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

[Jeff Law <law@redhat.com>]

On 06/25/14 21:38, Kito Cheng wrote:
> For example in arm-elf-eabi, movmem need word align, otherwise it will
> expand a libcall:
>
> And gcc configure with "--target=arm-elf-eabi --disable-nls
> --disable-shared --enable-languages=c,c++ --enable-threads=single
> --enable-lto --with-newlib"
>
> 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 alignment 
like you want?  I can see how that *section* gets its alignment, but I 
don't offhand see what in the ARM backend ensures that a CONSTRUCTOR 
tree has larger than normal known alignment.

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).

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.


Jeff