On 7/28/25 08:32, Tomasz Kaminski wrote:
In the __fwd_prod, __rev_prod I would also add the case for all-dynamic
extents, so we do not instantiate an array with all 1, i.e. we would have:
constexpr size_t __rank = _Extents::rank();
+ size_t __sta_prod = _RevProd<_Extents>::_S_value[__r];
+ return __extents_prod(__exts, __sta_prod, __r + 1, __rank);
if constexpr (__rank == 1)
return 1;
else if constexpr (__rank == 2)
return __i == 0 ? _M_extents.extent(1) : 1;
else if constexpr (Extents::rank_dynamic() == __rank)
return __extents_prod(__exts, __1u, __r + 1, __rank);
else
{
size_t __sta_prod = _RevProd<_Extents>::_S_value[__r];
return __extents_prod(__exts, __sta_prod, __r + 1, __rank);
}
True, purely dynamic extents, is another interesting case to optimize,
I suspect there's more potential in that case. For example the
indirection E[i] := D[k[i]] is not needed because k[i] == i.
Therefore, since the series already had several commits, I chose
to leave it for later.
On Mon, Jul 28, 2025 at 8:02 AM Tomasz Kaminski <tkami...@redhat.com> wrote:
On Sun, Jul 27, 2025 at 2:47 PM Luc Grosheintz <luc.groshei...@gmail.com>
wrote:
The methods layout_{left,right}::mapping::stride are defined
as
\prod_{i = 0}^r E[i]
\prod_{i = r+1}^n E[i]
This is computed as the product of a pre-comupted static product and the
product of the required dynamic extents.
Disassembly shows that even for low-rank extents, i.e. rank == 1 and
rank == 2, with at least one dynamic extent, the generated code loads
two values; and then runs the loop over at most one element, e.g.
220: 48 8b 0c f5 00 00 00 mov rcx,QWORD PTR [rsi*8+0x0]
227: 00
228: 48 8b 04 f5 00 00 00 mov rax,QWORD PTR [rsi*8+0x0]
22f: 00
230: 48 c1 e1 02 shl rcx,0x2
234: 74 1a je 250 <stride_left_d5+0x30>
236: 48 01 f9 add rcx,rdi
239: 0f 1f 80 00 00 00 00 nop DWORD PTR [rax+0x0]
240: 48 63 17 movsxd rdx,DWORD PTR [rdi]
243: 48 83 c7 04 add rdi,0x4
247: 48 0f af c2 imul rax,rdx
24b: 48 39 f9 cmp rcx,rdi
24e: 75 f0 jne 240 <stride_left_d5+0x20>
250: c3 ret
If there's no dynamic extents, it simply loads the precomputed product
of static extents.
For rank == 1 the answer is constant `1`; for rank == 2 it's either 1 or
extents.extent(k), with k == 0 for layout_left and k == 1 for
layout_right.
Consider,
using Ed = std::extents<int, dyn>;
int stride_left_d(const std::layout_left::mapping<Ed>& m, size_t r)
{ return m.stride(r); }
using E3d = std::extents<int, 3, dyn>;
int stride_left_3d(const std::layout_left::mapping<E3d>& m, size_t r)
{ return m.stride(r); }
using Ed5 = std::extents<int, dyn, 5>;
int stride_left_d5(const std::layout_left::mapping<Ed5>& m, size_t r)
{ return m.stride(r); }
The optimized code for these three cases is:
0000000000000060 <stride_left_d>:
60: b8 01 00 00 00 mov eax,0x1
65: c3 ret
0000000000000090 <stride_left_3d>:
90: 48 83 fe 01 cmp rsi,0x1
94: 19 c0 sbb eax,eax
96: 83 e0 fe and eax,0xfffffffe
99: 83 c0 03 add eax,0x3
9c: c3 ret
00000000000000a0 <stride_left_d5>:
a0: b8 01 00 00 00 mov eax,0x1
a5: 48 85 f6 test rsi,rsi
a8: 74 02 je ac <stride_left_d5+0xc>
aa: 8b 07 mov eax,DWORD PTR [rdi]
ac: c3 ret
For rank == 1 it simply returns 1 (as expected). For rank == 2, it
either implements a branchless formula, or conditionally loads one
value. In all cases involving a dynamic extent this seems like it's
always doing clearly less work, both in terms of computation and loads.
For rank == 2, it trades loading one value for a branchless sequence of
four instructions that don't require loading any values.
I will put this optimization into the __fwd_prod and __rev_pord functions,
so it will be applied for all uses. This will also avoid us creating this
caching
tables for such small ranks.
libstdc++-v3/ChangeLog:
* include/std/mdspan (layout_left::mapping::stride): Optimize
for rank <= 2.
(layout_right::mapping::stride): Ditto.
Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
---
libstdc++-v3/include/std/mdspan | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/include/std/mdspan
b/libstdc++-v3/include/std/mdspan
index 06ccf3e3827..f288af96cdb 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -652,7 +652,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
requires (extents_type::rank() > 0)
{
__glibcxx_assert(__i < extents_type::rank());
- return __mdspan::__fwd_prod(_M_extents, __i);
+ if constexpr (extents_type::rank() == 1)
+ return 1;
+ else if constexpr (extents_type::rank() == 2)
+ return __i == 0 ? 1 : _M_extents.extent(0);
+ else
+ return __mdspan::__fwd_prod(_M_extents, __i);
}
template<typename _OExtents>
@@ -797,7 +802,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
requires (extents_type::rank() > 0)
{
__glibcxx_assert(__i < extents_type::rank());
- return __mdspan::__rev_prod(_M_extents, __i);
+ if constexpr (extents_type::rank() == 1)
+ return 1;
+ else if constexpr (extents_type::rank() == 2)
+ return __i == 0 ? _M_extents.extent(1) : 1;
+ else
+ return __mdspan::__rev_prod(_M_extents, __i);
}
template<typename _OExtents>
--
2.50.0
