On Mon, Mar 1, 2021 at 5:30 AM Carlos O'Donell <carlos@redhat.com> wrote:
>
> On 2/2/21 2:12 PM, H.J. Lu via Libc-alpha wrote:
> > Since there is only "movq imm32s, mem64" and no "movq imm64, mem64", use
> > "movq reg64, mem64" to store 64-bit constant in TCB.
> > ---
> >  sysdeps/x86_64/nptl/tls.h | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/sysdeps/x86_64/nptl/tls.h b/sysdeps/x86_64/nptl/tls.h
> > index 20f0958780..9ec8703e45 100644
> > --- a/sysdeps/x86_64/nptl/tls.h
> > +++ b/sysdeps/x86_64/nptl/tls.h
> > @@ -269,6 +269,11 @@ _Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
> >         asm volatile ("movl %0,%%fs:%P1" :                                  \
> >                    : IMM_MODE (value),                                      \
> >                      "i" (offsetof (struct pthread, member)));              \
> > +     else if (__builtin_constant_p (value)                                 \
> > +           && (int64_t) (int32_t) (uintptr_t) value != (uintptr_t) value)  \
> > +       asm volatile ("movq %0,%%fs:%P1" :                                  \
> > +                  : "r" (value),                                           \
> > +                    "i" (offsetof (struct pthread, member)));              \
>
> (1) Move conditional into 'else /* 8 */' section.
>
> The blocks of conditionals are predicated on the size of the member we are
> about to write to.
>
> In the block below...
>
> >       else /* 8 */                                                          \
>
> ... here, we are about to write value into a member that is size 8.
>
> Your code changes the logical construction of the code, but in way that makes
> it more difficult to understand.
>
> We previously had:
>
> if (sizeof() == 1)
>
> else if (sizeof() == 4)
>
> else /* Assume 8 */
>
> In your case we must already be in the 'else /* Assume 8 */' because otherwise
> we've be writing a 64-bit constant into a < 64-bit structure member.
>
> I think we should put your code into the else clause.
>
> 272      else /* 8 */                                                             \
> 273        {                                                                      \
>
>              if (__builtin_constant_p (value)
>                  && ([the value is a >32-bit constant])
>                asm volatile ([use movq reg64, mem64]);
>              else
>
> 274          asm volatile ("movq %q0,%%fs:%P1" :                                  \
> 275                        : IMM_MODE ((uint64_t) cast_to_integer (value)),       \
> 276                          "i" (offsetof (struct pthread, member)));            \
> 277        }})
>
> (2) What if gcc can't prove it's constant?
>
> If the constant is >32-bit, but gcc can't prove it's constant, then don't we
> try to put a >32-bit constant into a 32-bit immediate?
>
> Shouldn't the code be structured the other way around?
>
> e.g.
>
> else /* 8 */
> {
>   if (__builtin_constant_p (value)
>       && ([the value is a <= 32-bit constant])
>     asm volatile ([use movq imm32, mem64]);
>   else
>     asm volatile ([use movq reg64, mem64]);
> }
>
> This way the code is always correct?

I changed it to

         if (!__builtin_constant_p (value)                                    \
             || (int64_t) (int32_t) (uintptr_t) value == (uintptr_t) value)   \
           asm volatile ("movq %q0,%%fs:%P1" :                                \
                         : IMM_MODE ((uint64_t) cast_to_integer (value)),     \
                           "i" (offsetof (struct pthread, member)));          \
         else                                                                 \
           asm volatile ("movq %0,%%fs:%P1" :                                 \
                         : "r" (value),                                       \
                           "i" (offsetof (struct pthread, member)));          \

> >         {                                                                   \
> >        asm volatile ("movq %q0,%%fs:%P1" :                                  \
> > @@ -294,6 +299,12 @@ _Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
> >                    : IMM_MODE (value),                                      \
> >                      "i" (offsetof (struct pthread, member[0])),            \
> >                      "r" (idx));                                            \
> > +     else if (__builtin_constant_p (value)                                 \
> > +           && (int64_t) (int32_t) (uintptr_t) value != (uintptr_t) value)  \
> > +       asm volatile ("movq %0,%%fs:%P1(,%q2,8)" :                          \
> > +                  : "r" (value),                                           \
> > +                    "i" (offsetof (struct pthread, member[0])),            \
> > +                    "r" (idx));                                            \
> >       else /* 8 */                                                          \
> >         {                                                                   \
> >        asm volatile ("movq %q0,%%fs:%P1(,%q2,8)" :                          \
> >
>

Here is the v2 patch.  OK for master?

Thanks.

-- 
H.J.