On Tue, May 19, 2009 at 2:44 PM, Andrew Haley <a...@redhat.com> wrote: > Dave Korn wrote: >> Ben Elliston wrote: >>> This patch silences the following warnings when building libgcc: >>> >>> unwind-dw2-fde.c:321: warning: dereferencing type-punned pointer will break >>> strict-aliasing rules >> >>> - const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) x->pc_begin; >>> - const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) y->pc_begin; >>> + _Unwind_Ptr x_ptr, y_ptr; >>> + memcpy (&x_ptr, x->pc_begin, sizeof (_Unwind_Ptr)); >>> + memcpy (&y_ptr, y->pc_begin, sizeof (_Unwind_Ptr)); >> >> Say, I've been in the habit in the past (GCC 3.x) of avoiding these >> warnings >> by doing like: >> >>> - const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) x->pc_begin; >>> - const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) y->pc_begin; >>> + const _Unwind_Ptr x_ptr = *(const _Unwind_Ptr *) (void *) x->pc_begin; >>> + const _Unwind_Ptr y_ptr = *(const _Unwind_Ptr *) (void *) y->pc_begin; >> >> ... which I see no longer works. Was that ever correct, or was I just losing >> information during the cast-to-void that meant GCC just couldn't diagnose a >> potential problem, but it was still there? I believed that casting through >> void would make the result could-alias-anything, which I thought would >> prevent >> any untoward consequences.
No it doesn't. > Casting via void* just shuts up a warning; the bad code is still there. > But this case should be fine AFAICS, because pc_begin is char[]. I'm not > sure why we get the warning. Because pc_begin being char[] doesn't make the access valid. The patch is ok. Thanks, Richard.
