Demo of Clang++'s implementation of C++ static local variable

command_prompt

$ clang++ -S -emit-llvm yy.cc

yy.cc

int g_counter = 0;

struct Dummy {
  int value;

  Dummy(): value(0) { g_counter++; }
};

int foo() {
  static Dummy dummy;
  return dummy.value++;
}

int main() {
  foo();
  foo();
  return 0;
}

yy.ll

; ModuleID = 'yy.cc'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.9.0"

%struct.Dummy = type { i32 }

@g_counter = global i32 0, align 4
@_ZZ3foovE5dummy = internal global %struct.Dummy zeroinitializer, align 4
@_ZGVZ3foovE5dummy = internal global i64 0

; Function Attrs: ssp uwtable
define i32 @_Z3foov() #0 {
  %1 = alloca i8*
  %2 = alloca i32
  %3 = load atomic i8* bitcast (i64* @_ZGVZ3foovE5dummy to i8*) acquire, align 1
  %4 = icmp eq i8 %3, 0
  br i1 %4, label %5, label %10

; <label>:5                                       ; preds = %0
  %6 = call i32 @__cxa_guard_acquire(i64* @_ZGVZ3foovE5dummy) #1
  %7 = icmp ne i32 %6, 0
  br i1 %7, label %8, label %10

; <label>:8                                       ; preds = %5
  invoke void @_ZN5DummyC1Ev(%struct.Dummy* @_ZZ3foovE5dummy)
          to label %9 unwind label %13

; <label>:9                                       ; preds = %8
  call void @__cxa_guard_release(i64* @_ZGVZ3foovE5dummy) #1
  br label %10

; <label>:10                                      ; preds = %9, %5, %0
  %11 = load i32* getelementptr inbounds (%struct.Dummy* @_ZZ3foovE5dummy, i32 0, i32 0), align 4
  %12 = add nsw i32 %11, 1
  store i32 %12, i32* getelementptr inbounds (%struct.Dummy* @_ZZ3foovE5dummy, i32 0, i32 0), align 4
  ret i32 %11

; <label>:13                                      ; preds = %8
  %14 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*)
          cleanup
  %15 = extractvalue { i8*, i32 } %14, 0
  store i8* %15, i8** %1
  %16 = extractvalue { i8*, i32 } %14, 1
  store i32 %16, i32* %2
  call void @__cxa_guard_abort(i64* @_ZGVZ3foovE5dummy) #1
  br label %17

; <label>:17                                      ; preds = %13
  %18 = load i8** %1
  %19 = load i32* %2
  %20 = insertvalue { i8*, i32 } undef, i8* %18, 0
  %21 = insertvalue { i8*, i32 } %20, i32 %19, 1
  resume { i8*, i32 } %21
}

; Function Attrs: nounwind
declare i32 @__cxa_guard_acquire(i64*) #1

; Function Attrs: ssp uwtable
define linkonce_odr void @_ZN5DummyC1Ev(%struct.Dummy* %this) unnamed_addr #0 align 2 {
  %1 = alloca %struct.Dummy*, align 8
  store %struct.Dummy* %this, %struct.Dummy** %1, align 8
  %2 = load %struct.Dummy** %1
  call void @_ZN5DummyC2Ev(%struct.Dummy* %2)
  ret void
}

declare i32 @__gxx_personality_v0(...)

; Function Attrs: nounwind
declare void @__cxa_guard_abort(i64*) #1

; Function Attrs: nounwind
declare void @__cxa_guard_release(i64*) #1

; Function Attrs: ssp uwtable
define i32 @main() #0 {
  %1 = alloca i32, align 4
  store i32 0, i32* %1
  %2 = call i32 @_Z3foov()
  %3 = call i32 @_Z3foov()
  ret i32 0
}

; Function Attrs: nounwind ssp uwtable
define linkonce_odr void @_ZN5DummyC2Ev(%struct.Dummy* %this) unnamed_addr #2 align 2 {
  %1 = alloca %struct.Dummy*, align 8
  store %struct.Dummy* %this, %struct.Dummy** %1, align 8
  %2 = load %struct.Dummy** %1
  %3 = getelementptr inbounds %struct.Dummy* %2, i32 0, i32 0
  store i32 0, i32* %3, align 4
  %4 = load i32* @g_counter, align 4
  %5 = add nsw i32 %4, 1
  store i32 %5, i32* @g_counter, align 4
  ret void
}

attributes #0 = { ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { nounwind }
attributes #2 = { nounwind ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)"}