libstdc++: Improve fully dynamic extents in mdspan.

In mdspan related code, for extents with no static extents, i.e. only
dynamic extents, the following simplifications can be made:

  - The array of dynamic extents has size rank.
  - The two arrays dynamic-index and dynamic-index-inv become
  trivial, e.g. k[i] == i.
  - All elements of the arrays __{fwd,rev}_partial_prods are 1.

This commits eliminates the arrays for dynamic-index, dynamic-index-inv
and __{fwd,rev}_partial_prods. It also removes the indirection k[i] == i
from the source code, which isn't as relevant because the optimizer is
(often) capable of eliminating the indirection.

To check if it's working we look at:

  using E2 = std::extents<int, dyn, dyn, dyn, dyn>;

  int stride_left_E2(const std::layout_left::mapping<E2>& m, size_t r)
  { return m.stride(r); }

which generates the following

  0000000000000190 <stride_left_E2>:
  190:  48 c1 e6 02            shl    rsi,0x2
  194:  74 22                  je     1b8 <stride_left_E2+0x28>
  196:  48 01 fe               add    rsi,rdi
  199:  b8 01 00 00 00         mov    eax,0x1
  19e:  66 90                  xchg   ax,ax
  1a0:  48 63 17               movsxd rdx,DWORD PTR [rdi]
  1a3:  48 83 c7 04            add    rdi,0x4
  1a7:  48 0f af c2            imul   rax,rdx
  1ab:  48 39 fe               cmp    rsi,rdi
  1ae:  75 f0                  jne    1a0 <stride_left_E2+0x10>
  1b0:  c3                     ret
  1b1:  0f 1f 80 00 00 00 00   nop    DWORD PTR [rax+0x0]
  1b8:  b8 01 00 00 00         mov    eax,0x1
  1bd:  c3                     ret

We see that:

  - There's no code to load the partial product of static extents.
  - There's no indirection D[k[i]], it's just D[i] (as before).

On a test file which computes both mapping::stride(r) and
mapping::required_span_size, we check for static storage with

  objdump -h

we don't see the NTTP _Extents, anything (anymore) related to
_StaticExtents, __fwd_partial_prods or __rev_partial_prods. We also
check that the size of the reference object file (described three
commits prior) reduced by a few percent from 41.9kB to 39.4kB.

libstdc++-v3/ChangeLog:

	* include/std/mdspan (__mdspan::__all_dynamic): New function.
	(__mdspan::_StaticExtents::_S_dynamic_index): Convert to method.
	(__mdspan::_StaticExtents::_S_dynamic_index_inv): Ditto.
	(__mdspan::_StaticExtents): New specialization for fully dynamic
	extents.
	(__mdspan::__fwd_prod): New constexpr if branch to avoid
	instantiating __fwd_partial_prods.
	(__mdspan::__rev_prod): Ditto.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

0 files changed, 0 insertions, 0 deletions