lkml.org 
[lkml]   [2020]   [May]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    Date
    SubjectRe: [PATCH] x86: support i386 with Clang
    On Mon, May 11, 2020 at 12:34 PM Brian Gerst <brgerst@gmail.com> wrote:
    >
    > On Mon, May 11, 2020 at 2:46 PM Nick Desaulniers
    > <ndesaulniers@google.com> wrote:
    > >
    > > On Mon, May 11, 2020 at 11:09 AM Brian Gerst <brgerst@gmail.com> wrote:
    > > > This looks like the same issue that we just discussed for bitops.h.
    > > > Add the "b" operand size modifier to force it to use the 8-bit
    > > > register names (and probably also needs the "w" modifier in the 16-bit
    > > > case).
    > >
    > > While it does feel familiar, it is slightly different.
    > > https://godbolt.org/z/Rme4Zg
    > > That case was both compilers validating the inline asm, yet generating
    > > assembly that the assembler would choke on. This case is validation
    > > in the front end failing.
    >
    > > long long ret;
    > > switch (sizeof(ret)) {
    > > case 1:
    > > asm ("movb $5, %0" : "=q" (ret));
    > > break;
    > > case 8:;
    > > }
    >
    > So if the issue here is that the output variable type is long long,
    > what code is using a 64-bit percpu variable on a 32-bit kernel? Can
    > you give a specific file that fails to build with Clang? If Clang is
    > choking on it it may be silently miscompiling on GCC.

    I'm not sure that's the case. Applying this patch, undoing the hunk
    in percpu_from_op() we get tons of errors. Looking at one:

    kernel/events/core.c:8679:8: error: invalid output size for constraint '=q'
    ./include/linux/percpu-defs.h:446:2: note: expanded from macro '__this_cpu_read'
    raw_cpu_read(pcp); \
    ^
    ...

    There's nothing wrong with this line, it's reading a percpu u64 into a
    local u64. The error comes from validating the inline asm in the dead
    branch.
    --
    Thanks,
    ~Nick Desaulniers

    \
     
     \ /
      Last update: 2020-05-11 22:18    [W:3.943 / U:0.208 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site