On Wed, 24 May 2023, Richard Biener wrote:
> 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? I don't think so, but if necessary we could add "and the result was truncated back to the base type": When the base type is narrower than @code{int}, element-wise shifts are performed as if operands underwent C integer promotions, and the result was truncated back to the base type, like in OpenCL. > 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). Here it depends on how we define the GIMPLE-level semantics of bit-shift operators for narrow types. To avoid changing lowering we could say that shifting by up to 31 bits is well-defined for narrow types. RTL-level semantics are also undocumented, unfortunately. > 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). Yeah. But let's constrain the optimizations first. > So, I don't see us making sure the hardware does the right thing for > out-of bound values. I think in practice it worked out even if GCC did not pay attention to it, because SIMD instructions had to facilitate autovectorization for C with corresponding shift semantics. Alexander > > 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 > > >