[llvm] ce3485a - [llvm][GlobalOpt] Remove empty atexit destructors/handlers (#88836)
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Mon Apr 29 17:29:38 PDT 2024
Author: Max Winkler
Date: 2024-04-29T20:29:34-04:00
New Revision: ce3485a0cd12b45c725f008a7836e71a1d72df49
URL: https://github.com/llvm/llvm-project/commit/ce3485a0cd12b45c725f008a7836e71a1d72df49
DIFF: https://github.com/llvm/llvm-project/commit/ce3485a0cd12b45c725f008a7836e71a1d72df49.diff
LOG: [llvm][GlobalOpt] Remove empty atexit destructors/handlers (#88836)
https://godbolt.org/z/frjhqMKqc for an example.
Removal of allocations due to empty `__cxa_atexit` destructor calls is
done by the following globalopt pass.
This pass currently does not look for `atexit` handlers generated for
platforms that do not use `__cxa_atexit`.
By default Win32 and AIX use `atexit`.
I don't see an easy way to only remove `atexit` calls that the compiler
generated without looking at the generated mangled name of the atexit
handler that is being registered.
However we can easily remove all `atexit` calls that register empty
handlers since it is trivial to ensure the removed call still returns
`0` which is the value for success.
Added:
llvm/test/Transforms/GlobalOpt/atexit-dtor.ll
Modified:
llvm/include/llvm/Analysis/TargetLibraryInfo.def
llvm/lib/Transforms/IPO/GlobalOpt.cpp
llvm/test/tools/llvm-tli-checker/ps4-tli-check.yaml
llvm/unittests/Analysis/TargetLibraryInfoTest.cpp
Removed:
################################################################################
diff --git a/llvm/include/llvm/Analysis/TargetLibraryInfo.def b/llvm/include/llvm/Analysis/TargetLibraryInfo.def
index 37221eb9e47115..717693a7cf63c1 100644
--- a/llvm/include/llvm/Analysis/TargetLibraryInfo.def
+++ b/llvm/include/llvm/Analysis/TargetLibraryInfo.def
@@ -471,6 +471,11 @@ TLI_DEFINE_ENUM_INTERNAL(cxa_atexit)
TLI_DEFINE_STRING_INTERNAL("__cxa_atexit")
TLI_DEFINE_SIG_INTERNAL(Int, Ptr, Ptr, Ptr)
+/// int atexit(void (*f)(void));
+TLI_DEFINE_ENUM_INTERNAL(atexit)
+TLI_DEFINE_STRING_INTERNAL("atexit")
+TLI_DEFINE_SIG_INTERNAL(Int, Ptr)
+
/// void __cxa_guard_abort(guard_t *guard);
/// guard_t is int64_t in Itanium ABI or int32_t on ARM eabi.
TLI_DEFINE_ENUM_INTERNAL(cxa_guard_abort)
diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index fbb83e787f6327..c8c835115a992a 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -87,6 +87,7 @@ STATISTIC(NumNestRemoved , "Number of nest attributes removed");
STATISTIC(NumAliasesResolved, "Number of global aliases resolved");
STATISTIC(NumAliasesRemoved, "Number of global aliases eliminated");
STATISTIC(NumCXXDtorsRemoved, "Number of global C++ destructors removed");
+STATISTIC(NumAtExitRemoved, "Number of atexit handlers removed");
STATISTIC(NumInternalFunc, "Number of internal functions");
STATISTIC(NumColdCC, "Number of functions marked coldcc");
STATISTIC(NumIFuncsResolved, "Number of statically resolved IFuncs");
@@ -2328,18 +2329,19 @@ OptimizeGlobalAliases(Module &M,
}
static Function *
-FindCXAAtExit(Module &M, function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
+FindAtExitLibFunc(Module &M,
+ function_ref<TargetLibraryInfo &(Function &)> GetTLI,
+ LibFunc Func) {
// Hack to get a default TLI before we have actual Function.
auto FuncIter = M.begin();
if (FuncIter == M.end())
return nullptr;
auto *TLI = &GetTLI(*FuncIter);
- LibFunc F = LibFunc_cxa_atexit;
- if (!TLI->has(F))
+ if (!TLI->has(Func))
return nullptr;
- Function *Fn = M.getFunction(TLI->getName(F));
+ Function *Fn = M.getFunction(TLI->getName(Func));
if (!Fn)
return nullptr;
@@ -2347,17 +2349,18 @@ FindCXAAtExit(Module &M, function_ref<TargetLibraryInfo &(Function &)> GetTLI) {
TLI = &GetTLI(*Fn);
// Make sure that the function has the correct prototype.
