Consider the following short program. If it is compiled MyFunc1 is vectorized
but MyFunc2 is not. Both fuctions differ only in the empty loop, which is a
comment in MyMunc2. My compiler is the latest gcc41 snapshot (20050618)
Michael Cieslinski
double MyFunc1 (int size)
{
int len = size + 1;
double Data[16] = {0};
// for (int i=3; i<len; i++) {} // empty loop
for (int i=0; i<len; i++)
Data[i] = Data[i]>=0 ? Data[i] : -Data[i];
return Data[1];
}
double MyFunc2 (int size)
{
int len = size + 1;
double Data[16] = {0};
for (int i=3; i<len; i++) {} // empty loop
for (int i=0; i<len; i++)
Data[i] = Data[i]>=0 ? Data[i] : -Data[i];
return Data[1];
}
Output from gcc:
g++41c -O2 -ftree-vectorize -c vec-test.cpp -ftree-vectorizer-verbose=5
vec-test.cpp:7: note: accesses have the same alignment.
vec-test.cpp:7: note: LOOP VECTORIZED.
vec-test.cpp:1: note: vectorized 1 loops in function.
vec-test.cpp:17: note: not vectorized: unsupported data-type
vec-test.cpp:19: note: not vectorized: number of iterations cannot be computed.
vec-test.cpp:13: note: vectorized 0 loops in function.
g++41c -v
Using built-in specs.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.1-20050618/configure --prefix=/usr/local/gcc41c --
program-suffix=41c --with-arch=opteron --enable-languages=c,c++ --enable-
checking
Thread model: posix
gcc version 4.1.0 20050618 (experimental)
--
Summary: tree vectorizer depends on context
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: micis at gmx dot de
CC: gcc-bugs at gcc dot gnu dot org
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22184
