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[LowerGlobalDtors] Skip __cxa_atexit call completely when arg0 is unused #68758

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Merged
merged 1 commit into from
Oct 23, 2023
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[LowerGlobalDtors] Skip __cxa_atexit call completely when arg0 is unused #68758

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2 changes: 1 addition & 1 deletion lld/test/wasm/init-fini-no-gc.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -25,7 +25,7 @@ entry:
}

define hidden i32 @__cxa_atexit(i32 %func, i32 %arg, i32 %dso_handle) {
ret i32 0
ret i32 %func
}

@llvm.global_ctors = appending global [1 x { i32, ptr, ptr }] [
Expand Down
35 changes: 18 additions & 17 deletions lld/test/wasm/init-fini.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -26,10 +26,7 @@ entry:
declare hidden void @externCtor()
declare hidden void @externDtor()
declare hidden void @__wasm_call_ctors()

define i32 @__cxa_atexit(i32 %func, i32 %arg, i32 %dso_handle) {
ret i32 0
}
declare i32 @__cxa_atexit(i32 %func, i32 %arg, i32 %dso_handle)

define hidden void @_start() {
entry:
Expand Down Expand Up @@ -57,13 +54,17 @@ entry:
; CHECK: - Type: IMPORT
; CHECK-NEXT: Imports:
; CHECK-NEXT: - Module: env
; CHECK-NEXT: Field: externDtor
; CHECK-NEXT: Field: __cxa_atexit
; CHECK-NEXT: Kind: FUNCTION
; CHECK-NEXT: SigIndex: 0
; CHECK-NEXT: - Module: env
; CHECK-NEXT: Field: externDtor
; CHECK-NEXT: Kind: FUNCTION
; CHECK-NEXT: SigIndex: 1
; CHECK-NEXT: - Module: env
; CHECK-NEXT: Field: externCtor
; CHECK-NEXT: Kind: FUNCTION
; CHECK-NEXT: SigIndex: 0
; CHECK-NEXT: SigIndex: 1
; CHECK: - Type: ELEM
; CHECK-NEXT: Segments:
; CHECK-NEXT: - Offset:
Expand All @@ -72,31 +73,31 @@ entry:
; CHECK-NEXT: Functions: [ 9, 11, 13, 17, 19, 21 ]
; CHECK-NEXT: - Type: CODE
; CHECK-NEXT: Functions:
; CHECK-NEXT: - Index: 2
; CHECK-NEXT: - Index: 3
; CHECK-NEXT: Locals:
; CHECK-NEXT: Body: 10031004100A100F1012100F10141003100C100F10161001100E0B
; CHECK-NEXT: Body: 10041005100A100F1012100F10141004100C100F10161002100E0B
; CHECK: - Index: 22
; CHECK-NEXT: Locals:
; CHECK-NEXT: Body: 02404186808080004100418088808000108780808000450D0000000B0B
; CHECK-NEXT: Body: 02404186808080004100418088808000108080808000450D0000000B0B
; CHECK-NEXT: - Type: CUSTOM
; CHECK-NEXT: Name: name
; CHECK-NEXT: FunctionNames:
; CHECK-NEXT: - Index: 0
; CHECK-NEXT: Name: externDtor
; CHECK-NEXT: Name: __cxa_atexit
; CHECK-NEXT: - Index: 1
; CHECK-NEXT: Name: externCtor
; CHECK-NEXT: Name: externDtor
; CHECK-NEXT: - Index: 2
; CHECK-NEXT: Name: __wasm_call_ctors
; CHECK-NEXT: Name: externCtor
; CHECK-NEXT: - Index: 3
; CHECK-NEXT: Name: func1
; CHECK-NEXT: Name: __wasm_call_ctors
; CHECK-NEXT: - Index: 4
; CHECK-NEXT: Name: func2
; CHECK-NEXT: Name: func1
; CHECK-NEXT: - Index: 5
; CHECK-NEXT: Name: func3
; CHECK-NEXT: Name: func2
; CHECK-NEXT: - Index: 6
; CHECK-NEXT: Name: func4
; CHECK-NEXT: Name: func3
; CHECK-NEXT: - Index: 7
; CHECK-NEXT: Name: __cxa_atexit
; CHECK-NEXT: Name: func4
; CHECK-NEXT: - Index: 8
; CHECK-NEXT: Name: _start
; CHECK-NEXT: - Index: 9
Expand Down
11 changes: 11 additions & 0 deletions llvm/lib/Transforms/Utils/LowerGlobalDtors.cpp
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Expand Up @@ -140,6 +140,17 @@ static bool runImpl(Module &M) {
{PointerType::get(AtExitFuncTy, 0), VoidStar, VoidStar},
/*isVarArg=*/false));

// If __cxa_atexit is defined (e.g. in the case of LTO) and arg0 is not
// actually used (i.e. it's dummy/stub function as used in emscripten when
// the program never exits) we can simply return early and clear out
// @llvm.global_dtors.
if (auto F = dyn_cast<Function>(AtExit.getCallee())) {
if (F && F->hasExactDefinition() && F->getArg(0)->getNumUses() == 0) {
GV->eraseFromParent();
return true;
}
}

// Declare __dso_local.
Type *DsoHandleTy = Type::getInt8Ty(C);
Constant *DsoHandle = M.getOrInsertGlobal("__dso_handle", DsoHandleTy, [&] {
Expand Down
17 changes: 17 additions & 0 deletions llvm/test/Transforms/LowerGlobalDestructors/lower-global-dtors-unused.ll
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Original file line number Diff line number Diff line change
@@ -0,0 +1,17 @@
; RUN: opt -passes=lower-global-dtors -S < %s | FileCheck %s --implicit-check-not=llvm.global_dtors

; Test that @llvm.global_dtors is completely removed if __cxa_atexit
; is a no-op (i.e. doesn't use its first argument).

declare void @orig_dtor()

define i32 @__cxa_atexit(ptr, ptr, ptr) {
ret i32 0
}

@llvm.global_dtors = appending global [1 x { i32, ptr, ptr }] [
{ i32, ptr, ptr } { i32 0, ptr @orig_dtor, ptr null }
]

; CHECK-NOT: @llvm.global_dtors
; CHECK-NOT: call void @orig_dtor()