- if (!TLI->getLibFunc(*Fn, F) || F != LibFunc_cxa_atexit)
+ LibFunc F;
+ if (!TLI->getLibFunc(*Fn, F) || F != Func)
return nullptr;
return Fn;
}
-/// Returns whether the given function is an empty C++ destructor and can
-/// therefore be eliminated.
-/// Note that we assume that other optimization passes have already simplified
-/// the code so we simply check for 'ret'.
-static bool cxxDtorIsEmpty(const Function &Fn) {
+/// Returns whether the given function is an empty C++ destructor or atexit
+/// handler and can therefore be eliminated. Note that we assume that other
+/// optimization passes have already simplified the code so we simply check for
+/// 'ret'.
+static bool IsEmptyAtExitFunction(const Function &Fn) {
// FIXME: We could eliminate C++ destructors if they're readonly/readnone and
// nounwind, but that doesn't seem worth doing.
if (Fn.isDeclaration())
@@ -2373,7 +2376,7 @@ static bool cxxDtorIsEmpty(const Function &Fn) {
return false;
}
-static bool OptimizeEmptyGlobalCXXDtors(Function *CXAAtExitFn) {
+static bool OptimizeEmptyGlobalAtExitDtors(Function *CXAAtExitFn, bool isCXX) {
/// Itanium C++ ABI p3.3.5:
///
/// After constructing a global (or local static) object, that will require
@@ -2386,8 +2389,8 @@ static bool OptimizeEmptyGlobalCXXDtors(Function *CXAAtExitFn) {
/// registered before this one. It returns zero if registration is
/// successful, nonzero on failure.
- // This pass will look for calls to __cxa_atexit where the function is trivial
- // and remove them.
+ // This pass will look for calls to __cxa_atexit or atexit where the function
+ // is trivial and remove them.
bool Changed = false;
for (User *U : llvm::make_early_inc_range(CXAAtExitFn->users())) {
@@ -2400,14 +2403,17 @@ static bool OptimizeEmptyGlobalCXXDtors(Function *CXAAtExitFn) {
Function *DtorFn =
dyn_cast<Function>(CI->getArgOperand(0)->stripPointerCasts());
- if (!DtorFn || !cxxDtorIsEmpty(*DtorFn))
+ if (!DtorFn || !IsEmptyAtExitFunction(*DtorFn))
continue;
// Just remove the call.
CI->replaceAllUsesWith(Constant::getNullValue(CI->getType()));
CI->eraseFromParent();
- ++NumCXXDtorsRemoved;
+ if (isCXX)
+ ++NumCXXDtorsRemoved;
+ else
+ ++NumAtExitRemoved;
Changed |= true;
}
@@ -2525,9 +2531,12 @@ optimizeGlobalsInModule(Module &M, const DataLayout &DL,
// Try to remove trivial global destructors if they are not removed
// already.
- Function *CXAAtExitFn = FindCXAAtExit(M, GetTLI);
- if (CXAAtExitFn)
- LocalChange |= OptimizeEmptyGlobalCXXDtors(CXAAtExitFn);
+ if (Function *CXAAtExitFn =
+ FindAtExitLibFunc(M, GetTLI, LibFunc_cxa_atexit))
+ LocalChange |= OptimizeEmptyGlobalAtExitDtors(CXAAtExitFn, true);
+
+ if (Function *AtExitFn = FindAtExitLibFunc(M, GetTLI, LibFunc_atexit))
+ LocalChange |= OptimizeEmptyGlobalAtExitDtors(AtExitFn, false);
// Optimize IFuncs whose callee's are statically known.
LocalChange |= OptimizeStaticIFuncs(M);
diff --git a/llvm/test/Transforms/GlobalOpt/atexit-dtor.ll b/llvm/test/Transforms/GlobalOpt/atexit-dtor.ll
new file mode 100644
index 00000000000000..bf78fd3d3ef49b
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/atexit-dtor.ll
@@ -0,0 +1,54 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt < %s -S -passes=globalopt | FileCheck %s
+
+declare dso_local i32 @atexit(ptr)
+
+define dso_local void @empty_atexit_handler() {
+; CHECK-LABEL: define dso_local void @empty_atexit_handler() local_unnamed_addr {
+; CHECK-NEXT: ret void
+;
+ ret void
+}
+
+; Check that `atexit` is removed if the handler is empty.
+; Check that a removed `atexit` call returns `0` which is the value that denotes success.
+define dso_local noundef i32 @register_atexit_handler() {
+; CHECK-LABEL: define dso_local noundef i32 @register_atexit_handler() local_unnamed_addr {
+; CHECK-NEXT: ret i32 0
+;
+ %1 = call i32 @atexit(ptr @empty_atexit_handler)
+ ret i32 %1
+}
+
+declare dso_local void @declared_atexit_handler()
+
+; Check that an atexit handler with only a declaration is not removed.
