On 01/20/2015 12:45 PM, Reid Kleckner wrote:> On Tue, Jan 20, 2015 at 9:52 AM, Rodney M. Bates <rodney_bates at lcwb.coop <mailto:rodney_bates at lcwb.coop>> wrote: > > 1) Larger structs are returned differently, via memcpy. Do > these methods of returning struct values show through in > the ultimately generated machine code? It seems hard to > imagine that, of the many different target code generators > in llvm, there would not be at least some with standardized > ABIs that differ in such respects. Does llvm make target- > dependent transformations for different targets to match > their ABIs? Or do I have to do that at the level of generating > llvm IR? > > > LLVM handles the low-level ABI details like what registers to use for arguments, but frontends unfortunately need to handle lots of ABI issues around struct passing. LLVM isn't really responsible for transforming IR to make it match any particular ABI. > > 2) To correctly return the value using a bitcast, as in the example, the > front end has to independently and correctly duplicate the layout that > llvm will produce. This seems both very fragile and difficult to > diagnose when it fails. My front end already does record layout, but > I had previously decided, after a discussion on this list, that it was > better to let llvm do it. Any advice on the best way here? > > > Personally, I wouldn't recommend letting LLVM do struct layout. I would recommend creating high-level LLVM struct types, but the frontend should use packed struct types to precisely control the layout. In that way, the frontend can still make assumptions about the exact layout in memory. Make sense? >So, if I build the llvm struct type with packed attribute, will it just put every field in the next available bit? I see the two ways of accessing fields (GEP) and insertvalue/extractvalue both identify the field with a field sequence number, so I would have to be sure I could control the way llvm laid the struct out.> In this particular case, probably all you need to know is the size of the struct, and notice that it is small. I would try to find the Sys V ABI docs to get the threshold or check the Clang source code. > > 3) I am also a little worried about the implications of returning an > i16, with alignment of 1. Won't this create trouble somewhere, or > at least lose some benefit of returning as a scalar? > > > First, the optimizer will typically remove the alloca and the load. Second, the low alignment on the alloca and load looks like a bug in Clang.-- Rodney Bates rodney.m.bates at acm.org
On Thu, Jan 22, 2015 at 1:56 PM, Rodney M. Bates <rodney_bates at lcwb.coop> wrote:> > So, if I build the llvm struct type with packed attribute, will it just > put every field in the > next available bit? I see the two ways of accessing fields (GEP) and > insertvalue/extractvalue > both identify the field with a field sequence number, so I would have to > be sure I could control > the way llvm laid the struct out.Yes, when you set the packed attribute, each field is laid out on the next available byte. I believe non-byte sized integers are rounded up in size the next byte. To handle padding gaps, the frontend needs to manually insert padding fields, and maintain a mapping from frontend field to LLVM field number. Padding is typically an [i8 x N] array where N is the appropriate size to bring you to the byte boundary of the next field. The advantage is that you can be 100% sure that LLVM and your frontend agree on the layout of the struct, while unpacked structs will have different layouts on different targets. The fact that your frontend seems to want to think about things in terms of bits suggests that your next question will be about bitfields. For bitfields, Clang will emit a large integer for all the bits and emit a wide load with extra masking code to extract the relevant bits. All the downstream optimizations are designed to handle this as input, so we should generate good code. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150122/2c9f0b40/attachment.html>
On 01/22/2015 04:10 PM, Reid Kleckner wrote:> On Thu, Jan 22, 2015 at 1:56 PM, Rodney M. Bates <rodney_bates at lcwb.coop <mailto:rodney_bates at lcwb.coop>> wrote: > > So, if I build the llvm struct type with packed attribute, will it just put every field in the > next available bit? I see the two ways of accessing fields (GEP) and insertvalue/extractvalue > both identify the field with a field sequence number, so I would have to be sure I could control > the way llvm laid the struct out. > > > Yes, when you set the packed attribute, each field is laid out on the next available byte. I believe non-byte sized integers are rounded up in size the next byte. To handle padding gaps, the frontend needs to manually insert padding fields, and maintain a mapping from frontend field to LLVM field number. Padding is typically an [i8 x N] array where N is the appropriate size to bring you to the byte boundary of the next field. The advantage is that you can be 100% sure that LLVM and your frontend agree on the layout of the struct, while unpacked structs will have different layouts on different targets. >Ah, bytes, not bits. But I can use that for all fields that start and end on byte boundaries. FWIW, the reason I care so much is that my front end IR operators that access fields use bit offsets within the struct and bit sizes, rather than anything that would identify a field. But I can de-lower (spell check hates me for that) this to field numbers if there is agreement on the layout.> The fact that your frontend seems to want to think about things in terms of bits suggests that your next question will be about bitfields. For bitfields, Clang will emit a large integer for all the bits and emit a wide load with extra masking code to extract the relevant bits. All the downstream optimizations are designed to handle this as input, so we should generate good code.Yes, that would have been my next question. I like the answer, as I was worried about losing optimizations if I generated shift-and-mask code, which I will use for non-whole-byte fields. So the next question is, what about fields that that occupy only whole bytes, but are not 2^n bytes or aren't aligned to their size. Should I treat these as bitfields and produce shift-and-mask operations to access them? Thanks for the advice. Rodney Bates rodney.m.bates at acm.org