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
