Re: New regression on ARM Linux

[Richard Biener <rguenther@suse.de>]

On Tue, 31 Mar 2015, Jakub Jelinek wrote:

> On Tue, Mar 31, 2015 at 11:47:37AM +0100, Alan Lawrence wrote:
> > Richard Biener wrote:
> > >
> > >But I find it odd that on ARM passing *((aligned_int *)p) as
> > >vararg (only as varargs?) changes calling conventions independent
> > >of the functions type signature.
> > 
> > Does it? Do you have a testcase, and compilation flags, that'll make this
> > show up in an RTL dump? I've tried numerous cases, including AFAICT yours,
> > and I always get the value being passed in the expected ("unaligned")
> > register?
> 
> If the integral type alignment right now matters, I'd try something like:
> 
> typedef int V __attribute__((aligned (8)));
> V x;
> 
> int foo (int x, ...)
> {
>   int z;
>   __builtin_va_list va;
>   __builtin_va_start (va, x);
>   switch (x)
>     {
>     case 1:
>     case 3:
>     case 6:
>       z = __builtin_va_arg (va, int);
>       break;
>     default:
>       z = __builtin_va_arg (va, V);
>       break;
>     }
>   __builtin_va_end (va);
>   return z;
> }
> 
> int
> bar (void)
> {
>   V v = 3;
>   int w = 3;
>   foo (1, (int) v);
>   foo (2, (V) w);
>   v = 3;
>   w = (int) v;
>   foo (3, w);
>   foo (4, (V) w);
>   v = (V) w;
>   foo (5, v);
>   foo (6, (int) v);
>   foo (7, x);
>   return 0;
> }
> 
> (of course, most likely with passing a different value each time and
> verification of the result).
> As the compiler treats all those casts there as useless, I'd expect
> that the types of the passed argument would be pretty much random.
> And, note that even on x86_64, the __builtin_va_arg with V expands into
>   # addr.1_3 = PHI <addr.1_27(9), _31(10)>
>   z_35 = MEM[(V * {ref-all})addr.1_3];
> using exactly the same address for int as well as V va_arg - if you increase
> the overalignment arbitrarily, it will surely be a wrong IL because nobody
> really guarantees anything about the overalignment.
> 
> So, I think the tree-sra.c patch is a good idea - try to keep using the main
> type variants as the types in the IL where possible except for the MEM_REF
> first argument (i.e. even the lhs of the load should IMHO not be
> overaligned).

Yeah, I'm testing it right now as it seems to fix the regression and
should be certainly safe.

Richard.

> As Eric Botcazou said, GCC right now isn't really prepared for under or
> overaligned scalars, only when they are in structs (or for middle-end in
> *MEM_REFs).
> 	Jakub
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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