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Re: [Qemu-devel] [RFC v6 10/14] softmmu: Simplify helper_*_st_name, wrap
From: |
alvise rigo |
Subject: |
Re: [Qemu-devel] [RFC v6 10/14] softmmu: Simplify helper_*_st_name, wrap unaligned code |
Date: |
Thu, 7 Jan 2016 17:54:29 +0100 |
On Thu, Jan 7, 2016 at 5:35 PM, Alex Bennée <address@hidden> wrote:
>
> alvise rigo <address@hidden> writes:
>
>> On Thu, Jan 7, 2016 at 3:46 PM, Alex Bennée <address@hidden> wrote:
>>>
>>> Alvise Rigo <address@hidden> 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 <address@hidden>
>>>> Suggested-by: Claudio Fontana <address@hidden>
>>>> Signed-off-by: Alvise Rigo <address@hidden>
>>>> ---
>>>> 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