>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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