Re: [Qemu-devel] [RFC v6 10/14] softmmu: Simplify helper_*_st_name, wrap unaligned code

[alvise rigo <a.rigo@virtualopensystems.com>]

On Thu, Jan 7, 2016 at 5:35 PM, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> alvise rigo <a.rigo@virtualopensystems.com> writes:
>
>> On Thu, Jan 7, 2016 at 3:46 PM, Alex Bennée <alex.bennee@linaro.org> wrote:
>>>
>>> Alvise Rigo <a.rigo@virtualopensystems.com> writes:
>>>
>>>> Attempting to simplify the helper_*_st_name, wrap the
>>>> do_unaligned_access code into an inline function.
>>>> Remove also the goto statement.
>>>
>>> As I said in the other thread I think these sort of clean-ups can come
>>> before the ll/sc implementations and potentially get merged ahead of the
>>> rest of it.
>>>
>>>>
>>>> Suggested-by: Jani Kokkonen <jani.kokkonen@huawei.com>
>>>> Suggested-by: Claudio Fontana <claudio.fontana@huawei.com>
>>>> Signed-off-by: Alvise Rigo <a.rigo@virtualopensystems.com>
>>>> ---
>>>>  softmmu_template.h | 96 ++++++++++++++++++++++++++++++++++--------------------
>>>>  1 file changed, 60 insertions(+), 36 deletions(-)
>>>>
>>>> diff --git a/softmmu_template.h b/softmmu_template.h
>>>> index d3d5902..92f92b1 100644
>>>> --- a/softmmu_template.h
>>>> +++ b/softmmu_template.h
>>>> @@ -370,6 +370,32 @@ static inline void glue(io_write, SUFFIX)(CPUArchState *env,
>>>>                                   iotlbentry->attrs);
>>>>  }
>>>>
>>>> +static inline void glue(helper_le_st_name, _do_unl_access)(CPUArchState *env,
>>>> +                                                           DATA_TYPE val,
>>>> +                                                           target_ulong addr,
>>>> +                                                           TCGMemOpIdx oi,
>>>> +                                                           unsigned mmu_idx,
>>>> +                                                           uintptr_t retaddr)
>>>> +{
>>>> +    int i;
>>>> +
>>>> +    if ((get_memop(oi) & MO_AMASK) == MO_ALIGN) {
>>>> +        cpu_unaligned_access(ENV_GET_CPU(env), addr, MMU_DATA_STORE,
>>>> +                             mmu_idx, retaddr);
>>>> +    }
>>>> +    /* XXX: not efficient, but simple */
>>>> +    /* Note: relies on the fact that tlb_fill() does not remove the
>>>> +     * previous page from the TLB cache.  */
>>>> +    for (i = DATA_SIZE - 1; i >= 0; i--) {
>>>> +        /* Little-endian extract.  */
>>>> +        uint8_t val8 = val >> (i * 8);
>>>> +        /* Note the adjustment at the beginning of the function.
>>>> +           Undo that for the recursion.  */
>>>> +        glue(helper_ret_stb, MMUSUFFIX)(env, addr + i, val8,
>>>> +                                        oi, retaddr + GETPC_ADJ);
>>>> +    }
>>>> +}
>>>
>>> There is still duplication of 99% of the code here which is silly given
>>
>> Then why should we keep this template-like design in the first place?
>> I tried to keep the code duplication for performance reasons
>> (otherwise how can we justify the two almost identical versions of the
>> helper?), while making the code more compact and readable.
>
> We shouldn't really - code duplication is bad for all the well known
> reasons. The main reason we need explicit helpers for the be/le case are
> because they are called directly from the TCG code which encodes the
> endianess decision in the call it makes. However that doesn't stop us
> making generic inline helpers (helpers for the helpers ;-) which the
> compiler can sort out.

I thought you wanted to make conditional all the le/be differences not
just those helpers for the helpers...
So, if we are allowed to introduce this small overhead, all the
helper_{le,be}_st_name_do_{unl,mmio,ram}_access can be squashed to
helper_generic_st_do_{unl,mmio,ram}_access. I think this is want you
proposed in the POC, right?

