lildmh marked an inline comment 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:
> > > > > > > > > > > > > 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 struсtures/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.
Hi Alexey,
Sure, let's discuss this in the mailing list. I'll summarize it and send it to
the mailing list later.
> We have recursive (actually, not recursive, because you cannot use types
> recursively) mappers calls anyway, it is nature of struсtures/classes.
We won't have recursive calls with inlining.
> We have a lot of similar code. And I'm not sure that it can be optimized out.
I think it's even harder to optimized these code out when we move them into the
runtime.
> Yes, but it means that we have n extra runtime calls, where n is the number
> of branches in the structure/class tree.
I don't quite understand. It's still equal to the number of mappers in any case.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D59474/new/
https://reviews.llvm.org/D59474
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
