https://gcc.gnu.org/g:0197c3b15853de18d7bce386caed6f7fa792fb33
commit r16-3315-g0197c3b15853de18d7bce386caed6f7fa792fb33
Author: Luc Grosheintz <luc.groshei...@gmail.com>
Date: Sun Aug 3 22:57:27 2025 +0200
libstdc++: Improve nearly fully dynamic extents in mdspan.
One previous commit optimized fully dynamic extents; and another
refactored __size such that __fwd_prod is valid for __r = 0, ..., rank
(exclusive).
Therefore, by noticing that __rev_prod (and __fwd_prod) never accesses
the first (or last) extent, one can avoid pre-computing partial products
of static extents in those cases, if all other extents are dynamic.
We check that the size of the reference object file decreases further
and the .rodata sections for
__fwd_prod<dyn, ..., dyn, 11>
__rev_prod<3, dyn, ..., dyn>
are absent.
libstdc++-v3/ChangeLog:
* include/std/mdspan (__fwd_prods): Relax condition for
fully-dynamic
extents to cover (dyn, ..., dyn, X).
(__rev_partial_prods): Analogous for (X, dyn, ..., dyn).
Reviewed-by: Tomasz Kamiński <tkami...@redhat.com>
Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
Diff:
---
libstdc++-v3/include/std/mdspan | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index d98b91d44192..a9168af4acce 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -469,7 +469,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return 1;
else if constexpr (__rank == 2)
return __r == 0 ? 1 : __exts.extent(0);
- else if constexpr (__all_dynamic(__sta_exts))
+ else if constexpr (__all_dynamic(std::span(__sta_exts).first(__rank-1)))
return __extents_prod(__exts, 1, 0, __r);
else
{
@@ -489,7 +489,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return 1;
else if constexpr (__rank == 2)
return __r == 0 ? __exts.extent(1) : 1;
- else if constexpr (__all_dynamic(__sta_exts))
+ else if constexpr (__all_dynamic(std::span(__sta_exts).last(__rank-1)))
return __extents_prod(__exts, 1, __r + 1, __rank);
else
{
