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

[Stefan Kristiansson]

On Sat, Aug 18, 2012 at 12:46 AM, Eli Friedman <eli.friedman at gmail.com> wrote:
>
> It looks like the call to getFirstFieldInTransparentUnion isn't in the
> right place on x86-32.
>

Well spotted, attached is a an updated patch with the call to
getFirstFieldInTransparentUnion moved into
X86_32ABIInfo::classifyArgumentTypeWithReg.

> I'm suspicious that ABIArgInfo::getDirect() isn't going to do the
> "right" thing in all cases (say, for example, a union with a "void*"
> and a "double"); that said, it probably doesn't in practice, given the
> expected usage of transparent_union.
>

I can't say that I'm able to think out the possible cases where it
might go wrong, I'm willing to take a look at anything you throw
at me though (if it's worth bending over backwards to handle all kinds of
possible and impossible non-real use-cases, that's another story ;)).

The void*, double union example looks like this:
$ cat transparent-union.c
typedef union {
  void *a;
  double b;
} ARG __attribute__ ((__transparent_union__));

void f0(ARG u);

void f1(void *a)
{
  f0(a);
}

void f2(double b)
{
  f0(b);
}


$ clang -O2 -c -S transparent-union.c -emit-llvm -o -
; ModuleID = 'transparent-union.c'
target datalayout =
"e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

define void @f1(i8* %a) nounwind uwtable {
entry:
  tail call void @f0(i8* %a) nounwind
  ret void
}

declare void @f0(i8*)

define void @f2(double %b) nounwind uwtable {
entry:
  %0 = bitcast double %b to i64
  %1 = inttoptr i64 %0 to i8*
  tail call void @f0(i8* %1) nounwind
  ret void
}


Which seems OK to me.

Stefan
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