Subject: Re: [PATCH v4 1/2] powerpc/uaccess: Implement unsafe_put_user() using 'asm goto'

From: Segher Boessenkool <[email-masked]>
Date: Thu, 11 Jun 2020 18:52:56 -0500

On Thu, Jun 11, 2020 at 03:43:55PM -0700, Nick Desaulniers wrote:
> Segher, Cristophe, I suspect Clang is missing support for the %L and %U
> output templates [1].

The arch/powerpc kernel first used the %U output modifier in 0c176fa80fdf
(from 2016), and %L in b8b572e1015f (2008).  include/asm-ppc (and ppc64)
have had %U since 2005 (1da177e4c3f4), and %L as well (0c541b4406a6).

> I've implemented support for some of these before
> in Clang via the documentation at [2], but these seem to be machine
> specific?

Yes, almost all output modifiers are.  Only %l, %a, %n, and part of %c
are generic (and %% and %= and on some targets, %{, %|, %}).

> Can you please point me to documentation/unit tests/source for
> these so that I can figure out what they should be doing, and look into
> implementing them in Clang?

The PowerPC part of
https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints
(sorry, no anchor) documents %U.

Traditionally the source code is the documentation for this.  The code
here starts with the comment
      /* Write second word of DImode or DFmode reference.  Works on register
         or non-indexed memory only.  */
(which is very out-of-date itself, it works fine for e.g. TImode as well,
but alas).

Unit tests are completely unsuitable for most compiler things like this.

The source code is gcc/config/rs6000/rs6000.c, easiest is to search for
'L' (with those quotes).  Function print_operand.

HtH,

Segher