The following testcase demonstrates where reassociation/regrouping of
expressions could result in greater parallelism for processors that have
multiple arithmetic execution units.
int myfunction (int a, int b, int c, int d, int e, int f, int g, int h) {
int ret;
ret = a + b + c + d + e + f + g + h;
return ret;
}
Compiling with -O3 results in a series of dependent add instructions to
accumulate the sum.
add 4,3,4
add 4,4,5
add 4,4,6
add 4,4,7
add 4,4,8
add 4,4,9
add 4,4,10
If we regrouped to (a+b)+(c+d)+... we can do multiple adds in parallel on
different execution units.
--
Summary: Reassociate expressions for greater parallelism
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: pthaugen at gcc dot gnu dot org
GCC build triplet: powerpc64-unknown-linux-gnu
GCC host triplet: powerpc64-unknown-linux-gnu
GCC target triplet: powerpc64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45671
