ bool clang::isStackNearlyExhausted() {
void clang::runWithSufficientStackSpaceSlow(llvm::function_ref<void()> Diag,
llvm::function_ref<void()> Fn) {
llvm::CrashRecoveryContext CRC;
- CRC.RunSafelyOnThread([&] {
- noteBottomOfStack();
+ // Preserve the BottomOfStack in case RunSafelyOnNewStack uses split stacks.
+ uintptr_t PrevBottom = BottomOfStack;
+ CRC.RunSafelyOnNewStack([&] {
+ noteBottomOfStack(true);
Diag();
Fn();
}, DesiredStackSize);
+ BottomOfStack = PrevBottom;
}
diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 91093d3ccb84c..33babdead55ba 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -1275,7 +1275,7 @@ compileModuleImpl(CompilerInstance &ImportingInstance, SourceLocation ImportLoc,
// Execute the action to actually build the module in-place. Use a separate
// thread so that we get a stack large enough.
- bool Crashed = !llvm::CrashRecoveryContext().RunSafelyOnThread(
+ bool Crashed = !llvm::CrashRecoveryContext().RunSafelyOnNewStack(
[&]() {
GenerateModuleFromModuleMapAction Action;
Instance.ExecuteAction(Action);
diff --git a/llvm/include/llvm/Support/CrashRecoveryContext.h b/llvm/include/llvm/Support/CrashRecoveryContext.h
index 26ddf97b3ef02..31293d6715757 100644
--- a/llvm/include/llvm/Support/CrashRecoveryContext.h
+++ b/llvm/include/llvm/Support/CrashRecoveryContext.h
@@ -97,6 +97,9 @@ class CrashRecoveryContext {
return RunSafelyOnThread([&]() { Fn(UserData); }, RequestedStackSize);
}
+ bool RunSafelyOnNewStack(function_ref<void()>,
+ unsigned RequestedStackSize = 0);
+
/// Explicitly trigger a crash recovery in the current process, and
/// return failure from RunSafely(). This function does not return.
[[noreturn]] void HandleExit(int RetCode);
diff --git a/llvm/include/llvm/Support/ProgramStack.h b/llvm/include/llvm/Support/ProgramStack.h
new file mode 100644
index 0000000000000..cc8fe98d7a8d1
--- /dev/null
+++ b/llvm/include/llvm/Support/ProgramStack.h
@@ -0,0 +1,45 @@
+//===--- ProgramStack.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 LLVM_SUPPORT_PROGRAMSTACK_H
+#define LLVM_SUPPORT_PROGRAMSTACK_H
+
+#include "llvm/ADT/STLFunctionalExtras.h"
+
+namespace llvm {
+
+/// \returns an address close to the current value of the stack pointer.
+///
+/// The value is not guaranteed to point to anything specific. It can be used to
+/// estimate how much stack space has been used since the previous call.
+uintptr_t getStackPointer();
+
+/// \returns the default stack size for this platform.
+///
+/// Based on \p RLIMIT_STACK or the equivalent.
+unsigned getDefaultStackSize();
+
+/// Runs Fn on a new stack of at least the given size.
+///
+/// \param StackSize requested stack size. A size of 0 uses the default stack
+/// size of the platform.
+///
+/// The preferred implementation is split stacks on platforms that have a good
+/// debugging experience for them. On other platforms a new thread is used.
