[PATCH] D59474: [OpenMP 5.0] Codegen support for user-defined mappers

[Lingda Li via Phabricator via cfe-commits]

lildmh marked 2 inline comments as done.
lildmh added inline comments.


================
Comment at: lib/CodeGen/CGOpenMPRuntime.cpp:8739
+///     // For each component specified by this mapper:
+///     if (currentComponent.hasMapper())
+///       (*currentComponent.Mapper())(rt_mapper_handle, arg_base, arg_begin,
----------------
ABataev wrote:
> lildmh wrote:
> > ABataev wrote:
> > > lildmh wrote:
> > > > ABataev wrote:
> > > > > lildmh wrote:
> > > > > > ABataev wrote:
> > > > > > > lildmh wrote:
> > > > > > > > ABataev wrote:
> > > > > > > > > lildmh wrote:
> > > > > > > > > > ABataev wrote:
> > > > > > > > > > > lildmh wrote:
> > > > > > > > > > > > ABataev wrote:
> > > > > > > > > > > > > Currently `currentComponent` is generated by the compiler. But can we instead pass this data as an extra parameter to this `omp_mapper` function.
> > > > > > > > > > > > Emm, I think this scheme will be very difficult and inefficient. If we pass components as an argument of `omp_mapper` function, it means that the runtime needs to generate all components related to a map clause. I don't think the runtime is able to do that efficiently. On the other hand, in the current scheme, these components are naturally generated by the compiler, and the runtime only needs to know the base pointer, pointer, type, size. etc.
> > > > > > > > > > > With the current scheme, we may end with the code blowout. We need to generate very similar code for different types and variables. The worst thing here is that we will be unable to optimize this huge amount of code because the codegen relies on the runtime functions and the code cannot be inlined. That's why I would like to move as much as possible code to the runtime rather than to emit it in the compiler. 
> > > > > > > > > > I understand your concerns. I think this is the best we can do right now.
> > > > > > > > > > 
> > > > > > > > > > The most worrisome case will be when we have nested mappers within each other. In this case, a mapper function will call another mapper function. We can inline the inner mapper functions in this scenario, so that these mapper function can be properly optimized. As a result, I think the performance should be fine.
> > > > > > > > > Instead, we can use indirect function calls passed in the array to the runtime. Do you think it is going to be slower? In your current scheme, we generate many runtime calls instead. Could you try to estimate the number of calls in cases if we'll call the mappers through the indirect function calls and in your cuurent scheme, where we need to call the runtime functions many times in each particular mapper?
> > > > > > > > Hi Alexey,
> > > > > > > > 
> > > > > > > > Sorry I don't understand your idea. What indirect function calls do you propose to be passed to the runtime? What are these functions supposed to do?
> > > > > > > > 
> > > > > > > > The number of function calls will be exactly equal to the number of components mapped, no matter whether there are nested mappers or not. The number of components depend on the program. E.g., if we map a large array section, then there will be many more function calls.
> > > > > > > I mean the pointers to the mapper function, generated by the compiler. In your comment, it is `c.Mapper()`
> > > > > > If we pass nested mapper functions to the runtime, I think it will slow down execution because of the extra level of indirect function calls. E.g., the runtime will call `omp_mapper1`, which calls the runtime back, which calls `omp_mapper2`, .... This can result in a deep call stack.
> > > > > > 
> > > > > > I think the current implementation will be more efficient, which doesn't pass nested mappers to the runtime. One call to the outer most mapper function will have all data mapping done. The call stack will be 2 level deep (the first level is the mapper function, and the second level is `__tgt_push_mapper_component`) in this case from the runtime. There are also more compiler optimization space when we inline all nested mapper functions.
> > > > > Yes, if we leave it as is. But if instead of the bunch unique functions we'll have the common one, that accept list if indirect pointers to functions additionally, and move it to the runtime library, we won't need those 2 functions we have currently. We'll have full access to the mapping data vector in the runtime library and won't need to use those 2 accessors we have currently. Instead, we'll need just one runtime functions, which implements the whole mapping logic. We still need to call it recursively, but I assume the number of calls will remain the same as in the current scheme. Did you understand the idea? If yes, it would good if you coild try to estimate the number of function calls in current scheme and in this new scheme to estimate possible pros and cons.
> > > > Hi Alexey,
> > > > 
> > > > Could you give an example for this scheme? 1) I don't understand how the mapper function can have full access to the mapping data vector without providing these 2 accessors. 2) I don't think it is possible to have a common function instead of bunch of unique functions for each mapper declared.
> > > Hi Lingda, something like this.
> > > ```
> > > void __tgt_mapper(void *base, void *begin, size_t size, int64_t type, auto components[]) {
> > >   // Allocate space for an array section first.
> > >   if (size > 1 && !maptype.IsDelete)
> > >      <push>(base, begin, size*sizeof(Ty), clearToFrom(type));
> > >    
> > >   // Map members.
> > >   for (unsigned i = 0; i < size; i++) {
> > >      // For each component specified by this mapper:
> > >      for (auto c : components) {
> > >        if (c.hasMapper())
> > >          (*c.Mapper())(c.arg_base, c.arg_begin, c.arg_size, c.arg_type);
> > >        else
> > >          <push>(c.arg_base, c.arg_begin, c.arg_size, c.arg_type);
> > >      }
> > >   }
> > >   // Delete the array section.
> > >   if (size > 1 && maptype.IsDelete)
> > >     <push>(base, begin, size*sizeof(Ty), clearToFrom(type));
> > > }
> > > 
> > > void <type>.mapper(void *base, void *begin, size_t size, int64_t type) {
> > >  auto sub_components[] = {...};
> > >  __tgt_mapper(base, begin, size, type, sub_components);
> > > }
> > > ```
> > > 
> > Hi Alexey,
> > 
> > I don't think this scheme is more efficient than the current scheme. My reasons are:
> > 
> > 1) Most code here is essentially to generate `components`, i.e., we need to generate `c.arg_base, c.arg_begin, c.arg_size, c.arg_type` for each `c` in `components`, so there will still be a lot of code in `<type>.mapper`. It will not reduce the mapper function code, i.e., we will still have a bunch of unique mapper functions.
> > 
> > 2) This scheme will prevent a lot of compiler optimization from happening. In reality, a lot of computation should be redundant. E.g., for two components `c1` and `c2`, `c1`'s base may be the same as `c2`'s begin, so the compiler will be able to eliminate these reduction computation, especially when we inline all nested mapper functions together. If we move these computation into the runtime, the compiler will not be able to do such optimization.
> > 
> > 3) In terms of the number of `push` function calls, this scheme has the exact same number of calls as the current scheme, so I don't think this scheme can bring performance benefits. The scheme should perform worse than the current scheme, because it reduces the opportunities of compiler optimization as mentioned above.
> Hi Lingda, I'm trying to simplify the code generated by clang and avoid some unnecessary code duplications. If the complexity of this scheme is the same as proposed by you, I would prefer to use this scheme unless there are some other opinions.
> 1. It is not a problem. This code is unique and is not duplicated in the different mappers.
> 2. Inlining is no solution here. We still generate to much code, which is almost the same in many cases and it will lead to very ineffective codegen because we still end up with a lot of almost the same code. This also might lead to poor performance.
> 3. Yes, the number of pushes is always the same, in all possible schemes. It would be good to compare somehow the performance of both schemes, at least preliminary.
> 
> Also, this solution reduces the number of required runtime functions, instead of 2 we need just 1 and, thus, we need to make fewer runtime functions calls.
> 
> I think it would better to propose this scheme as an alternate design and discuss it in the OpenMP telecon. What do you think? Or we can try to discuss it in the offline mode via the e-mail with other members.
> I'm not trying to convince you to implement this scheme right now, but it would be good to discuss it. Maybe it will lead to some better ideas from others?
Hi Alexey,

