[cfe-commits] [PATCH] CodeGen support for transparent_union

[Stefan Kristiansson]

On Fri, Aug 10, 2012 at 1:49 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Wed, Aug 8, 2012 at 11:52 PM, Stefan Kristiansson
> <stefan.kristiansson at saunalahti.fi> wrote:
>> Ping?
>
> Quick review:
>

Thanks a lot for taking the time to look at this!

> The changes to lib/CodeGen/CodeGenFunction.h and
> lib/CodeGen/CodeGenTypes.cpp don't make sense; transparent_union
> doesn't affect the representation of values of the given type, just
> the calling convention.  What issue are you trying to fix with those
> changes?
>

To be honest, when looking at this again I can't come up with a good
defense for those changes.
Without them something like this:

struct c {
  int a[256];
};

typedef union {
  struct c a;
} ARG __attribute__ ((__transparent_union__));

void foo(ARG u);

void bar(struct c a)
{
  foo(a);
}

turns into:
%struct.c = type { [256 x i32] }
%union.ARG = type { %struct.c }

define void @bar(%struct.c* nocapture byval align 8 %a) nounwind uwtable {
entry:
  %tmpcast = bitcast %struct.c* %a to %union.ARG*
  call void @foo(%union.ARG* byval align 8 %tmpcast) nounwind
  ret void
}

declare void @foo(%union.ARG* byval align 8)

and with them into:
%struct.c = type { [256 x i32] }

define void @bar(%struct.c* nocapture byval align 8 %a) nounwind uwtable {
entry:
  call void @foo(%struct.c* byval align 8 %a) nounwind
  ret void
}

declare void @foo(%struct.c* byval align 8)


but this looks like an non-issue since ultimately the calling
convention stays the same.

Attached is an updated patch with the changes to
lib/CodeGen/CodeGenFunction.h and lib/CodeGen/CodeGenTypes.cpp
removed.

> getFirstFieldInTransparentUnion is ugly, but transparent_union is
> really an ABI thing, and there isn't any obvious alternative place to
> handle that.
>

Ok, so I guess that's an necessary evil then.

> (BTW, you don't have to wait a whole month to send a ping; a few days
> is usually appropriate.)
>

I'll remember that :)

Stefan
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