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
>
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