I still prefer the current scheme, because:
1) I don't like recursive mapper calls, which goes back to my original scheme a little bit. I really think inlining can make a big difference when we have nested mappers. These compiler optimizations are the keys to have better performance for mappers.
2) I don't think the codegen here is inefficient. Yes there is duplicated code across different mapper functions, but why that will lead to poor performance?
3) Although we have 2 runtime functions now, the `__tgt_mapper_num_components` is called only once per mapper. It should have very negligible performance impact.

But if you have a different option, we can discuss it next time in the meeting. I do have a time constraint to work on the mapper implementation. I'll no longer work in this project starting this September, and I have about 30% of my time working on it until then.


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Comment at: lib/CodeGen/CGOpenMPRuntime.cpp:747
+  // Call to void __tgt_push_mapper_component(void *rt_mapper_handle, void
+  // *base, void *begin, size_t size, int64_t type);
+  OMPRTL__tgt_push_mapper_component,
----------------
ABataev wrote:
> lildmh wrote:
> > ABataev wrote:
> > > Check the declaration of the runtime function in the runtime patch and here. `size` parameter here has type `size_t`, while in runtime part it is `int64_t`
> > The runtime part uses `int64_t` because I see every other runtime function use `int64_t` instead of `size_t` for `size`, e.g., `__tgt_target, __tgt_target_teams`, etc., despite that they are declared to be `size_t` in Clang codegen. So I guess it is done on purpose? Otherwise we need to modify all these runtime function interface in the future.
> I recently committed the patch that fixes this problem in clang. If you're using `int64_t` in the runtime, the same type must be used in clang codegen.
That's nice to know. Then I will change the type of `size` to `int64_t` as well.


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https://reviews.llvm.org/D59474