Reposting from here: https://gcc.gnu.org/ml/gcc-help/2016-05/msg00003.html
Not sure if this applies: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54210 If I compile on a k8 Opteron 248 with -march=native, I do not see -mprfchw listed in the options in -fverbose-asm. In the assembly, I see this: prefetcht0 (%rax) # ivtmp.1160 prefetcht0 304(%rcx) # prefetcht0 (%rax) # ivtmp.1160 If I compile on a bdver2 Opteron 6386 SE with -march=k8 (thus trying to target the older system), I do see it listed in the options in -fverbose-asm. In the assembly, I see this: prefetcht0 (%rax) # ivtmp.1160 prefetcht0 304(%rcx) # prefetchw (%rax) # ivtmp.1160 (The third line is the only difference) In both cases, I'm using gcc 4.9.3. Which is correct for a k8 Opteron 248? Also, FWIW: 1) The march=native version that uses prefetcht0 is very repeatably faster by about 15% in the particular test case I'm looking at. 2) The compilers in both instances are not just the same version, they are the same compiler binary installed on an NFS mount and shared to both computers.
