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Re: Postpone expanding va_arg until pass_stdarg


On 10-02-15 17:57, Michael Matz wrote:
Hi,

On Tue, 10 Feb 2015, Tom de Vries wrote:

I've added two modifications to gimplify_modify_expr:
- the WITH_SIZE_EXPR in which the CALL_TREE is wrapped, is dropped after
   gimplification, but we need the size expression at expansion in pass_stdarg.
   So I added the size expression as argument to the internal function.
   [ And at pass_stdarg::execute, we wrap the result of
gimplify_va_arg_internal
   in a WITH_SIZE_EXPR before generating the assign to the lhs ]

Hmm, why do you need the WITH_SIZE_EXPR actually?  For variable-sized
types returned by va_arg?


Yep.

- we detect after gimplify_arg (&ap) whether it created a copy ap.1 of ap,
   rather than use ap itself, and if so, we copy the value back from ap.1 to ap
   after va_arg.

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.

In gimplify_modify_expr, we're already doing effort not to generate temporaries for call lhs, see comment:
...
     To
     prevent gimplify_expr from trying to create a new temporary for
     foo's LHS, we tell it that it should only gimplify until it
     reaches the CALL_EXPR.  On return from gimplify_expr, the newly
     created GIMPLE_CALL <foo> will be the last statement in *PRE_P
     and all we need to do here is set 'a' to be its LHS
...

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.

Thanks,
- Tom



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