[PATCH v3 5/8] libstdc++: Improve nearly fully dynamic extents in mdspan.

[Tomasz Kamiński]

From: Luc Grosheintz <luc.grosheintz@gmail.com>

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>
---
Patch v3 updates passes span for __all_dynamic.

 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 d98b91d4419..a9168af4acc 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
 	  {
-- 
2.49.0