+define dso_local noundef i32 @register_declared_atexit_handler() {
+; CHECK-LABEL: define dso_local noundef i32 @register_declared_atexit_handler() local_unnamed_addr {
+; CHECK-NEXT: [[TMP1:%.*]] = call i32 @atexit(ptr @declared_atexit_handler)
+; CHECK-NEXT: ret i32 [[TMP1]]
+;
+ %1 = call i32 @atexit(ptr @declared_atexit_handler)
+ ret i32 %1
+}
+
+declare dso_local void @external_exit_func()
+
+define dso_local void @nonempty_atexit_handler() {
+; CHECK-LABEL: define dso_local void @nonempty_atexit_handler() {
+; CHECK-NEXT: call void @external_exit_func()
+; CHECK-NEXT: ret void
+;
+ call void @external_exit_func()
+ ret void
+}
+
+; Check that an atexit handler that consists of any instructions other than `ret` is considered nonempty and not removed.
+define dso_local noundef i32 @register_nonempty_atexit_handler() {
+; CHECK-LABEL: define dso_local noundef i32 @register_nonempty_atexit_handler() local_unnamed_addr {
+; CHECK-NEXT: [[TMP1:%.*]] = call i32 @atexit(ptr @nonempty_atexit_handler)
+; CHECK-NEXT: ret i32 [[TMP1]]
+;
+ %1 = call i32 @atexit(ptr @nonempty_atexit_handler)
+ ret i32 %1
+}
diff --git a/llvm/test/tools/llvm-tli-checker/ps4-tli-check.yaml b/llvm/test/tools/llvm-tli-checker/ps4-tli-check.yaml
index 46f6a2d0a554ac..95c23007b1a05a 100644
--- a/llvm/test/tools/llvm-tli-checker/ps4-tli-check.yaml
+++ b/llvm/test/tools/llvm-tli-checker/ps4-tli-check.yaml
@@ -34,21 +34,21 @@
#
# CHECK: << Total TLI yes SDK no: 8
# CHECK: >> Total TLI no SDK yes: 0
-# CHECK: == Total TLI yes SDK yes: 238
+# CHECK: == Total TLI yes SDK yes: 239
#
# WRONG_DETAIL: << TLI yes SDK no : '_ZdaPv' aka operator delete[](void*)
# WRONG_DETAIL: >> TLI no SDK yes: '_ZdaPvj' aka operator delete[](void*, unsigned int)
# WRONG_DETAIL-COUNT-8: << TLI yes SDK no : {{.*}}__hot_cold_t
# WRONG_SUMMARY: << Total TLI yes SDK no: 9{{$}}
# WRONG_SUMMARY: >> Total TLI no SDK yes: 1{{$}}
-# WRONG_SUMMARY: == Total TLI yes SDK yes: 237
+# WRONG_SUMMARY: == Total TLI yes SDK yes: 238
#
## The -COUNT suffix doesn't care if there are too many matches, so check
## the exact count first; the two directives should add up to that.
## Yes, this means additions to TLI will fail this test, but the argument
## to -COUNT can't be an expression.
-# AVAIL: TLI knows 479 symbols, 246 available
-# AVAIL-COUNT-246: {{^}} available
+# AVAIL: TLI knows 480 symbols, 247 available
+# AVAIL-COUNT-247: {{^}} available
# AVAIL-NOT: {{^}} available
# UNAVAIL-COUNT-233: not available
# UNAVAIL-NOT: not available
@@ -263,6 +263,10 @@ DynamicSymbols:
Type: STT_FUNC
Section: .text
Binding: STB_GLOBAL
+ - Name: atexit
+ Type: STT_FUNC
+ Section: .text
+ Binding: STB_GLOBAL
- Name: atof
Type: STT_FUNC
Section: .text
diff --git a/llvm/unittests/Analysis/TargetLibraryInfoTest.cpp b/llvm/unittests/Analysis/TargetLibraryInfoTest.cpp
index 8e3fe3b44a84a9..1fe94e2aae059e 100644
--- a/llvm/unittests/Analysis/TargetLibraryInfoTest.cpp
+++ b/llvm/unittests/Analysis/TargetLibraryInfoTest.cpp
@@ -495,6 +495,8 @@ TEST_F(TargetLibraryInfoTest, ValidProto) {
"declare i32 @__cxa_guard_acquire(%struct*)\n"
"declare void @__cxa_guard_release(%struct*)\n"
+ "declare i32 @atexit(void ()*)\n"
+
"declare i32 @__nvvm_reflect(i8*)\n"
"declare i8* @__memcpy_chk(i8*, i8*, i64, i64)\n"
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