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doc: clarify semantics of vector bitwise shifts

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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.
This commit is contained in:
Alexander Monakov 2023-05-24 15:48:29 +03:00
parent bffc52838e
commit 9f926f3a0c
1 changed files with 8 additions and 1 deletions
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gcc/doc/extend.texi
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@ -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