On Mon, Sep 05, 2022 at 11:25:21AM +0200, Richard Biener wrote: > On Fri, Sep 2, 2022 at 7:24 PM Segher Boessenkool > <seg...@kernel.crashing.org> wrote: > > On Fri, Sep 02, 2022 at 12:02:54PM -0500, Peter Bergner wrote: > > > On 9/2/22 11:31 AM, Segher Boessenkool wrote: > > > > (Did you also look at non-MMA VIEW_CONVERT_EXPR uses btw?) > > > > > > I did. It seemed they were all related to pointers to vectors and I > > > remember > > > you mentioning that as one of the reasons for using VIEW_CONVERT_EXPR over > > > NOP_EXPR, so I left them alone to be safe. > > > > Huh? I have no idea what you mean here. > > > > Casting from one pointer type to another never needs it. Casting from a > > scalar integer type to a pointer type not either AFAIKi. But I am not a > > Gimple expert, all this might be wrong, it isn't documented anywbere :-( > > NOP_EXPR is for conversions between types with the same kind > (and pointer-to-integer and integer-to-pointer > conversions when pointer and integer are of the same size). > When used on vectors it converts the vector elements. When you want to > re-interpret V4SI as V4SF you need VIEW_CONVERT_EXPR (bit_cast), > likewise V4SI interpreted as V16QI needs that. > > Think of VIEW_CONVERT as bit_cast and NOP_EXPR as conversion. > Of course for some conversions (like unsigned int to int) you can also > use a VIEW_CONVERT since it's semantically the same. In those cases > we canonicalize to NOP_EXPR via folding.
Thanks for the explanation! About that last point... You say VIEW_CONVERT_EXPR is folded to NOP_EXPR where possible. Does that happen in all cases / can we depend on that? So that in target code like what started this thread the only real difference is documentation of intent? (Which never is unimportant of course!) Segher
