[llvm-dev] help me understand how nounwind attribute on functions works?

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Wed Feb 8 10:58:06 PST 2017

I think this behavior is intended to allow better LTO between C and C++.
Consider this kind of code:

// foo.h
extern "C" int doThing(bool canThrow);
// foo.cpp
int doThing(bool canThrow) {
  if (hadError) {
    if (canThrow) throw MyException; else return -1;
// bar.c
#include "foo.h"
void f() {
  doThing(false); // don't declare doThing as nounwind
// baz.cpp

Basically, compiling a C declaration for a function is not an assertion
that it never throws an exception. However, *defining* a function in a C TU
implies that it will not throw an exception.

On Sun, Feb 5, 2017 at 8:32 AM, Andrew Kelley via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> from http://llvm.org/docs/LangRef.html:
> nounwind
>> This function attribute indicates that the function never raises an
>> exception. If the function does raise an exception, its runtime behavior is
>> undefined. However, functions marked nounwind may still trap or generate
>> asynchronous exceptions. Exception handling schemes that are recognized by
>> LLVM to handle asynchronous exceptions, such as SEH, will still provide
>> their implementation defined semantics.
> Some things I noticed by looking at various C test programs with clang -S
> -emit-llvm:
>  * If a function definition is provided, it gets nounwind, even if it
> calls a non-nounwind function
>  * If a function is external (no definition provided), it does not get
> nounwind
> What I don't understand is:
>  * A non-nounwind function that calls a nounwind function, wouldn't the
> ability for an exception to be thrown transfer? I thought if you don't
> catch an exception, it bubbles up to the caller and so on.
>  * More importantly, if compiling in C mode, why doesn't every function
> have nounwind? I thought that C does not have exceptions, so calling a
> function and having it throw an exception would be undefined behavior
> anyway. Wouldn't that mean every function should be nounwind?
> Regards,
> Andrew
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