[Bug c++/20280] [4.0/4.1 regression] ICE in create_tmp_var, at gimplify.c:368

aoliva at redhat dot com gcc-bugzilla@gcc.gnu.org
Fri Mar 4 06:01:00 GMT 2005

------- Additional Comments From aoliva at gcc dot gnu dot org  2005-03-04 06:01 -------
Subject: Re: [PR c++/20280] hoist indirect_ref out of addressable cond_exprs

On Mar  3, 2005, Mark Mitchell <mark@codesourcery.com> wrote:

> Alexandre Oliva wrote:
> \
>> I went ahead and verified that I didn't break bit-field lvalues in
>> conditional expressions (my first attempt did), but I was surprised to
>> find out that the calls to h() pass.  I understand why they do (we
>> create a temporary and bind to that), but I'm not sure this is correct
>> behavior.  Opinions?

>> +  // Hmm...  I don't think these should be accepted.  The conditional
>> +  // expressions are lvalues for sure, and 8.5.3/5 exempts lvalues
>> +  // that are bit-fields, but not lvalues that are conditional
>> +  // expressions involving bit-fields.
>> +  h (b ? x.i : x.j);
>> +  h (b ? x.i : x.k);
>> +  h (b ? x.j : x.k);

> That's legal because "h" takes a "const &", which permits the compiler
> to create a temporary.

Yeah, it permits, but only in certain circumstances that AFAICT aren't
met.  This expression AFAICT is an lvalue that isn't a bit-field, so
it has to bind directly, per the first bullet in 8.5.3/5.  Since it
meets the conditions of this first bullet, it doesn't get to use the
`otherwise' portion of that paragraph, that creates a temporary.  Or
am I misreading anything?

> And, I think these kinds of transformations (if necessary) should be
> done in a langhook during gimplification, not at COND_EXPR-creation
> time.  We really want the C++ front-end's data structures to be an
> accurate mirror of the input program for as long as possible.

Err...  But in what sense does my patch change that?  See, what I'm
doing is hoisting the indirect_refs that are inserted by
stabilize_reference out of the cond_expr.  They weren't in the
original code.  There's no dereferencing going on unless the whole
expression undergoes lvalue-to-rvalue decay, so I'd argue that the
transformation I'm proposing actually matches even more accurately the
meaning of the original source code.



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