------- Comment #10 from joseph at codesourcery dot com 2009-04-29 11:33 ------- Subject: Re: [4.5 Regression] Revision 146817 caused unaligned access in gcc.dg/torture/pr26565.c
On Wed, 29 Apr 2009, matz at gcc dot gnu dot org wrote: > The user should have the possibility to announce the unalignedness to the > compiler via casts, like so: > > memcpy((char*)&outdata->tv, tp, sizeof outdata->tv); > > (or void* or whatever). This doesn't work currently, as the emitted gimple > code loses that cast. > > So, there're two things: explicit alignment changing casts are lost and > the type of ADDR_EXPR of non-naturally aligned fields has the wrong pointer > type (losing the unalignedness). > > The former problem is a bit problematic to solve, as parameter passing > always has an implicit conversion to the formal parameter type (void* in this > case). We don't want to lose alignment info just because of that conversion, > only for explicit ones. In ISO C terms casts change alignment information in the opposite direction to the one you want - if the conversion sequence contains anywhere a pointer to an N-byte aligned type, you can assume that the pointer is N-byte aligned, so you take the highest alignment from the sequence of types, not the lowest. The problem is representing the address of an unaligned field as a pointer-to-aligned-type. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39954
