[Mesa-dev] [RFC PATCH 00/16] A new IR for Mesa

[Roland Scheidegger]

Am 16.08.2014 02:12, schrieb Connor Abbott:
> I know what you might be thinking right now. "Wait, *another* IR? Don't
> we already have like 5 of those, not counting all the driver-specific
> ones? Isn't this stuff complicated enough already?" Well, there are some
> pretty good reasons to start afresh (again...). In the years we've been
> using GLSL IR, we've come to realize that, in fact, it's not what we
> want *at all* to do optimizations on. Ian has done a talk at FOSDEM that
> highlights some of the problems they've run into:
> 
> https://urldefense.proofpoint.com/v1/url?u=https://video.fosdem.org/2014/H1301_Cornil/Saturday/Three_Years_Experience_with_a_Treelike_Shader_IR.webm&k=oIvRg1%2BdGAgOoM1BIlLLqw%3D%3D%0A&r=F4msKE2WxRzA%2BwN%2B25muztFm5TSPwE8HKJfWfR2NgfY%3D%0A&m=iXhCeAYmidPDc1lFo757Cc9V0PvWAN4n3X%2Fw%2B%2F7Lx%2Fs%3D%0A&s=f103fb26bf53eee64318a490517d1ee9ab88ecd29fcdbe49d54b5a27e7581c2e
> 
> But here's the summary:
> 
> * GLSL IR is way too much of a memory hog, since it has to make a new
> variable for each temporary the compiler creates and then each time you
> want to dereference that temporary you need to create an
> ir_dereference_variable that points to it which is also very
> cache-unfriendly ("downright cache-mean!").
> 
> * The expression trees were originally added so that we could do
> pattern matching to automatically optimize things, but this turned out
> to be both very difficult to do and not very helpful. Instead, all it
> does is add more complexity to the IR without much benefit - with SSA or
> having proper use-def chains, we could get back what the trees give us
> while also being able to do lots more optimizations.
> 
> * We don't have the concept of basic blocks in GLSL IR, which makes a
> lot of optimizations harder because they were originally designed with
> basic blocks in mind - take, for example, my SSA series. I had to map a
> whole lot of concepts that were based on the control flow graph to this
> tree of statements that GLSL IR uses, and the end result wound up
> looking nothing at all like the original paper. This problem gets even
> worse for things like e.g. Global Code Motion that depend upon having
> the dominance tree.
> 
> I originally wanted to modify GLSL IR to fix these problems by adding
> new instruction types that would address these issues and then
> converting back and forth between the old and the new form, but I
> realized that fixing all the problems would basically mean a complete
> rewrite - and if that's the case, then why don't we start from scratch?
> So I took Ken's suggestions and started designing, and then at Intel
> over the summer started implementing, a completely new IR which I call
> NIR that's at a lower level than GLSL IR, but still high-level enough to
> be mostly device-independant (different drivers may have different
> passes and different ways of lowering e.g.  matrix multiplies) so that
> we can do generic optimizations on it. Having support for SSA from the
> beginning was also a must, because lots of optimisations that we really
> want for cleaning up DX9-translated games are either a lot easier in or
> made possible by SSA. I also made the decision for it to be typeless,
> because that's what the cool kids are all doing :) and for a
> lower-level, flat IR it seemed like the thing to do (it could have gone
> either way, though). So the key design points of NIR (pronounced either
> like "near" as in "NIR is near!" or to rhyme with "burr") are:
> 
> * It's flat (no expression trees)
> 
> * It's typeless
> 
> * Modifiers (abs, negate, saturate), swizzles, and write masks are part
> of ALU instructions
> 
> * It includes enough GLSL-like things (variables that you can load from
> or store to, function calls) to be hardware-agnostic (although we don't
> have a way to represent matrix multiplies right now, but that could
> easily be added) to be able to do optimizations at a high level, while
> having lowering passes that convert variables to registers and
> input/output/uniform loads/stores that will open up more opportunities
> for optimization and save memory while being more hardware-specific.
> 
> * Control flow consists of a tree of if statements and loops, like in
> GLSL IR, except the leaves of the tree are now basic blocks instead of
> instructions. Also, each basic block keeps track of its successors and
> predecessors, so the control flow graph is explicit in the IR.
> 
> * SSA is natively supported, and SSA uses point directly to the SSA
> definition, which means that the use-def chains are always there, and
> def-use chains are kept by tracking the set of all uses for each
> definition.
> 
> * It's written in C.
> 
> (see the README in patch 3 and nir.h in patch 4 for more details)
> 
> Some things that are missing or could be improved:
> 
> * There's currently no alias tracking for inputs, outputs, and uniforms.
> This is especially important for uniforms because we don't pack them
> like we pack inputs and outputs.
> 
> * We need a way to represent matrix multiplies so that we can do
> matrix-flipping optimizations in NIR (currently GLSL IR does this for
> us).
> 
> * I'm not entirely happy about how we represent loads and stores in the
> IR. Right now, they're intrinsics, but that means we need a different
> intrinsic for each size and combination of arguments (indirect vs. not
> indirect, etc.) and we might run into a combinatorial explosion problem
> in the future, so we might need to make separate load/store instructions
> like what I did for textures.
> 
> * Right now, we only have a pass that lowers variables for scalar
> backends. We need to write a similar pass for vector backends that uses
> std140 packing or something similar, as well as porting
> lower_ubo_reference to NIR and changing it to output offsets in the
> hardware-native units instead of bytes.
> 
> * We'll need to write a pass that splits up vector expressions for
> scalar backends.

Interesting. I think conceptually this makes sense (I'm far from an
expert in that area though), though I wonder if we actually even should
have our own IR? GL NG will specify a common shading language
intermediate representation, and I suspect there'd be benefits if we'd
just use that? Obviously I don't have any idea how that's going to look
like but maybe it will be just like SPIR (which is llvm ir essentially)?
Granted a lot of stuff in your isa is conceptually similar (such as
being based around basic blocks).

Roland