On Wed, May 24, 2023 at 2:54 PM Alexander Monakov via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > Explicitly say that bitwise shifts for narrow types work similar to > element-wise C shifts with integer promotions, which coincides with > OpenCL semantics.
Do we need to clarify that v << w with v being a vector of shorts still yields a vector of shorts and not a vector of ints? Btw, I don't see this promotion reflected in the IL. For typedef short v8hi __attribute__((vector_size(16))); v8hi foo (v8hi a, v8hi b) { return a << b; } I get no masking of 'b' and vector lowering if the target doens't handle it yields short int _5; short int _6; _5 = BIT_FIELD_REF <a_1(D), 16, 0>; _6 = BIT_FIELD_REF <b_2(D), 16, 0>; _7 = _5 << _6; which we could derive ranges from for _6 (apparantly we don't yet). Even typedef int v8hi __attribute__((vector_size(16))); v8hi x; int foo (v8hi a, v8hi b) { x = a << b; return (b[0] > 33); } isn't optimized currently (but could - note I've used 'int' elements here). So, I don't see us making sure the hardware does the right thing for out-of bound values. Richard. > gcc/ChangeLog: > > * doc/extend.texi (Vector Extensions): Clarify bitwise shift > semantics. > --- > gcc/doc/extend.texi | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi > index e426a2eb7d..6b4e94b6a1 100644 > --- a/gcc/doc/extend.texi > +++ b/gcc/doc/extend.texi > @@ -12026,7 +12026,12 @@ 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@}}@. When the base type is narrower than @code{int}, > +element-wise shifts are performed as if operands underwent C integer > +promotions, like in OpenCL. This makes vector shifts by up to 31 bits > +well-defined for vectors with @code{char} and @code{short} base types. > + > +Operands of binary vector operations must have the same number of > elements. > > For convenience, it is allowed to use a binary vector operation > -- > 2.39.2 >