Re: [PATCH v1 1/5] riscv/cmpxchg: Deduplicate xchg() asm functions

Boqun Feng
2024-01-05T05:18:20Z

On Fri, Jan 05, 2024 at 01:45:42AM -0300, Leonardo Bras wrote:
[...]
> > > According to gcc.gnu.org:
> > >
> > > ---
> > > "memory" [clobber]:
> > >
> > > The "memory" clobber tells the compiler that the assembly code
> > > performs memory reads or writes to items other than those listed in
> > > the input and output operands (for example, accessing the memory
> > > pointed to by one of the input parameters). To ensure memory contains
> >
> > Note here it says "other than those listed in the input and output
> > operands", and in the above asm block, the memory pointed by "__ptr" is
> > already marked as read-and-write by the asm block via "+A" (*__ptr), so
> > the compiler knows the asm block may modify the memory pointed by
> > "__ptr", therefore in _relaxed() case, "memory" clobber can be avoided.
>
> Thanks for pointing that out!
> That helped me improve my understanding on constraints for asm operands :)
> (I ended up getting even more info from the gcc manual)
>
> So "+A" constraints means the operand will get read/write and it's an
> address stored into a register.
>
> >
> > Here is an example showing the difference, considering the follow case:
> >
> > this_val = *this;
> > that_val = *that;
> > xchg_relaxed(this, 1);
> > reread_this = *this;
> >
> > by the semantics of _relaxed, compilers can optimize the above into
> >
> > this_val = *this;
> > xchg_relaxed(this, 1);
> > that_val = *that;
> > reread_this = *this;
> >
>
> Seems correct, since there is no barrier().
>
> > but the "memory" clobber in the xchg_relexed() will provide this.
>
> By 'this' here you mean the barrier? I mean, IIUC "memory" clobber will
> avoid the above optimization, right?
>

Right, seems I mis-typed "provide" (I meant "prevent")

> > Needless to say the '"+A" (*__ptr)' prevents compiler from the following
> > optimization:
> >
> > this_val = *this;
> > that_val = *that;
> > xchg_relaxed(this, 1);
> > reread_this = this_val;
> >
> > since the compiler knows the asm block will read and write *this.
>
> Right, the compiler knows that address will be wrote by the asm block, and
> so it reloads the value instead of re-using the old one.
>

Correct.

>
> A question, though:

Good question ;-)

> Do we need the "memory" clobber in any other xchg / cmpxchg asm?

The "memory" clobber is needed for others, see below:

> I mean, usually the only write to memory will happen in the *__ptr, which
> should be safe by "+A".
>
> I understand that since the others are not "relaxed" they will need to
> have a barrier, but is not the compiler supposed to understand the barrier
> instruction and avoid compiler reordering / optimizations across given
> instruction ?
>

The barrier semantics (ACQUIRE/RELEASE/FULL) is provided by the combined
effort of both 1) preventing compiler optimization by "memory" clobber
and 2) preventing CPU/memory reordering by arch-specific instructions.

In other words, an asm block contains a hardware barrier instruction
should always have the "memory" clobber, otherwise, there are
possiblities that compilers reorder the asm block therefore break the
ordering provided by the hardware instructions.

