ABataev 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,
----------------
lildmh wrote:
> 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.
Lingda,
1. We have recursive (actually, not recursive, because you cannot use types
recursively) mappers calls anyway, it is nature of struŃtures/classes.
2. We have a lot of similar code. And I'm not sure that it can be optimized out.
3. Yes, but it means that we have n extra runtime calls, where n is the number
of branches in the structure/class tree.
I see :(. I understand your concern. In this case, we could try to discuss it
offline, in the mailing list, to make it a little bit faster. We just need to
hear other opinions on this matter, maybe there are some other pros and cons
for these schemes.
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