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Re: Using associativity for optimization


It works fine for my test case.
Thanks

Richard Biener <richard.guenther@gmail.com> wrote:

>On Tue, Dec 2, 2014 at 12:11 AM, shmeel gutl
><shmeelgutl@shmuelhome.mine.nu> wrote:
>> While testing my implementation of passing arguments in registers, I noticed
>> that gcc 4.7 creates instruction dependencies when it doesn't have to.
>> Consider:
>>
>> int foo(int a1, int a2, int a3, int a4)
>> {
>>     return a1|a2|a3|a4;
>> }
>>
>> gcc, even with -O2 generated code that was equivalent to
>>
>> temp1 = a1 | a2;
>> temp2 = temp1 | a3;
>> temp3 = temp2 | a4;
>>
>> return temp3;
>>
>> This code must be executed serially.
>>
>> Could I create patterns, or enable optimizations that would cause the
>> compiler to generate
>>
>> temp1 = a1 | a2;
>> temp2 = a3 | a4;
>> temp3 = temp1 | temp2;
>>
>> Thereby allowing the scheduler to compute temp1 and temp2 in parallel.
>
>You can tune it with --param tree-reassoc-width=N, not sure if that
>was implemented for 4.7 already.
>
>Richard.

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