[gcc r17-2588] docs: Document that scatters are left-to-right.

https://gcc.gnu.org/g:066b2291bcafe507f8576d01518028532fce954f

commit r17-2588-g066b2291bcafe507f8576d01518028532fce954f
Author: Robin Dapp <[email protected]>
Date:   Tue Jul 21 10:37:46 2026 +0200

    docs: Document that scatters are left-to-right.
    
    This changes the docs to reflect the vectorizer's assumption that
    scatter stores are ordered.  Both, scatters and strided stores are
    affected.
    
    gcc/ChangeLog:
    
            * doc/md.texi: Document that scatters are left-to-right.

Diff:
---
 gcc/doc/md.texi | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index a98a572cc017..67c036d6bcb3 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -7439,11 +7439,11 @@ Mask elements @var{i} with @var{i} > (operand 4 + operand 5) are ignored.
 
 @mdindex scatter_store@var{m}@var{n}
 @item @samp{scatter_store@var{m}@var{n}}
-Store a vector of mode @var{m} into several distinct memory locations.
-Operand 0 is a scalar base address and operand 1 is a vector of mode
-@var{n} containing offsets from that base.  Operand 4 is the vector of
-values that should be stored, which has the same number of elements as
-@var{n}.  For each element index @var{i}:
+Store a vector of mode @var{m} into several, not necessarily distinct,
+memory locations.  Operand 0 is a scalar base address and operand 1 is
+a vector of mode @var{n} containing offsets from that base.  Operand 4
+is the vector of values that should be stored, which has the same number
+of elements as @var{n}.  For each element index @var{i}:
 
 @itemize @bullet
 @item
@@ -7460,6 +7460,10 @@ store element @var{i} of operand 4 to that address.
 The value of operand 2 does not matter if the offsets are already
 address width.
 
+If two or more offset elements point to the same location, it is expected that
+the one with the highest element index is being written last.  This implies a
+left-to-right ordering of the store.
+
 @mdindex mask_scatter_store@var{m}@var{n}
 @item @samp{mask_scatter_store@var{m}@var{n}}
 Like @samp{scatter_store@var{m}@var{n}}, but takes an extra mask operand as
@@ -7476,7 +7480,7 @@ Mask elements @var{i} with @var{i} > (operand 6 + operand 7) are ignored.
 
 @mdindex mask_len_strided_store@var{m}
 @item @samp{mask_len_strided_store@var{m}}
-Store a vector of mode m into several distinct memory locations.
+Store a vector of mode m into several, not necessarily distinct, memory locations.
 Operand 0 is a scalar base address and operand 1 is scalar stride of Pmode.
 Operand 2 is the vector of values that should be stored, which is of mode @var{m}.
 operand 3 is mask operand, operand 4 is length operand and operand 5 is bias operand.