On Tue, Apr 14, 2015 at 9:58 AM, Jeff Law <l...@redhat.com> wrote: > On 04/14/2015 10:30 AM, Steve Ellcey wrote: >> >> On Mon, 2015-04-13 at 23:18 -0600, Jeff Law wrote: >> >>> But I don't see how using alloca ensures that you're going to have an >>> aligned spill slot. It can get you an aligned stack pointer, but that >>> doesn't ensure alignment of any particular spill slot IIRC. >> >> >> It doesn't. I found a big hole in my idea because as soon as you do an >> alloca >> then frame_pointer_needed is set to true and spills are done relative to >> the frame pointer, not the stack pointer. Thus having an aligned stack >> pointer at that point doesn't help at all with the alignment of spills. > > Right. I almost mentioned something about the frame pointer here... But > yes, once you do alloca, spills are going to be frame relative. > > I don't recall if we can have a different alignment requirements for the > frame and stack. Even if we can get an aligned frame, that's still not a > guarantee of an aligned spill slot. > >> Yes, I am trying to look at how the x86 does dynamic stack alignment but >> it is difficult to untangle the generic concepts from the parts tied >> specifically to the x86 calling convention. No other platform appears >> to do dynamic stack alignment. > > I'm not aware of any other target doing dynamic stack realignment. x86 has > a long history of alignment issues (*), so folks have been thinking about > how to do it there for a long time. > > I'm a bit surprised this hasn't come up on other architectures given the > proliferation of wider and wider vectors. If you can see a path to > generalization of what the x86 is doing so that other targets can use it, > it'd definitely be a win.
We have done just that in GCC 4.4 to implement dynamic stack alignment on x86 :-). Some of x86 backend changes for dynamic stack alignment are x86 psABI specific. Others are historical, like -mstackrealign. which was the old attempt for dynamic stack alignment. -- H.J.
