augustine.sterl...@gmail.com wrote: > On Fri, Mar 13, 2015 at 7:54 AM, Max Filippov <jcmvb...@gmail.com> wrote: [...] > > 2. alloca seems to make an additional 16-bytes padding to each stack > > allocation: alloca(1) results in moving sp down by 32 bytes, > > alloca(17) > > moves it by 48 bytes, etc. This padding looks unnecessary to me: > > either > > this space is not used (previous register frame is not spilled), or > > alloca > > exception handler will take care about reloading or moving spilled > > registers > > to a new location. In both cases after movsp this space is just > > wasted. > > Do you know why this padding may be needed? > > Answering this question definitively requires some time with the ABI > manual, which I don't have. You may be right, but I would check what > XCC does in this case. It is far better tested.
Other than the required 16-byte stack alignment, there's nothing in the ABI that requires these extra 16 bytes. Perhaps there was a bad implementation of the alloca exception handler at some point a long time ago that prompted the extra 16 bytes? Today XCC doesn't add the extra 16 bytes. alloca(n) with n in a2 comes out as this: 0x60000490 <+12>: movi.n a8, -16 0x60000492 <+14>: addi.n a3, a2, 15 0x60000494 <+16>: and a3, a3, a8 0x60000497 <+19>: sub a3, a1, a3 0x6000049a <+22>: movsp a1, a3 which just rounds up to 16 bytes. -Marc
