[gcc r14-1494] doc: clarify semantics of vector bitwise shifts

[Alexander Monakov <amonakov@gcc.gnu.org>]

https://gcc.gnu.org/g:9f926f3a0c8dee7edb6807323a1fe2a92b33bfaa

commit r14-1494-g9f926f3a0c8dee7edb6807323a1fe2a92b33bfaa
Author: Alexander Monakov <amonakov@ispras.ru>
Date:   Wed May 24 15:48:29 2023 +0300

    doc: clarify semantics of vector bitwise shifts
    
    Explicitly say that attempted shift past element bit width is UB for
    vector types.  Mention that integer promotions do not happen.
    
    gcc/ChangeLog:
    
            * doc/extend.texi (Vector Extensions): Clarify bitwise shift
            semantics.

Diff:
---
 gcc/doc/extend.texi | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index f9d13b495ad..cdbd4b34a35 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -12026,7 +12026,14 @@ elements in the operand.
 It is possible to use shifting operators @code{<<}, @code{>>} on
 integer-type vectors. The operation is defined as following: @code{@{a0,
 a1, @dots{}, an@} >> @{b0, b1, @dots{}, bn@} == @{a0 >> b0, a1 >> b1,
-@dots{}, an >> bn@}}@. Vector operands must have the same number of
+@dots{}, an >> bn@}}@.  Unlike OpenCL, values of @code{b} are not
+implicitly taken modulo bit width of the base type @code{B}, and the behavior
+is undefined if any @code{bi} is greater than or equal to @code{B}.
+
+In contrast to scalar operations in C and C++, operands of integer vector
+operations do not undergo integer promotions.
+
+Operands of binary vector operations must have the same number of
 elements. 
 
 For convenience, it is allowed to use a binary vector operation