[llvm-dev] Confusions around nocapture and sret

[Johannes Doerfert via llvm-dev]

On 2/22/21 11:31 AM, Jameson Nash via llvm-dev wrote:
>> end up generating this IR after optimisation
> AFAICT, that's the IR before optimization, as seen from `clang -S
> -emit-llvm -O0 -o - -x c -`.
>
>> I strongly suggest to emit nocapture with sret in the frontend instead.
> It seems like `clang -x c` doesn't emit nocapture for even trivial cases I
> tried (like the above, but without function `x`). Is that a bug then, or
> just not applicable to C?

I think one of my earlier statement was wrong as sret doesn't imply 
nocapture
the was we use it. The frontend could know if it does and place 
nocapture, but
it is not as straight forward as I hoped it would be. Unclear if it is worth
to add the logic to Clang but certainly it is good if frontends emit as many
attributes as possible.


> Regardless, I don't think this should be a verifier error for passing
> nocapture to maycapture, since it may be valid for a source language to
> know that a particular use of an argument is nocapture, even if the general
> function contract doesn't promise it.

Agreed. You can do that in C/C++ through clang as well.


>
> On Mon, Feb 22, 2021 at 4:51 AM David Chisnall via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> On 21/02/2021 18:39, Johannes Doerfert via llvm-dev wrote:
>>> I strongly suggest to emit nocapture with sret in the frontend
>>> instead.
>> I don't think that is actually feasible.  For example, consider this C++
>> file:
>>
>> ```c++
>> #include <set>
>>
>> struct Example;
>> std::set<Example*> live_examples;
>> struct Example {
>>          Example()
>>          {
>>                  live_examples.insert(this);
>>          }
>>          ~Example()
>>          {
>>                  live_examples.erase(live_examples.find(this));
>>          }
>> };
>>
>> Example somefn()
>> {
>>          Example e;
>>          return e;
>> }
>> ```
>>
>> In this example, guaranteed copy elision means that somefn allocates `e`
>> in the space provided for it in the caller, calling the constructor,
>> which then captures the value.  In the generated IR, the space for `e`
>> has the `sret` attribute but it is definitely not nocapture.
>>
>> You can also trigger this in C, though in the C case it is undefined
>> behaviour.  Consider this example:
>>
>> ```c
>> struct Foo
>> {
>>           int a[5];
>> };
>>
>> int x(struct Foo *);
>>
>> struct Foo f(void)
>> {
>>           struct Foo foo;
>>           x(&foo);
>>           return foo;
>> }
>> ```
>>
>> The source-language semantics guarantee that no pointers to `foo`
>> outlive the invocation of `f`, which implies that `x` must not capture
>> the argument.  The optimisers take advantage of the fact that it would
>> be UB to compare the address of foo after the end of `f` to any other
>> allocation and we end up generating this IR after optimisation, eliding
>> the copy:
>>
>> ```
>> ; Function Attrs: nounwind uwtable
>> define dso_local void @f(%struct.Foo* noalias sret(%struct.Foo) align 4
>> %0) local_unnamed_addr #0 {
>>     %2 = tail call i32 @x(%struct.Foo* %0) #2
>>     ret void
>> }
>> ```
>>
>> Nothing in the IR says that `x`'s argument is nocapture.  Whether this
>> is permitted depends on what we want nocapture to mean.  There are two
>> possible interpretations:
>>
>>    - The callee does not capture the argument, if the callee does capture
>> the argument then the IR is ill-formed and we have a compiler bug.
>>    - The caller is free to assume that the callee does not capture the
>> argument, if the callee does capture the argument then it is UB.
>>
>> The former allows the absence of nocapture to be interpreted as 'we
>> can't statically prove that the argument is not captured'.  This is very
>> useful for memory-safety work, because it allows us to trust `nocapture`
>> as a security property: we can emit any further analysis.
>>
>> The latter allows optimisations to be more aggressive but will sometimes
>> generate more surprising code for users and may break some security
>> properties if security-related transforms depend on this information.
>>
>> My personal bias is towards the former: we would like to be able to use
>> `nocapture` in stack temporal safety work as a strong guarantee.  As
>> such, the front end could not insert it because transforms may later
>> insert a capture.  Alternatively, the module verifier should be updated
>> to ensure that a nocapture argument is not passed to any other function
>> except via a nocapture argument.
>>
>> David
>>
>>
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