http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48732
Summary: Nested loops with small iteration count gobble time in
"tree reassociation"
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: arthur.j.odw...@gmail.com
Created attachment 24076
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24076
Output of "gcc-4.5 -w -O1 -S test.c -Q -v"
The following test case takes inordinately long to compile on my machine
(Ubuntu 10.10, 64-bit), starting with gcc-4.5.
cat >test.c <<EOF
void func_47()
{
int a, b, c, d, e;
for (a=0; a < 8; ++a) {
for (b=0; b < 8; ++b) {
for (c=0; c < 8; ++c) {
for (d=0; d < 8; ++d) {
int l_752[8];
int j;
for (j = 0; j < 8; j++)
l_752[j] = 0;
}
}
}
}
}
EOF
time gcc -w -O1 -S test.c -Q
On my machine, the timings are as follows:
With gcc-4.4: real 0.027s
With gcc-4.5: real 5.801s, spent 94% of that time in "tree reassociation"
With trunk: real 3.567s, spent 77% of that time in "tree reassociation"
The only difference in the assembly output is that gcc-4.4 uses "ret" and
gcc-4.5 uses "rep ret".
Adding a fifth nested loop with 8 iterations makes compilation take more than
300 seconds; I killed it at that point.
