C ++静态初始化

Question

what should be the behavior in the following case:

class C {
    boost::mutex mutex_;
    std::map<...> data_;
};

C& get() {
    static C c;
    return c;
}

int main() {
    get(); // is compiler free to optimize out the call? 
    ....
}

is compiler allowed to optimize out the call to get() ?

the idea was to touch static variable to initialize it before multithreaded operations needed it

is this a better option?:

C& get() {
    static C *c = new C();
    return *c;
}

Answer 1

The C and C++ standards operate under a rather simple principle generally known as the "as-if rule" -- basically, that the compiler is free to do almost anything as long as no conforming code can discern the difference between what it did and what was officially required.

I don't see a way for conforming code to discern whether get was actually called in this case, so it looks to me like it's free to optimize it out.

Answer 2

Based on your edits, here's an improved version, with the same results.

Input:

struct C { 
    int myfrob;
    int frob();
    C(int f);
 };
C::C(int f) : myfrob(f) {}
int C::frob() { return myfrob; }

C& get() {
    static C *c = new C(5);
    return *c;
}

int main() {
    return get().frob(); // is compiler free to optimize out the call? 

}

Output:

; ModuleID = '/tmp/webcompile/_28088_0.bc'
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-linux-gnu"

%struct.C = type { i32 }

@guard variable for get()::c = internal global i64 0            ; <i64*> [#uses=4]

declare i32 @__cxa_guard_acquire(i64*) nounwind

declare i8* @operator new(unsigned long)(i64)

declare void @__cxa_guard_release(i64*) nounwind

declare i8* @llvm.eh.exception() nounwind readonly

declare i32 @llvm.eh.selector(i8*, i8*, ...) nounwind

declare void @__cxa_guard_abort(i64*) nounwind

declare i32 @__gxx_personality_v0(...)

declare void @_Unwind_Resume_or_Rethrow(i8*)

define i32 @main() {
entry:
  %0 = load i8* bitcast (i64* @guard variable for get()::c to i8*), align 8 ; <i8> [#uses=1]
  %1 = icmp eq i8 %0, 0                           ; <i1> [#uses=1]
  br i1 %1, label %bb.i, label %_Z3getv.exit

bb.i:                                             ; preds = %entry
  %2 = tail call i32 @__cxa_guard_acquire(i64* @guard variable for get()::c) nounwind ; <i32> [#uses=1]
  %3 = icmp eq i32 %2, 0                          ; <i1> [#uses=1]
  br i1 %3, label %_Z3getv.exit, label %bb1.i

bb1.i:                                            ; preds = %bb.i
  %4 = invoke i8* @operator new(unsigned long)(i64 4)
          to label %invcont.i unwind label %lpad.i ; <i8*> [#uses=2]

invcont.i:                                        ; preds = %bb1.i
  %5 = bitcast i8* %4 to %struct.C*               ; <%struct.C*> [#uses=1]
  %6 = bitcast i8* %4 to i32*                     ; <i32*> [#uses=1]
  store i32 5, i32* %6, align 4
  tail call void @__cxa_guard_release(i64* @guard variable for get()::c) nounwind
  br label %_Z3getv.exit

lpad.i:                                           ; preds = %bb1.i
  %eh_ptr.i = tail call i8* @llvm.eh.exception()  ; <i8*> [#uses=2]
  %eh_select12.i = tail call i32 (i8*, i8*, ...)* @llvm.eh.selector(i8* %eh_ptr.i, i8* bitcast (i32 (...)* @__gxx_personality_v0 to i8*), i8* null) ; <i32> [#uses=0]
  tail call void @__cxa_guard_abort(i64* @guard variable for get()::c) nounwind
  tail call void @_Unwind_Resume_or_Rethrow(i8* %eh_ptr.i)
  unreachable

_Z3getv.exit:                                     ; preds = %invcont.i, %bb.i, %entry
  %_ZZ3getvE1c.0 = phi %struct.C* [ null, %bb.i ], [ %5, %invcont.i ], [ null, %entry ] ; <%struct.C*> [#uses=1]
  %7 = getelementptr inbounds %struct.C* %_ZZ3getvE1c.0, i64 0, i32 0 ; <i32*> [#uses=1]
  %8 = load i32* %7, align 4                      ; <i32> [#uses=1]
  ret i32 %8
}

Noteworth, no code is emitted for ::get, but main still allocates ::get::c (at %4) with a guard variable as needed (at %2 and at the end of invcont.i and lpad.i). llvm here is inlining all of that stuff.

tl;dr: Don't worry about it, the optimizer normally gets this stuff right. Are you seeing an error?

Answer 3

Whether the compiler optimizes the function call or not is basically unspecified behavior as per the Standard. An unspecified behavior is basically a behavior which is chosen from a set of finite possibilities, but the choice may not be consistent every time. In this case, the choice is 'to optimize' or 'not', which the Standard does not specify and the implementation is also not supposed to document, as it is a choice which may not be consistently taken by a given implementation.

If the idea is just to 'touch', will it help if we just add a dummy volatile variable and dummy increment it in each call

eg

C& getC(){
   volatile int dummy;
   dummy++;
   // rest of the code
}