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libstdc++: Improve nearly fully dynamic extents in mdspan.

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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 <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
This commit is contained in:
Luc Grosheintz
2025-08-21 10:20:57 +02:00
committed by Tomasz Kamiński
parent 5bcaee96c6
commit 0197c3b158
1 changed files with 2 additions and 2 deletions
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libstdc++-v3/include/std/mdspan
+2 -2
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@@ -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
{