Regards,
Boqun

>
> Thanks!
> Leo
>
> > Regards,
> > Boqun
> >
> > > correct values, GCC may need to flush specific register values to
> > > memory before executing the asm. Further, the compiler does not assume
> > > that any values read from memory before an asm remain unchanged after
> > > that asm ; it reloads them as needed. Using the "memory" clobber
> > > effectively forms a read/write memory barrier for the compiler.
> > >
> > > Note that this clobber does not prevent the processor from doing
> > > speculative reads past the asm statement. To prevent that, you need
> > > processor-specific fence instructions.
> > > ---
> > >
> > > IIUC above text says that having memory accesses to *__ptr would require
> > > above asm to have the "memory" clobber, so memory accesses don't get
> > > reordered by the compiler.
> > >
> > > By above affirmation, all asm in this file should have the "memory"
> > > clobber, since all atomic operations will change memory pointed by an input
> > > ptr. Is that correct?
> > >
> > > Thanks!
> > > Leo
> > >
> > >
> > > >
> > > > Regards,
> > > > Boqun
> > > >
> > > > > - break; \
> > > > > - case 8: \
> > > > > - __asm__ __volatile__ ( \
> > > > > - " amoswap.d %0, %2, %1\n" \
> > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > - : "r" (__new) \
> > > > > - : "memory"); \
> > > > > - break; \
> > > > > - default: \
> > > > > - BUILD_BUG(); \
> > > > > - } \
> > > > > - __ret; \
> > > > > -})
> > > > > -
> > > > > -#define arch_xchg_relaxed(ptr, x) \
> > > > > -({ \
> > > > > - __typeof__(*(ptr)) _x_ = (x); \
> > > > > - (__typeof__(*(ptr))) __xchg_relaxed((ptr), \
> > > > > - _x_, sizeof(*(ptr))); \
> > > > > + __asm__ __volatile__ ( \
> > > > > + prepend \
> > > > > + " amoswap" sfx " %0, %2, %1\n" \
> > > > > + append \
> > > > > + : "=r" (r), "+A" (*(p)) \
> > > > > + : "r" (n) \
> > > > > + : "memory"); \
> > > > > })
> > > > >
> > > > > -#define __xchg_acquire(ptr, new, size) \
> > > > > +#define _arch_xchg(ptr, new, sfx, prepend, append) \
> > > > > ({ \
> > > > > __typeof__(ptr) __ptr = (ptr); \
> > > > > - __typeof__(new) __new = (new); \
> > > > > - __typeof__(*(ptr)) __ret; \
> > > > > - switch (size) { \
> > > > > + __typeof__(*(__ptr)) __new = (new); \
> > > > > + __typeof__(*(__ptr)) __ret; \
> > > > > + switch (sizeof(*__ptr)) { \
> > > > > case 4: \
> > > > > - __asm__ __volatile__ ( \
> > > > > - " amoswap.w %0, %2, %1\n" \
> > > > > - RISCV_ACQUIRE_BARRIER \
> > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > - : "r" (__new) \
> > > > > - : "memory"); \
> > > > > + __arch_xchg(".w" sfx, prepend, append, \
> > > > > + __ret, __ptr, __new); \
> > > > > break; \
> > > > > case 8: \
> > > > > - __asm__ __volatile__ ( \
> > > > > - " amoswap.d %0, %2, %1\n" \
> > > > > - RISCV_ACQUIRE_BARRIER \
> > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > - : "r" (__new) \
> > > > > - : "memory"); \
> > > > > + __arch_xchg(".d" sfx, prepend, append, \
> > > > > + __ret, __ptr, __new); \
> > > > > break; \
> > > > > default: \
> > > > > BUILD_BUG(); \
> > > > > } \
> > > > > - __ret; \
> > > > > + (__typeof__(*(__ptr)))__ret; \
> > > > > })
> > > > >
> > > > > -#define arch_xchg_acquire(ptr, x) \
> > > > > -({ \
> > > > > - __typeof__(*(ptr)) _x_ = (x); \
> > > > > - (__typeof__(*(ptr))) __xchg_acquire((ptr), \
> > > > > - _x_, sizeof(*(ptr))); \
> > > > > -})
> > > > > +#define arch_xchg_relaxed(ptr, x) \
> > > > > + _arch_xchg(ptr, x, "", "", "")
> > > > >
> > > > > -#define __xchg_release(ptr, new, size) \
> > > > > -({ \
> > > > > - __typeof__(ptr) __ptr = (ptr); \
> > > > > - __typeof__(new) __new = (new); \
> > > > > - __typeof__(*(ptr)) __ret; \
> > > > > - switch (size) { \
> > > > > - case 4: \
> > > > > - __asm__ __volatile__ ( \
> > > > > - RISCV_RELEASE_BARRIER \
> > > > > - " amoswap.w %0, %2, %1\n" \
> > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > - : "r" (__new) \
> > > > > - : "memory"); \
> > > > > - break; \
> > > > > - case 8: \
> > > > > - __asm__ __volatile__ ( \
> > > > > - RISCV_RELEASE_BARRIER \
> > > > > - " amoswap.d %0, %2, %1\n" \
> > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > - : "r" (__new) \
> > > > > - : "memory"); \
> > > > > - break; \
> > > > > - default: \
> > > > > - BUILD_BUG(); \
> > > > > - } \
> > > > > - __ret; \
> > > > > -})
> > > > > +#define arch_xchg_acquire(ptr, x) \
> > > > > + _arch_xchg(ptr, x, "", "", RISCV_ACQUIRE_BARRIER)
> > > > >
> > > > > #define arch_xchg_release(ptr, x) \
> > > > > -({ \
> > > > > - __typeof__(*(ptr)) _x_ = (x); \
> > > > > - (__typeof__(*(ptr))) __xchg_release((ptr), \
> > > > > - _x_, sizeof(*(ptr))); \
> > > > > -})
> > > > > -
> > > > > -#define __arch_xchg(ptr, new, size) \
> > > > > -({ \
> > > > > - __typeof__(ptr) __ptr = (ptr); \
> > > > > - __typeof__(new) __new = (new); \
> > > > > - __typeof__(*(ptr)) __ret; \
> > > > > - switch (size) { \
> > > > > - case 4: \
> > > > > - __asm__ __volatile__ ( \
> > > > > - " amoswap.w.aqrl %0, %2, %1\n" \
> > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > - : "r" (__new) \
> > > > > - : "memory"); \
> > > > > - break; \
> > > > > - case 8: \
> > > > > - __asm__ __volatile__ ( \
> > > > > - " amoswap.d.aqrl %0, %2, %1\n" \
> > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > - : "r" (__new) \
> > > > > - : "memory"); \
> > > > > - break; \
> > > > > - default: \
> > > > > - BUILD_BUG(); \
> > > > > - } \
> > > > > - __ret; \
> > > > > -})
> > > > > + _arch_xchg(ptr, x, "", RISCV_RELEASE_BARRIER, "")
> > > > >
> > > > > #define arch_xchg(ptr, x) \
> > > > > -({ \
> > > > > - __typeof__(*(ptr)) _x_ = (x); \
> > > > > - (__typeof__(*(ptr))) __arch_xchg((ptr), _x_, sizeof(*(ptr))); \
> > > > > -})
> > > > > + _arch_xchg(ptr, x, ".aqrl", "", "")
> > > > >
> > > > > #define xchg32(ptr, x) \
> > > > > ({ \
> > > > > --
> > > > > 2.43.0
> > > > >
> > > >
> > >
> >
>