[Bug target/58744] Illegal Memory Access on 3-byte packed struct ARCH: x86_64

["marcovanotti15+gcc at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58744

--- Comment #3 from Marco Vanotti <marcovanotti15+gcc at gmail dot com> ---
(In reply to Richard Biener from comment #2)
> Confirmed.  On i?86 we properly do a 16byte and a 8byte access (but we copy
> to stack anyway).
>

Yes, if the value is passed on the stack, it gets copied right. (For example,
if it is the seventh parameter of a function, it will be passed on the stack
and will be copied right).

The thing is that in the x86_64 calling convention it has to be passed on
registers while the are available (rdi, rsi, rdx, rcx, r8 and r9).

Reading the source code, precisely the gcc/calls.c file:
http://gcc.gnu.org/viewcvs/gcc/trunk/gcc/calls.c?revision=203967&view=markup

---

The params that are passed on the stack are handled in line 3027, which says:

/* Now store (and compute if necessary) all non-register parms.
These come before register parms, since they can require block-moves,
which could clobber the registers used for register parms.
Parms which have partial registers are not stored here,
but we do preallocate space here if they want that.  */

Assuming that the registers may not require block-moves.

It uses the function "store_one_arg" to store the arg in the stack (it doesn't
work with a non-partial register).

---

After a while, the function "load_register_parameters" (line 1860) is called,
in this function, it falls in the case:

move_block_to_reg (REGNO (reg), mem, nregs, args[i].mode);

where nregs == 1.

So a whole register is copied.

---

I don't know how this issue should be fixed, should we copy the register into
pseudos before the "load_register_parameters" ? Or should we change the
move_block_to_reg function to make the right size of move instructions, for
x86_64 we don't need "backup-registers", but maybe this bug is also in another
arch. 

size 3:

mov di, [rax]
sal rdi, 16
mov dil, [rax]

---

size 5:

mov edi, [rax]
sal rdi, 8
mov dil, [rax]

---

size 6:

mov edi, [rax]
sal rdi, 16
mov di, [rax]

---

size 7:

mov edi, [rax] ;move 4
sal rdi, 24
mov di, [rax]  ;move 3
sal rdi, 16
mov dil, [rax]

-----------------------------

I would gladly submit a patch if I can get some advice on how this should be
fixed :)