+void runOnNewStack(unsigned StackSize, function_ref<void()> Fn);
+
+template <typename R, typename... Ts>
+R runOnNewStack(unsigned StackSize, function_ref<R(Ts...)> Fn, Ts &&...Args) {
+ std::optional<R> Ret;
+ runOnNewStack(StackSize, [&]() { Ret = Fn(std::forward<Ts>(Args)...); });
+ return std::move(*Ret);
+}
+
+} // namespace llvm
+
+#endif // LLVM_SUPPORT_PROGRAMSTACK_H
diff --git a/llvm/lib/Support/CMakeLists.txt b/llvm/lib/Support/CMakeLists.txt
index 98ffd829d80b8..def37f3f278d0 100644
--- a/llvm/lib/Support/CMakeLists.txt
+++ b/llvm/lib/Support/CMakeLists.txt
@@ -295,6 +295,7 @@ add_llvm_component_library(LLVMSupport
Path.cpp
Process.cpp
Program.cpp
+ ProgramStack.cpp
RWMutex.cpp
Signals.cpp
Threading.cpp
diff --git a/llvm/lib/Support/CrashRecoveryContext.cpp b/llvm/lib/Support/CrashRecoveryContext.cpp
index f53aea177d612..ca0c8744a398c 100644
--- a/llvm/lib/Support/CrashRecoveryContext.cpp
+++ b/llvm/lib/Support/CrashRecoveryContext.cpp
@@ -10,6 +10,7 @@
#include "llvm/Config/llvm-config.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/ExitCodes.h"
+#include "llvm/Support/ProgramStack.h"
#include "llvm/Support/Signals.h"
#include "llvm/Support/thread.h"
#include <cassert>
@@ -523,3 +524,21 @@ bool CrashRecoveryContext::RunSafelyOnThread(function_ref<void()> Fn,
CRC->setSwitchedThread();
return Info.Result;
}
+
+bool CrashRecoveryContext::RunSafelyOnNewStack(function_ref<void()> Fn,
+ unsigned RequestedStackSize) {
+ // If crash recovery is disabled, do nothing.
+ if (gCrashRecoveryEnabled) {
+ assert(!Impl && "Crash recovery context already initialized!");
+ CrashRecoveryContextImpl *CRCI = new CrashRecoveryContextImpl(this);
+ Impl = CRCI;
+
+ CRCI->ValidJumpBuffer = true;
+ if (setjmp(CRCI->JumpBuffer) != 0) {
+ return false;
+ }
+ }
+
+ runOnNewStack(RequestedStackSize, Fn);
+ return true;
+}
diff --git a/llvm/lib/Support/ProgramStack.cpp b/llvm/lib/Support/ProgramStack.cpp
new file mode 100644
index 0000000000000..cc4a356d63918
--- /dev/null
+++ b/llvm/lib/Support/ProgramStack.cpp
@@ -0,0 +1,123 @@
+//===--- RunOnNewStack.cpp - Crash Recovery -------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/ProgramStack.h"
+#include "llvm/Config/config.h"
+#include "llvm/Support/Compiler.h"
+
+#ifdef LLVM_ON_UNIX
+# include <sys/resource.h> // for getrlimit
+#endif
+
+#ifdef _MSC_VER
+# include <intrin.h> // for _AddressOfReturnAddress
+#endif
+
+// Currently only Apple AArch64 is known to support split stacks in the debugger
+// and other tooling.
+#if defined(__APPLE__) && defined(__aarch64__) && \
+ LLVM_HAS_CPP_ATTRIBUTE(gnu::naked) && __has_extension(gnu_asm)
+# define LLVM_HAS_SPLIT_STACKS
+# define LLVM_HAS_SPLIT_STACKS_AARCH64
+#include <sys/mman.h>
+#endif
+
+#ifndef LLVM_HAS_SPLIT_STACKS
+# include "llvm/Support/thread.h"
+#endif
+
+using namespace llvm;
+
+uintptr_t llvm::getStackPointer() {
+#if __GNUC__ || __has_builtin(__builtin_frame_address)
+ return (uintptr_t)__builtin_frame_address(0);
+#elif defined(_MSC_VER)
+ return (uintptr_t)_AddressOfReturnAddress();
+#else
+ volatile char CharOnStack = 0;
+ // The volatile store here is intended to escape the local variable, to
+ // prevent the compiler from optimizing CharOnStack into anything other
+ // than a char on the stack.
+ //
+ // Tested on: MSVC 2015 - 2019, GCC 4.9 - 9, Clang 3.2 - 9, ICC 13 - 19.
+ char *volatile Ptr = &CharOnStack;
+ return (uintptr_t)Ptr;
+#endif
+}
+
+unsigned llvm::getDefaultStackSize() {
+#ifdef LLVM_ON_UNIX
+ rlimit RL;
+ getrlimit(RLIMIT_STACK, &RL);
+ return RL.rlim_cur;
+#else
+ // Clang recursively parses, instantiates templates, and evaluates constant
+ // expressions. We've found 8MiB to be a reasonable stack size given the way
+ // Clang works and the way C++ is commonly written.
