Can someone explain the reason why icc is not doing loop invariant code motion (of moving pointer assignment of a) in code 1 below. I see a 40% improvement in performance when the a pointer assignment is moved out of the t6 loop as done in code 2. I tried to mark the pointer 'a' as a const and restrict, to let the compiler know that the pointer 'a' will not be changed inside t6 loop. I am compiling code with '-ansi-alias' , '-03' and '-ipo' option in icc.
[cpp]
//code 1
for(t4=256*t1; t4<=256*t1+254; t4++){
int lbv=256*t2; int ubv=256*t2+255;
for(t6=lbv; t6<=ubv; t6++){
double*restrictconst a= a_trans[lbv/256];//loop invariant code
a[t6-lbv]=a[t6-lbv]/a[t6-lbv];
}
}
//code 2
for(t4=256*t1; t4<=256*t1+254; t4++){
int lbv=256*t2; int ubv=256*t2+255;
double*restrict const a= a_trans[lbv/256];
for(t6=lbv; t6<=ubv; t6++){
a[t6-lbv]=a[t6-lbv]/a[t6-lbv];
}}
[/cpp]
This code is generated by a source to source translator. It is very tedious to manually apply this transformation for lots of loops. I have marked 'a_trans' as const/restrict with in the function.
Is there any other keyword that I can use to let icc know that 'a_trans' will not be changed with loop?
