Re: Use C99 designated initializers for some structs

Hi,

On 2018-08-29 20:35:57 -0400, Chapman Flack wrote:
> On 08/29/18 18:51, Tom Lane wrote:
> 
> > As against that, of course, explicitly zeroing fields that you know very
> > well are already zero eats some cycles.  I've occasionally wondered if
> 
> I haven't checked what a smart C99 compiler actually emits for a
> designated initializer giving a field a compile-time known constant zero.
> Is it sure to eat any more cycles than the same initializer with the field
> unmentioned?

It's unlikely that any compiler worth its salt will emit redundant zero
initializations after a struct initialization (it's dead trivial to
recognize than in any SSA like form, which I think most compilers use
these days, certainly gcc and clang).  What it can't optimize away
however is the x = makeNode(SomeType); x->foo = EquivalentToZero;
case.  Currently the compiler has no way to know that the memory is zero
initialized (except for the type member, which the compiler can see
being set).

Greetings,

Andres Freund