+ return 8 << 20;
+#endif
+}
+
+namespace {
+#ifdef LLVM_HAS_SPLIT_STACKS_AARCH64
+[[gnu::naked]] void runOnNewStackImpl(void *Stack, void (*Fn)(void *),
+ void *Ctx) {
+ __asm__ volatile(
+ "mov x16, sp\n\t"
+ "sub x0, x0, #0x20\n\t" // subtract space from stack
+ "stp xzr, x16, [x0, #0x00]\n\t" // save old sp
+ "stp x29, x30, [x0, #0x10]\n\t" // save fp, lr
+ "mov sp, x0\n\t" // switch to new stack
+ "add x29, x0, #0x10\n\t" // switch to new frame
+ ".cfi_def_cfa w29, 16\n\t"
+ ".cfi_offset w30, -8\n\t" // lr
+ ".cfi_offset w29, -16\n\t" // fp
+
+ "mov x0, x2\n\t" // Ctx is the only argument
+ "blr x1\n\t" // call Fn
+
+ "ldp x29, x30, [sp, #0x10]\n\t" // restore fp, lr
+ "ldp xzr, x16, [sp, #0x00]\n\t" // load old sp
+ "mov sp, x16\n\t"
+ "ret"
+ );
+}
+#endif
+
+#ifdef LLVM_HAS_SPLIT_STACKS
+void callback(void *Ctx) {
+ (*reinterpret_cast<function_ref<void()> *>(Ctx))();
+}
+#endif
+} // namespace
+
+#ifdef LLVM_HAS_SPLIT_STACKS
+void llvm::runOnNewStack(unsigned StackSize, function_ref<void()> Fn) {
+ if (StackSize == 0)
+ StackSize = getDefaultStackSize();
+
+ // We use malloc here instead of mmap because:
+ // - it's simpler,
+ // - many malloc implementations will reuse the allocation in cases where
+ // we're bouncing accross the edge of a stack boundry, and
+ // - many malloc implemenations will already provide guard pages for
+ // allocations this large.
+ void *Stack = malloc(StackSize);
+ void *BottomOfStack = (char *)Stack + StackSize;
+
+ runOnNewStackImpl(BottomOfStack, callback, &Fn);
+
+ free(Stack);
+}
+#else
+void llvm::runOnNewStack(unsigned StackSize, function_ref<void()> Fn) {
+ llvm::thread Thread(
+ StackSize == 0 ? std::nullopt : std::optional<unsigned>(StackSize), Fn);
+ Thread.join();
+}
+#endif
diff --git a/llvm/unittests/Support/CMakeLists.txt b/llvm/unittests/Support/CMakeLists.txt
index 6c4e7cb689b20..e5bf820fb4d1c 100644
--- a/llvm/unittests/Support/CMakeLists.txt
+++ b/llvm/unittests/Support/CMakeLists.txt
@@ -70,6 +70,7 @@ add_llvm_unittest(SupportTests
PerThreadBumpPtrAllocatorTest.cpp
ProcessTest.cpp
ProgramTest.cpp
+ ProgramStackTest.cpp
RecyclerTest.cpp
RegexTest.cpp
ReverseIterationTest.cpp
diff --git a/llvm/unittests/Support/ProgramStackTest.cpp b/llvm/unittests/Support/ProgramStackTest.cpp
new file mode 100644
index 0000000000000..1b4a071739139
--- /dev/null
+++ b/llvm/unittests/Support/ProgramStackTest.cpp
@@ -0,0 +1,29 @@
+//===- unittest/Support/ProgramStackTest.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
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/ProgramStack.h"
+#include "llvm/Support/Process.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+static uintptr_t func(int &A) {
+ A = 7;
+ return getStackPointer();
+}
+
+TEST(ProgramStackTest, runOnNewStack) {
+ int A = 0;
+ uintptr_t Stack = runOnNewStack(0, function_ref<uintptr_t(int &)>(func), A);
+ EXPECT_EQ(A, 7);
+ intptr_t StackDiff = (intptr_t)llvm::getStackPointer() - (intptr_t)Stack;
+ size_t StackDistance = (size_t)std::abs(StackDiff);
+ // Page size is used as it's large enough to guarantee were not on the same
+ // stack but not too large to cause spurious failures.
+ EXPECT_GT(StackDistance, llvm::sys::Process::getPageSizeEstimate());
+}
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