Re: Unaligned accesses in sha1dc

On 1 June 2017 at 12:08, Andreas Schwab [off-list ref] wrote:

On Jun 01 2017, Junio C Hamano [off-list ref] wrote:

quoted

Depending on the model of "ARM" (or "SPARC") emulated with QEMU, and
depending on the OS that runs on such an "ARM" or "SPARC", we may
not see this---if the emulated OS has the "software unaligned-access
emulation" our userland may not see a SIGBUS.

Even if the architecture implements unaligned accesses in hardware, it
is still undefined behaviour, and the compiler will (eventually) take
advantage of it.

I tried to optically follow the macros and ended up on line 87/89 in
lib/sha1.c of the sha1dc-library, where there is undefined behavior if
the address is unaligned, which it seems it could be. Maybe Git uses
some particular combination of macro-definitions and I went down the
wrong path... There might also be other spots; I haven't thrown UBSan
at the code.

Using memcpy on those lines should not be a performance problem on
platforms where unaligned access is ok, of course assuming the
compiler sees the opportunity.

Martin