With gcc-4.6 I see a different bug. When compiling without -flto, ie gcc test.c -o test -O1 -fopenmp -std=c99 everything is fine. With -flto: $ gcc test.c -o test -O1 -fopenmp -flto -std=c99 /tmp/cckxIhdE.ltrans0.ltrans.o: In function `main._omp_fn.0.2060.2020': cckxIhdE.ltrans0.o:(.text+0x2): undefined reference to `omp_get_num_threads' cckxIhdE.ltrans0.o:(.text+0x9): undefined reference to `omp_get_thread_num' /tmp/cckxIhdE.ltrans0.ltrans.o: In function `main': cckxIhdE.ltrans0.o:(.text+0x87): undefined reference to `GOMP_parallel_start' cckxIhdE.ltrans0.o:(.text+0x8c): undefined reference to `GOMP_parallel_end' collect2: ld returned 1 exit status $ gcc test.c -o test -O1 -fopenmp -flto -std=c99 -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.1-4ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-multiarch --with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib/x86_64-linux-gnu --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-4ubuntu1) COLLECT_GCC_OPTIONS='-o' 'test' '-O1' '-fopenmp' '-flto' '-std=c99' '-v' '-mtune=generic' '-march=x86-64' '-pthread' /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/cc1 -quiet -v -D_REENTRANT test.c -quiet -dumpbase test.c -mtune=generic -march=x86-64 -auxbase test -O1 -std=c99 -version -fopenmp -flto -fstack-protector -o /tmp/ccjn2A1G.s GNU C (Ubuntu/Linaro 4.6.1-4ubuntu1) version 4.6.1 (x86_64-linux-gnu) compiled by GNU C version 4.6.1, GMP version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9 GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/include /usr/local/include /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/include-fixed /usr/include/x86_64-linux-gnu /usr/include End of search list. GNU C (Ubuntu/Linaro 4.6.1-4ubuntu1) version 4.6.1 (x86_64-linux-gnu) compiled by GNU C version 4.6.1, GMP version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9 GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 Compiler executable checksum: f4dac7f0dc853914603e09c85c5ce51e COLLECT_GCC_OPTIONS='-o' 'test' '-O1' '-fopenmp' '-flto' '-std=c99' '-v' '-mtune=generic' '-march=x86-64' '-pthread' as --64 -o /tmp/ccLtmwIC.o /tmp/ccjn2A1G.s COMPILER_PATH=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/ Reading specs from /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/libgomp.spec COLLECT_GCC_OPTIONS='-o' 'test' '-O1' '-fopenmp' '-flto' '-std=c99' '-v' '-mtune=generic' '-march=x86-64' '-pthread' /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/collect2 -plugin /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/liblto_plugin.so -plugin-opt=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper -plugin-opt=-fresolution=/tmp/ccLuLary.res -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lpthread -plugin-opt=-pass-through=-lc -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s -flto --build-id --no-add-needed --as-needed --eh-frame-hdr -m elf_x86_64 --hash-style=gnu -dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro -o test /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../crt1.o /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../crti.o /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/crtbegin.o -L/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1 -L/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../.. -L/usr/lib/x86_64-linux-gnu /tmp/ccLtmwIC.o -lgomp -lgcc --as-needed -lgcc_s --no-as-needed -lpthread -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/crtend.o /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../crtn.o gcc @/tmp/ccoBOoSF.args Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.1-4ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-multiarch --with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib/x86_64-linux-gnu --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-4ubuntu1) COLLECT_GCC_OPTIONS='-c' '-O1' '-fopenmp' '-std=c99' '-v' '-mtune=generic' '-march=x86-64' '-pthread' '-dumpdir' './' '-dumpbase' 