>
>>
>>> the compiler inlines the code anyway. If we gave the helper a more
>>> generic name and passed the endianess in via args I would hope the
>>> compiler did the sensible thing and constant fold the code. Something
>>> like:
>>>
>>> static inline void glue(helper_generic_st_name, _do_unl_access)
>>>                         (CPUArchState *env,
>>>                         bool little_endian,
>>>                         DATA_TYPE val,
>>>                         target_ulong addr,
>>>                         TCGMemOpIdx oi,
>>>                         unsigned mmu_idx,
>>>                         uintptr_t retaddr)
>>> {
>>>     int i;
>>>
>>>     if ((get_memop(oi) & MO_AMASK) == MO_ALIGN) {
>>>         cpu_unaligned_access(ENV_GET_CPU(env), addr, MMU_DATA_STORE,
>>>                              mmu_idx, retaddr);
>>>     }
>>>     /* Note: relies on the fact that tlb_fill() does not remove the
>>>      * previous page from the TLB cache.  */
>>>     for (i = DATA_SIZE - 1; i >= 0; i--) {
>>>         if (little_endian) {
>>
>> little_endian will have >99% of the time the same value, does it make
>> sense to have a branch here?
>
> The compiler should detect that little_endian is constant when it
> inlines the code and not bother generating a test/branch case for
> something that will never happen.
>
> Even if it did though I doubt a local branch would stall the processor
> that much, have you counted how many instructions we execute once we are
> on the slow path?

Too many :)

