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Re: Postpone expanding va_arg until pass_stdarg
- From: Michael Matz <matz at suse dot de>
- To: Tom de Vries <Tom_deVries at mentor dot com>
- Cc: Richard Biener <richard dot guenther at gmail dot com>, Jakub Jelinek <jakub at redhat dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Thu, 12 Feb 2015 14:57:39 +0100 (CET)
- Subject: Re: Postpone expanding va_arg until pass_stdarg
- Authentication-results: sourceware.org; auth=none
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Hi,
On Wed, 11 Feb 2015, Tom de Vries wrote:
> > My idea was to not generate temporaries and hence copies for
> > non-scalar types, but rather construct the "result" of va_arg directly
> > into the original LHS (that would then also trivially solve the
> > problem of nno-copyable types).
>
> The copy mentioned here is of ap, not of the result of va_arg.
Whoops, I misread, yes. Thanks.
> > > I'm not really sure yet why std_gimplify_va_arg_expr has a part
> > > commented out. Michael, can you comment?
> >
> > I think I did that because of SSA form. The old sequence calculated
> >
> > vatmp = valist;
> > vatmp = vatmp + boundary-1
> > vatmp = vatmp & -boundary
> >
> > (where the local variable in that function 'valist_tmp' is the tree
> > VAR_DECL 'vatmp') and then continue to use valist_tmp. When in SSA form
> > the gimplifier will rewrite this into:
> >
> > vatmp_1 = valist;
> > vatmp_2 = vatmp_1 + boundary-1
> > vatmp_3 = vatmp_2 & -boundary
> >
> > but the local valist_tmp variable will continue to be the VAR_DECL, not
> > the vatmp_3 ssa name. Basically whenever one gimplifies a MODIFY_EXPR
> > while in SSA form it's suspicious. So the new code simply build the
> > expression:
> >
> > ((valist + bound-1) & -bound)
> >
> > gimplifies that into an rvalue (most probably an SSA name) and uses that
> > to go on generating code by making valist_tmp be that returned rvalue.
> >
> > I think you'll find that removing that code will make the SSA verifier
> > scream or generate invalid code with -m32 when that hook is used.
> >
>
> Thanks for the detailed explanation. I'm not sure I understand the
> problem well enough, so I'll try to trigger it and investigate.
Actually the above fails to mention what the real problem is :-) The
problem is that the local variable valist_tmp will be used to generate
further code after the above expression is generated. Without my patch it
will continue to point to the VAR_DECL, not to the SSA name that actually
holds the computed value in the generated code.
Ciao,
Michael.