int f(void);
void acceptloop_th(int *t) {
int options = 0;
if (f()) options |= 0x1 << 0;
if (f()) options |= 0x1 << 1;
if (f()) options |= 0x1 << 2;
if (f()) options |= 0x1 << 3;
if (f()) options |= 0x1 << 4;
if (f()) options |= 0x1 << 5;
if (f()) options |= 0x1 << 6;
if (f()) options |= 0x1 << 7;
if (f()) options |= 0x1 << 8;
if (f()) options |= 0x1 << 9;
if (f()) options |= 0x1 << 10;
if (f()) options |= 0x1 << 11;
if (f()) options |= 0x1 << 12;
if (f()) options |= 0x1 << 13;
if (f()) options |= 0x1 << 14;
if (f()) options |= 0x1 << 15;
if (f()) options |= 0x1 << 16;
if (f()) options |= 0x1 << 17;
if (f()) options |= 0x1 << 18;
if (f()) options |= 0x1 << 19;
if (f()) options |= 0x1 << 20;
if (f()) options |= 0x1 << 21;
if (f()) options |= 0x1 << 22;
if (f()) options |= 0x1 << 23;
if (f()) options |= 0x1 << 24;
if (f()) options |= 0x1 << 25;
if (f()) options |= 0x1 << 26;
*t = options;
}
takes many minutes to compile with 4.3. No problem with 4.2. Timing report
shows PRE, and -fno-tree-pre makes it go really fast.
Some timings, where the first number is the number of "if"-lines:
10 gcc -c -O3 mini2.c -DN=$n 0.17s user 0.01s system 97% cpu 0.184 total
11 gcc -c -O3 mini2.c -DN=$n 0.20s user 0.02s system 97% cpu 0.221 total
12 gcc -c -O3 mini2.c -DN=$n 0.28s user 0.01s system 95% cpu 0.306 total
13 gcc -c -O3 mini2.c -DN=$n 0.42s user 0.03s system 97% cpu 0.463 total
14 gcc -c -O3 mini2.c -DN=$n 0.76s user 0.02s system 97% cpu 0.805 total
15 gcc -c -O3 mini2.c -DN=$n 1.52s user 0.03s system 97% cpu 1.604 total
16 gcc -c -O3 mini2.c -DN=$n 3.24s user 0.07s system 97% cpu 3.396 total
17 gcc -c -O3 mini2.c -DN=$n 7.00s user 0.12s system 97% cpu 7.314 total
18 gcc -c -O3 mini2.c -DN=$n 15.01s user 0.21s system 96% cpu 15.748 total
19 gcc -c -O3 mini2.c -DN=$n 33.21s user 0.49s system 94% cpu 35.600 total
20 gcc -c -O3 mini2.c -DN=$n 76.29s user 0.87s system 96% cpu 1:19.94 total
I'll also attach the original source this test case was extracted from.
--
Summary: Exponential time behavior in PRE
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Keywords: compile-time-hog
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: falk at debian dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32540
