Re: Postpone expanding va_arg until pass_stdarg

[Michael Matz <matz at suse dot de>]

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?

> - 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).

> 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.


Ciao,
Michael.