[LLVMdev] A limitation of LLVM with regard to marking sret functions as readonly.

[Nicholas Chapman]

On 08/03/2015 18:07, Daniel Berlin wrote:
>
>
> On Sun, Mar 8, 2015 at 9:55 AM, Nicholas Chapman 
> <admin at indigorenderer.com <mailto:admin at indigorenderer.com>> wrote:
>
>     Hi all,
>     I have identified what seems to be a limitation of LLVM with
>     regard to marking 'sret functions' as pure/readonly.
>
>     For some context - I have some JITed code produced by LLVM, and
>     that code calls back into the host application occasionally.
>     Since my language is purely functional, no functions have
>     side-effects.  Therefore I would like to be able to cache the
>     value of identical calls back into the host application, e.g.
>     if f is a function exported by the host application,
>     f(1) + f(1)
>     should only call f(1) once, then cache and reuse the value instead
>     of making a second call.
>
>     The problem is is that some of the exported functions need to
>     return structures.  As such a pointer to the returned structure is
>     passed as the first argument and marked with the sret attribute.
>     However due to this, that function can no longer be marked with
>     the 'readonly' attribute (as it needs to write to the returned
>     structure).  (I tried marking the function as readonly anyway and
>     the function is incorrectly compiled to 'ret void' or similar)
>
>     There seems to be a similar problem with C++ code:
>
>     ------------------------------------
>     class s
>     {
>       public:
>       float x;
>       float y;
>     };
>
>     extern s g(void* env)  __attribute__ ((pure));
>
>
>     float func(float x, void* env)
>     {
>       return g(env).x + g(env).x;
>     }
>     ------------------------------------
>
>     Compiles the function 'func' to just make a single call to g. 
>     Note that 's' is returned directly instead of being converted into
>     an SRET arg, due to the small size of the class 's'.  See
>     http://goo.gl/ezXxrI
>
>     If we make the class bigger, so it gets returned by SRET:
>
>     ----------------------------------
>     class s
>     {
>       public:
>       float x;
>       float y;
>
>       int a;
>       int b;
>       int c;
>     };
>
>     extern s g(void* env)  __attribute__ ((pure));
>
>
>     float func(float x, void* env)
>     {
>       return g(env).x + g(env).x;
>     }
>     ---------------------------------
>
>     Then LLVM compiles 'func' to have 2 calls to g, and g is no longer
>     marked as 'readonly'  See http://goo.gl/YW0n3V
>
> You mean clang does.
> Because it is clang that is turning off the attributes, deliberately 
> (CGCall.cpp)
>
>
>     So it doesn't seem possible to me to mark an SRET function as
>     readonly.
>     To me this seems like a problem.
>
>
>
> I agree with this one that we should be able to CSE sret'd functions.
>
>
>     One way to fix this could be to change the semantics of the
>     readonly attribute - it could be changed to allow writing through
>     the SRET pointer argument only.
>
>
> Depends on what the actual semantics of readonly are supposed to be.
>
> Pure functions in GCC are allowed to write to local, 
> not-visible-to-caller memory (IIRC, it has been a while), but not 
> global memory.
> So your function is "pure" as defined by GCC (because it does not 
> access *global* memory, and has no side-effects). GCC, at least at the 
> level it does most pure optimization, would see it as byval and never 
> see it as  memory writing.
>
>
> LLVM has a more well specified definition of readonly:
> "
> On a function, this attribute indicates that the function does not 
> write through an pointer arguments (including byval arguments) or 
> otherwise modify any state (e.g. memory, control registers, etc) 
> visible to caller functions. It may dereference pointer arguments and 
> read state that may be set in the caller. A readonly function always 
> returns the same value (or unwinds an exception identically) when 
> called with the same set of arguments and global state. It cannot 
> unwind an exception by calling the C++ exception throwing methods."
>
> I don't think adding "ps it can write to sret arguments" would make 
> this definition particularly sound, but it's certainly arguable you 
> could add "it may write local stack memory in order to return 
> structure values only, but may not read that same local memory" or 
> something.
>
> But in situations like this, unless you can essentially prove that no 
> optimizer would ever change as a result of your definition change 
> (besides it making theoretical sense), it often makes more sense to 
> add a new attribute, and add it to the optimizers that can use it (or 
> abstract it where it makes sense so a new something like mayBeCSE'd 
> returns true for both readonly and readonly_except_for_sret, and have 
> the optimizers use this instead, but the optimizers that care about 
> the difference can still test the individual flags)
>
> Now, you are of course, welcome to make such an argument - go through 
> the optimizers, see what/where they use readonly, and whether it'd be 
> affected, and say "actually, they'd all still do the right thing, we 
> should change readonly".
>
> But if you wanted to make that argument, i feel like you'd have to do 
> the legwork.
>
> (It's probably worth starting by seeing why CGCall turns off the 
> attributes for sret. i'd find the commit that did this, and see if 
> there is a bugreport of some bug this caused, or if it was just done 
> because someone thought it was wrong without any actual effect.
> Because if you get to a bugreport, this will be a great way to figure 
> out what optimizer/etc would be affected by a change here)
>
Hi Daniel,
I think modifying the semantics of readonly would make more sense than 
introducing a new attribute called readonly_except_for_sret.
As it stands, readonly is completely useless for sret functions.  It 
can't be applied to them or the function is not compiled correctly. 
(Which is reasonable given the current readonly semantics)
I think the best approach would be to modify the readonly semantics, 
then update optimisation passes to respect the new semantics.

I had a look at SVN blame for the CGCall.cpp code, it wasn't very 
enlightening however as the last changes were just 'no functional 
change' commits.  I'm not super keen to dig back further, when the 
purpose of the code seems fairly obvious - without stripping the 
readonly attribute, sret functions will miscompile.

I'm not (currently) volunteering to do the work for this in LLVM - I'm 
just bringing the issue to the attention of the community.

Thanks,
     Nick
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