'test.wpa' '-fltrans-output-list=/tmp/ccqnqk6J.ltrans.out' '-fwpa' '-fresolution=/tmp/ccLuLary.res' '-pthread' /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/lto1 -quiet -dumpdir ./ -dumpbase test.wpa -mtune=generic -march=x86-64 -auxbase ccLtmwIC -O1 -std=c99 -version -fopenmp -fltrans-output-list=/tmp/ccqnqk6J.ltrans.out -fwpa -fresolution=/tmp/ccLuLary.res @/tmp/ccOuUCCK GNU GIMPLE (Ubuntu/Linaro 4.6.1-4ubuntu1) version 4.6.1 (x86_64-linux-gnu) compiled by GNU C version 4.6.1, GMP version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9 GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 GNU GIMPLE (Ubuntu/Linaro 4.6.1-4ubuntu1) version 4.6.1 (x86_64-linux-gnu) compiled by GNU C version 4.6.1, GMP version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9 GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 COMPILER_PATH=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/ COLLECT_GCC_OPTIONS='-c' '-O1' '-fopenmp' '-std=c99' '-v' '-mtune=generic' '-march=x86-64' '-pthread' '-dumpdir' './' '-dumpbase' 'test.wpa' '-fltrans-output-list=/tmp/ccqnqk6J.ltrans.out' '-fwpa' '-fresolution=/tmp/ccLuLary.res' '-pthread' gcc @/tmp/ccantxtx.args Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/lto-wrapper Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro 4.6.1-4ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-multiarch --with-multiarch-defaults=x86_64-linux-gnu --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib/x86_64-linux-gnu --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib/x86_64-linux-gnu --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc --disable-werror --with-arch-32=i686 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu Thread model: posix gcc version 4.6.1 (Ubuntu/Linaro 4.6.1-4ubuntu1) COLLECT_GCC_OPTIONS='-c' '-O1' '-fopenmp' '-std=c99' '-v' '-mtune=generic' '-march=x86-64' '-pthread' '-dumpdir' './' '-dumpbase' 'test.ltrans0' '-fltrans' '-o' '/tmp/ccqnqk6J.ltrans0.ltrans.o' '-pthread' /usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/lto1 -quiet -dumpdir ./ -dumpbase test.ltrans0 -mtune=generic -march=x86-64 -auxbase-strip /tmp/ccqnqk6J.ltrans0.ltrans.o -O1 -std=c99 -version -fopenmp -fltrans @/tmp/cckb2oPN -o /tmp/cclzJJEJ.s GNU GIMPLE (Ubuntu/Linaro 4.6.1-4ubuntu1) version 4.6.1 (x86_64-linux-gnu) compiled by GNU C version 4.6.1, GMP version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9 GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 GNU GIMPLE (Ubuntu/Linaro 4.6.1-4ubuntu1) version 4.6.1 (x86_64-linux-gnu) compiled by GNU C version 4.6.1, GMP version 5.0.1, MPFR version 3.0.1-p3, MPC version 0.9 GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 COLLECT_GCC_OPTIONS='-c' '-O1' '-fopenmp' '-std=c99' '-v' '-mtune=generic' '-march=x86-64' '-pthread' '-dumpdir' './' '-dumpbase' 'test.ltrans0' '-fltrans' '-o' '/tmp/ccqnqk6J.ltrans0.ltrans.o' '-pthread' as --64 -o /tmp/ccqnqk6J.ltrans0.ltrans.o /tmp/cclzJJEJ.s COMPILER_PATH=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/:/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.6.1/../../../:/lib/:/usr/lib/:/usr/lib/x86_64-linux-gnu/ COLLECT_GCC_OPTIONS='-c' '-O1' '-fopenmp' '-std=c99' '-v' '-mtune=generic' '-march=x86-64' '-pthread' '-dumpdir' './' '-dumpbase' 'test.ltrans0' '-fltrans' '-o' '/tmp/ccqnqk6J.ltrans0.ltrans.o' '-pthread' /tmp/ccqnqk6J.ltrans0.ltrans.o: In function `main._omp_fn.0.2060.2020': ccqnqk6J.ltrans0.o:(.text+0x2): undefined reference to `omp_get_num_threads' ccqnqk6J.ltrans0.o:(.text+0x9): undefined reference to `omp_get_thread_num' /tmp/ccqnqk6J.ltrans0.ltrans.o: In function `main': ccqnqk6J.ltrans0.o:(.text+0x87): undefined reference to `GOMP_parallel_start' ccqnqk6J.ltrans0.o:(.text+0x8c): undefined reference to `GOMP_parallel_end' collect2: ld returned 1 exit status