http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50037
Bug #: 50037
Summary: Unroll factor exceeds max trip count
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: enkovich....@gmail.com
Created attachment 24971
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24971
Reproducer
Here is a small loop on which GCC performs inefficient unroll:
for ( count = ((*(hdrptr)) & 0xf) * 2; count > 0; count--, addr++ )
sum += *addr;
This loop has maximum 30 iterations. If we use -O3 then this loop is
vectorized. Resulting loop has maximum 30 / 8 = 3 iteration. Also vectorizer
generates prologue and epilogue loops. Each of them has maximum 7 iterations.
If we add -funroll-loops option then each of 3 generated by vectorizer loops is
unrolled with unroll factor 8. It creates a lot of code which is never executed
and also decreases performance due to additional checks and branches.
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc1/configure --prefix=/export/gcc-perf/install
--enable-languages=c,c++,fortran
Thread model: posix
gcc version 4.7.0 20110615 (experimental) (GCC)
COLLECT_GCC_OPTIONS='-O3' '-funroll-loops' '-S' '-v' '-mtune=generic'
'-march=x86-64'
/export/gcc-perf/install/libexec/gcc/x86_64-unknown-linux-gnu/4.7.0/cc1 -quiet
-v unroll_test.c -quiet -dumpbase unroll_test.c -mtune=generic -march=x86-64
-auxbase unroll_test -O3 -version -funroll-loops -o unroll_test.s
GNU C (GCC) version 4.7.0 20110615 (experimental) (x86_64-unknown-linux-gnu)
compiled by GNU C version 4.4.3, GMP version 4.3.1, MPFR version 2.4.2,
MPC version 0.8.1
GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096
