[PATCH] Fix PR79908

[Bill Schmidt]

> On Mar 14, 2017, at 9:25 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> On Tue, Mar 14, 2017 at 3:20 PM, Bill Schmidt
> <wschmidt@linux.vnet.ibm.com> wrote:
>> 
>>> On Mar 14, 2017, at 7:50 AM, Bill Schmidt <wschmidt@linux.vnet.ibm.com> wrote:
>>> 
>>> 
>>>> On Mar 14, 2017, at 3:57 AM, Richard Biener <richard.guenther@gmail.com> wrote:
>>>> 
>>>> On Tue, Mar 14, 2017 at 1:04 AM, Bill Schmidt
>>>> <wschmidt@linux.vnet.ibm.com> wrote:
>>>>> 
>>>>> Index: gcc/tree-stdarg.c
>>>>> ===================================================================
>>>>> --- gcc/tree-stdarg.c   (revision 246109)
>>>>> +++ gcc/tree-stdarg.c   (working copy)
>>>>> @@ -1057,7 +1057,7 @@ expand_ifn_va_arg_1 (function *fun)
>>>>>             types.  */
>>>>>          gimplify_assign (lhs, expr, &pre);
>>>>>        }
>>>>> -       else
>>>>> +       else if (is_gimple_addressable (expr))
>>>>>        gimplify_expr (&expr, &pre, &post, is_gimple_lvalue, fb_lvalue);
>>>> 
>>>> This is wrong - we lose side-effects this way and the only reason we gimplify
>>>> is to _not_ lose them.
>>>> 
>>>> Better is sth like
>>>> 
>>>> Index: gcc/tree-stdarg.c
>>>> ===================================================================
>>>> --- gcc/tree-stdarg.c   (revision 246082)
>>>> +++ gcc/tree-stdarg.c   (working copy)
>>>> @@ -1058,7 +1058,7 @@ expand_ifn_va_arg_1 (function *fun)
>>>>          gimplify_assign (lhs, expr, &pre);
>>>>        }
>>>>      else
>>>> -         gimplify_expr (&expr, &pre, &post, is_gimple_lvalue, fb_lvalue);
>>>> +         gimplify_expr (&expr, &pre, &post, is_gimple_val, fb_either);
>>>> 
>>>>      input_location = saved_location;
>>>>      pop_gimplify_context (NULL);
>>> 
>>> OK, thanks for the explanation.  Unfortunately this fails bootstrap with a failed
>>> assert a little later.  I'll dig further.
>> 
>> Looks like is_gimple_val is too restrictive for MEM_REFs, for which is_gimple_lvalue
>> passes.  Trying this now:
> 
> Hmm, it should simply gimplify to a load if it's not aggregate.  Can
> you share a testcase
> where it doesn't work?

So taking a step back...

The original problem is that most VA_ARGs get gimplified to a single SSA var,
which always passes the is_gimple_addressable check, so if the LHS has either
been cast away or gone dead, we get a nice lvalue to work with and everyone is
happy.

For a VA_ARG of _Complex type, gimplification instead produces a
COMPLEX_EXPR <a, b> where a and b have been reduced to SSA vars.
The is_gimple_addressable check does not pass in this case, so we can't
find the mandated lvalue and ICE in the gimplifier after doing all the work.

I suppose we could paper over this with:

Index: gcc/tree-stdarg.c                                                        
===================================================================
--- gcc/tree-stdarg.c   (revision 246109)                                       
+++ gcc/tree-stdarg.c   (working copy)                                          
@@ -1058,7 +1058,10 @@ expand_ifn_va_arg_1 (function *fun)
            gimplify_assign (lhs, expr, &pre);                                  
          }                                                                     
        else                                                                    
-         gimplify_expr (&expr, &pre, &post, is_gimple_lvalue, fb_lvalue);      
+         gimplify_expr (&expr, &pre, &post, is_gimple_lvalue,                  
+                        fb_lvalue | fb_mayfail);                               

...but that is perhaps opening the gate too wide (though we could throw
an error if gimplify_expr fails and expr is not a COMPLEX_EXPR).  The 
COMPLEX_EXPR really is not addressable, but forcing any COMPLEX_EXPR
to a variable in gimplify_expr also seems like overkill.  (All of the tcc_binary
expressions are treated the same; we would fail if somebody asked for an
lvalue of a PLUS_EXPR just the same.)  So offhand I don't see anything
better than the above.

Thoughts?

Thanks,
Bill