Re: [PATCH] Pass correct memory attributes for build constant

[Kito Cheng <kito.cheng@gmail.com>]

>>test.c:
>>extern bar(unsigned char p[3][2]);
>>void foo(int i)
>>{
>>     unsigned char data[3][2] = {{1,1}, {1,0}, {1,1}};
>>
>>     bar(data);
>>}
> First, note, I'm not an ARM expert.  However, the first question I
> have is are we sure the initializer is always going to be suitably
> aligned?   What guarantees this initializer is going to have 32 bit

It's a ARRAY_TYPE for the data and ARM require 32-bit align for that.
(Aligned by DATA_ALIGNMENT as Jan say.)

> I think that needs to be settled first, then we need to verify that
> the trees are correctly carrying that alignment requirement around
> and that the code uses it appropriately (and I have my doubts
> because EXP is a CONSTRUCTOR element and those seem to be largely
> ignored in the code we're looking to change).

The key problem is `exp` don't have right alignment info, but `decl` have,
 we can observe this in the code:

varasm.c
3166   /* Construct the VAR_DECL associated with the constant.  */
3167   decl = build_decl (UNKNOWN_LOCATION, VAR_DECL, get_identifier (label),
3168                      TREE_TYPE (exp));
3169   DECL_ARTIFICIAL (decl) = 1;
3170   DECL_IGNORED_P (decl) = 1;
3171   TREE_READONLY (decl) = 1;
3172   TREE_STATIC (decl) = 1;
3173   TREE_ADDRESSABLE (decl) = 1;
...
3181   if (TREE_CODE (exp) == STRING_CST)
3182     {
3183 #ifdef CONSTANT_ALIGNMENT
3184       DECL_ALIGN (decl) = CONSTANT_ALIGNMENT (exp, DECL_ALIGN (decl));
3185 #endif
3186     }
3187   else
3188     align_variable (decl, 0);

`decl` get alignment info here but `exp` doesn't.

...
3203   rtl = gen_const_mem (TYPE_MODE (TREE_TYPE (exp)), symbol);
3204   set_mem_attributes (rtl, exp, 1);
but here, we use `exp` to set memory attribute for MEM rtl.

> I would also strongly recommend turning your testcase into something
> we can add to the testsuite.
>
> If you look in gcc/testsuite/gcc.target/arm you'll see several
> examples.  I think you just want to compile this down to assembly
> code with the optimizer enabled, then verify there is no call to
> memcpy in the resulting output.  20030909-1.c would seem to be a
> reasonable example of a test that does something similar.

Hmmm, it's not target dependent problem, but this problem only can
observe by some target since not every target use MEM_ALIGN info for code gen,
the most common case is movmem pattern, they use alignment info to
decide expand or not.

So I am not sure is does it reasonable to make a testcase for target?