Using option "-WOPT:icopy=off:copy=off" to disable all copy-propagation?
2013/6/28 Yiran Wang <yiran.w...@gmail.com>
> Thanks for your comments.
>
> No, the assembly looks the same.
>
> Good or bad, the compiler is able to clean up the temporary completely,
> say, copy propagation and DSE.
>
> Regards,
> Yiran
>
>
>
> On Thu, Jun 27, 2013 at 12:26 AM, Jian-Xin Lai <laij...@gmail.com> wrote:
>
>> From your description, if the code is change a little:
>>
>> for(i = 0; i< j; i++)
>> {
>> int t = N*N;
>> x += t << 3;
>> z = x + N;
>> y = y + *x + *z;
>> }
>>
>> Will the N*N be hoisted?
>>
>>
>> 2013/6/27 Yiran Wang <yiran.w...@gmail.com>
>>
>>> Hi All,
>>>
>>> This one looks somewhat similar to the last example, but is different.
>>>
>>> int foo(int N, int j, int *x, int *z)
>>> {
>>> int y = N;
>>> N += 7;
>>> N >>= 3;
>>> int i;
>>> for(i = 0; i< j; i++)
>>> {
>>> x += N*N << 3;
>>> z = x + N;
>>> y = y + *x + *z;
>>> }
>>> return y;
>>> }
>>>
>>> Assembly of the loop at -O3.
>>> .p2align 4,,15
>>> .Lt_0_3586:
>>> #<loop> Loop body line 7, nesting depth: 1, estimated iterations: 1000
>>> .loc 1 9 0
>>> # 8 {
>>> # 9 x += N*N << 3;
>>> movl %eax,%ebx # [0]
>>> .loc 1 11 0
>>> # 10 z = x + N;
>>> # 11 y = y + *x + *z;
>>> addl $1,%ebp # [0]
>>> .loc 1 9 0
>>> imull %eax,%ebx # [1]
>>> shll $3,%ebx # [4]
>>> shll $2,%ebx # [5]
>>> addl %ebx,%edi # [6]
>>> addl %ebx,%esi # [6]
>>> .loc 1 11 0
>>> movl 0(%edi),%ecx # [7] id:23
>>> addl 0(%esi),%ecx # [10]
>>> addl %ecx,%edx # [13]
>>> cmpl 36(%esp),%ebp # [13] j
>>> jl .Lt_0_3586 # [16]
>>>
>>> As we see, the imul instruction remains in the loop.
>>> (and two consequent shll instructions, my guess is that CG is thinking
>>> there should not be such input from WOPT, so it is not optimized in CG,
>>> though it is simple. )
>>>
>>> It looks like SSA PRE omitted the rhs of Iv_update statement x+= N*N<<3,
>>> and VNFRE is only doing one level of CSE, say, promoting the ASHR + LDC 3
>>> out of the loop.
>>>
>>> I am curious why SSA PRE is omitting the expression here. By disabling
>>> this in opt_etable.cxx, the result looks good for this test case. I wonder
>>> if there is any correctness issue for some other test case, or performance
>>> issue?
>>>
>>> It should be noted one strength reduction transformation is done for z
>>> for this case. Also replacing "N>>=3;" with "N*=5;" results in similar
>>> sub-optimal code.
>>>
>>> Best Regards,
>>> Yiran Wang
>>>
>>>
>>>
>>>
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>>
>>
>> --
>> Regards,
>> Lai Jian-Xin
>>
>
>
--
Regards,
Lai Jian-Xin
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