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Re: Invariant is not moved out of loop
- From: Alexander Monakov <amonakov at ispras dot ru>
- To: Astor Piaz <appiazzolla at gmail dot com>
- Cc: gcc-help at gcc dot gnu dot org
- Date: Tue, 25 Jul 2017 18:11:51 +0300 (MSK)
- Subject: Re: Invariant is not moved out of loop
- Authentication-results: sourceware.org; auth=none
- References: <CAFbmpW=QGdCC+Sd7PxaOzr=95TjJ89kdxqjurpu9-S_VjsEu6Q@mail.gmail.com>
Hi,
I think what you're asking is not the "usual" loop invariant motion, but rather
applying distributive law to reductions. In your example you want the optimizer
to replace
R = 0;
for ( ... )
R += X * C;
by
R = 0;
for ( ... )
R += X;
R *= C;
We don't do that even for integer operands, the following isn't optimized either:
(neither do Clang and ICC according to my experiments on gcc.godbolt.org)
int f(int *a)
{
int r = 0;
for (int i = 0; i < 1024; i++)
r += a[i] * 5;
return r;
}
Alexander