$ cat shift.c #include <stdio.h> #define OFFSET 4 #define MASK 0xf0
int main() { unsigned int f,g; scanf("%u",&f); if ((f & MASK) >> OFFSET == 1) { printf("success\n"); } return 0; } $ gcc -S -O2 -fdump-tree-optimized shift.c $ tail -20 shift.c.t66.optimized unsigned int g; unsigned int f; int D.1807; unsigned int D.1806; unsigned int D.1805; unsigned int f.10; <bb 0>: scanf (&"%u"[0], &f); if ((f & 240) >> 4 == 1) goto <L0>; else goto <L1>; <L0>:; printf (&"success\n"[0]); <L1>:; return 0; } $ gcc -v Using built-in specs. Target: ia64-unknown-linux-gnu Configured with: ../gcc-4.1-20050306/configure --prefix=/home/zfkts --enable-languages=c,f95 --disable-optimization Thread model: posix gcc version 4.1.0 20050306 (experimental) The expression (f & 0xf0) >> 4 == 1 could be optimized to f & 0xf == 1 << 4. Code like that occurs in the libgfortan library. -- Summary: bit shift/mask optimization potential Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P2 Component: middle-end AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: Thomas dot Koenig at online dot de CC: gcc-bugs at gcc dot gnu dot org http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20517