Re: Postpone expanding va_arg until pass_stdarg

[Tom de Vries <Tom_deVries at mentor dot com>]

On 12-02-15 14:57, Michael Matz wrote:
> 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.

Hi,

Btw, I'm not happy about the ap copies, but I haven't been able to get rid of them.

> > > > 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.

I have not been able to reproduce this problem (with a bootstrap build on x86_64 
for all languages, and {unix/,unix/-m32} testing), so I've dropped this bit for now.

I've pushed the latest status to vries/expand-va-arg-at-pass-stdarg.

-ftree-stdarg-opt (the va_list_gpr/fpr_size optimization) has been renabled 
again. I needed patch "Always check phi-ops in optimize_va_list_gpr_fpr_size" 
for that.

With a similar bootstrap and reg-test as described above, there's only one 
failure left:
...
FAIL: gcc.dg/tree-ssa/stdarg-2.c scan-tree-dump stdarg "f15: va_list escapes 0, 
needs to save [148] GPR units and [1-9][0-9]* FPR units"
...
And this is due to the ap copy, which is classified as escape.

[ We're still expanding ifn_va_arg before the va_list_gpr/fpr_size optimization. ]

Thanks,
- Tom