On Tue, Mar 2, 2010 at 10:55 AM, Andrew Haley <a...@redhat.com> wrote: > On 03/02/2010 08:55 AM, Alain Ketterlin wrote: >> >> It looks like gcc sometimes produces "useless" conditional branches. >> I've found code like this: >> >> xor %edx,%edx >> ; code with no effect on edx (see full code below) >> test %edx,%edx >> jne <somewhere else> >> >> The branch on the last line is never taken. Why does gcc generate such >> code sequences? Is this patched at runtime, or something? Am I missing >> something obvious here? > >> Let me know if more detail is needed. > > We really need a test case, with source, that illustrates the problem. > When we have that, we can treat is as a missed-optimization bug.
I can't reproduce it with 4.3 nor 4.5 but indeed 4.4 has this interesting code sequence. It looks like a missed jump threading opportunity. Richard. > Andrew. >
