[PATCH u-boot 14/39] lib: crc32: make the crc_table variable non-const

[Pali Rohár]

On Sunday 07 March 2021 13:26:36 Marek Behun wrote:
> On Sun, 7 Mar 2021 06:02:16 +0100
> Marek Vasut <marex at denx.de> wrote:
> 
> > On 3/7/21 5:58 AM, Marek Behun wrote:
> > > On Sun, 7 Mar 2021 05:46:24 +0100
> > > Marek Vasut <marex at denx.de> wrote:
> > >   
> > >> On 3/7/21 5:25 AM, Marek Behún wrote:  
> > >>> When compiling with LTO, the compiler fails with an error saying that
> > >>> `crc_table` causes a section type conflict with `efi_var_buf`.
> > >>>
> > >>> This is because both are declared to be in the same section (via macro
> > >>> `__efi_runtime_data`), but one is const while the other is not.
> > >>>
> > >>> Make this variable non-const in order to fix this.  
> > >>
> > >> This does not look right, the crc32 array is constant.
> > >> Maybe what you want to do instead if create some __efi_constant_data
> > >> section ?  
> > > 
> > > Yes, this was the easier solution, and maybe is not ideal.
> > > 
> > > I thought it would not be much of a problem since this array can be
> > > nonconstant (generated after boot) if CONFIG_DYNAMIC_CRC_TABLE is
> > > enabled.
> > > 
> > > Anyway I don't much understand the EFI code so I wanted to poke into it
> > > as little as possible.  
> > 
> > Isn't the compiler capable of better optimization on constant stuff ?
> > That's pretty much what prompted my suggestion to add separate section.
> 
> Yes, but
> - for this case I don't think the compiler actually can do any
>   significat optimizations when declaring the table const, since it has
>   to access
>     tab[(crc ^ (x)) & 255]
>   I tried with arm compiler just now, with -O2 and -Os, and it
>   generated the same code either way
> - when using LTO, the compiler theoretically should be able to deduce
>   that the table is never written to
> 
> But if people insist on declaring the table const, I will look into
> this...

If you try to overwrite a const variable from the same code unit then
compiler throw an error. So declaring read-only variable as a const
could prevent people to unintentionally overwrite read-only variable.
And detect possible bad code.