Regards,
alvise

>
>>
>> Thank you,
>> alvise
>>
>>>                 /* Little-endian extract.  */
>>>                 uint8_t val8 = val >> (i * 8);
>>>         } else {
>>>                 /* Big-endian extract.  */
>>>                 uint8_t val8 = val >> (((DATA_SIZE - 1) * 8) - (i * 8));
>>>         }
>>>         /* Note the adjustment at the beginning of the function.
>>>            Undo that for the recursion.  */
>>>         glue(helper_ret_stb, MMUSUFFIX)(env, addr + i, val8,
>>>                                         oi, retaddr + GETPC_ADJ);
>>>     }
>>> }
>>>
>>>
>>>> +
>>>>  void helper_le_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
>>>>                         TCGMemOpIdx oi, uintptr_t retaddr)
>>>>  {
>>>> @@ -433,7 +459,8 @@ void helper_le_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
>>>>              return;
>>>>          } else {
>>>>              if ((addr & (DATA_SIZE - 1)) != 0) {
>>>> -                goto do_unaligned_access;
>>>> +                glue(helper_le_st_name, _do_unl_access)(env, val, addr, mmu_idx,
>>>> +                                                        oi, retaddr);
>>>>              }
>>>>              iotlbentry = &env->iotlb[mmu_idx][index];
>>>>
>>>> @@ -449,23 +476,8 @@ void helper_le_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
>>>>      if (DATA_SIZE > 1
>>>>          && unlikely((addr & ~TARGET_PAGE_MASK) + DATA_SIZE - 1
>>>>                       >= TARGET_PAGE_SIZE)) {
>>>> -        int i;
>>>> -    do_unaligned_access:
>>>> -        if ((get_memop(oi) & MO_AMASK) == MO_ALIGN) {
>>>> -            cpu_unaligned_access(ENV_GET_CPU(env), addr, MMU_DATA_STORE,
>>>> -                                 mmu_idx, retaddr);
>>>> -        }
>>>> -        /* XXX: not efficient, but simple */
>>>> -        /* Note: relies on the fact that tlb_fill() does not remove the
>>>> -         * previous page from the TLB cache.  */
>>>> -        for (i = DATA_SIZE - 1; i >= 0; i--) {
>>>> -            /* Little-endian extract.  */
>>>> -            uint8_t val8 = val >> (i * 8);
>>>> -            /* Note the adjustment at the beginning of the function.
>>>> -               Undo that for the recursion.  */
>>>> -            glue(helper_ret_stb, MMUSUFFIX)(env, addr + i, val8,
>>>> -                                            oi, retaddr + GETPC_ADJ);
>>>> -        }
>>>> +        glue(helper_le_st_name, _do_unl_access)(env, val, addr, oi, mmu_idx,
>>>> +                                                retaddr);
>>>>          return;
>>>>      }
>>>>
>>>> @@ -485,6 +497,32 @@ void helper_le_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
>>>>  }
>>>>
>>>>  #if DATA_SIZE > 1
>>>> +static inline void glue(helper_be_st_name, _do_unl_access)(CPUArchState *env,
>>>> +                                                           DATA_TYPE val,
>>>> +                                                           target_ulong addr,
>>>> +                                                           TCGMemOpIdx oi,
>>>> +                                                           unsigned mmu_idx,
>>>> +                                                           uintptr_t retaddr)
>>>> +{
>>>> +    int i;
>>>> +
>>>> +    if ((get_memop(oi) & MO_AMASK) == MO_ALIGN) {
>>>> +        cpu_unaligned_access(ENV_GET_CPU(env), addr, MMU_DATA_STORE,
>>>> +                             mmu_idx, retaddr);
>>>> +    }
>>>> +    /* XXX: not efficient, but simple */
>>>> +    /* Note: relies on the fact that tlb_fill() does not remove the
>>>> +     * previous page from the TLB cache.  */
>>>> +    for (i = DATA_SIZE - 1; i >= 0; i--) {
>>>> +        /* Big-endian extract.  */
>>>> +        uint8_t val8 = val >> (((DATA_SIZE - 1) * 8) - (i * 8));
>>>> +        /* Note the adjustment at the beginning of the function.
>>>> +           Undo that for the recursion.  */
>>>> +        glue(helper_ret_stb, MMUSUFFIX)(env, addr + i, val8,
>>>> +                                        oi, retaddr + GETPC_ADJ);
>>>> +    }
>>>> +}
>>>
>>> Not that it matters if you combine to two as suggested because anything
>>> not called shouldn't generate the code.
>>>
>>>>  void helper_be_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
>>>>                         TCGMemOpIdx oi, uintptr_t retaddr)
>>>>  {
>>>> @@ -548,7 +586,8 @@ void helper_be_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
>>>>              return;
>>>>          } else {
>>>>              if ((addr & (DATA_SIZE - 1)) != 0) {
>>>> -                goto do_unaligned_access;
>>>> +                glue(helper_be_st_name, _do_unl_access)(env, val, addr, mmu_idx,
>>>> +                                                        oi, retaddr);
>>>>              }
>>>>              iotlbentry = &env->iotlb[mmu_idx][index];
>>>>
>>>> @@ -564,23 +603,8 @@ void helper_be_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
>>>>      if (DATA_SIZE > 1
>>>>          && unlikely((addr & ~TARGET_PAGE_MASK) + DATA_SIZE - 1
>>>>                       >= TARGET_PAGE_SIZE)) {
>>>> -        int i;
>>>> -    do_unaligned_access:
>>>> -        if ((get_memop(oi) & MO_AMASK) == MO_ALIGN) {
>>>> -            cpu_unaligned_access(ENV_GET_CPU(env), addr, MMU_DATA_STORE,
>>>> -                                 mmu_idx, retaddr);
>>>> -        }
>>>> -        /* XXX: not efficient, but simple */
>>>> -        /* Note: relies on the fact that tlb_fill() does not remove the
>>>> -         * previous page from the TLB cache.  */
>>>> -        for (i = DATA_SIZE - 1; i >= 0; i--) {
>>>> -            /* Big-endian extract.  */
>>>> -            uint8_t val8 = val >> (((DATA_SIZE - 1) * 8) - (i * 8));
>>>> -            /* Note the adjustment at the beginning of the function.
>>>> -               Undo that for the recursion.  */
>>>> -            glue(helper_ret_stb, MMUSUFFIX)(env, addr + i, val8,
>>>> -                                            oi, retaddr + GETPC_ADJ);
>>>> -        }
>>>> +        glue(helper_be_st_name, _do_unl_access)(env, val, addr, oi, mmu_idx,
>>>> +                                                retaddr);
>>>>          return;
>>>>      }
>>>
>>>
>>> --
>>> Alex Bennée
>
>
> --
> Alex Bennée