Chris Lattner wrote:
I'm interested in understanding the extent of the assumptions which llvm makes about the types of hardware it is capable of targeting.
Different pieces of the compiler make different assumptions. In particular, the code generator we ship is good for targetting certain classes of devices, but isn't fully general (it doesn't help if you're synthesizing a netlist from llvm, for example).
In particular, I'm investigating a proposal by Zack Rusin to use llvm as
the shader compilation engine within Mesa, targeting GPU backends.
Ok
It seems that LLVA and by extension Vector-LLVA assumes that looping and
branching control flow can be expressed in terms of a simple "br" branch
operation.
LLVA is not a part of LLVM, so I won't answer for it.
OK, I guess I misunderstood the papers I pulled down - my impression was that at some stage programs in llvm would be represented in LLVA.
What, out of interest, is the relationship between LLVM and LLVA?
Typically GPU environments cannot provide such a facility as they tend
to run 16, 32 or 64 simd threads all with the same program counter.
Though this is a wide vector environment, each of the threads is
typically a scalar program and at any branch point, some of those
threads may take the branch and some not. So, to provide dynamic
branching facilities in this environment, you end up with per-channel
execution masks, and opcodes like "IF", "THEN", and "ELSE" which
manipulate those per-channel masks, and use stack semantics for pushing
and popping masks to emulate nested control structures.
Right, it's basically a form of predication.
Yes, a set of high-level-ish instructions layered on dynamic predication to give a very close match to the normally understood dynamic branching and looping constructs we're all familiar with.
It's initially quite odd to see something like "ELSE" or "WHILE" as hardware opcodes, but it makes sense under the hood.
This is probably all very familiar to anybody who's thought about simd
program execution. But it means that GPUs, and low-level GPU
abstractions tend not to have branch instructions.
The question then, is to what extent it is possible to target this type
of execution environment with LLVM and the LLVA/Vector-LLVA ISAs???
Is it necessary (or feasible) to try to analyse LLVA programs and
extract IF/THEN/ELSE semantics from a set of arbitary branch instructions?
Is it possible to extend LLVA with these 'high level' control flow
instructions and end up generating those instead of branches, and if so
how does that affect the rest of LLVM?
The code generator and llvm should be able to handle this just fine, with only minimal extensions.
Basically, you want to model this as predicated execution, and you want the code generator to predicate away as many branches etc as possible.
One observation can be made though: there will always be some programs that you can't map onto the hardware. For example, if you don't have branches, you can't do loops that execute for a variable number of iterations.
Actually, you can - there is a program counter, the loop keeps executing until the execution mask reaches zero. Likewise branches are dynamic.
As such, I'd structure the compiler as a typical code generator with an early predication pass that flattens branches. If you get to the end of the codegen and have some dynamic branches left, you detect the error condition and reject the shader from the hardware path (so you have to emulate it in software).
The hardware *does* support dynamic branching, and looping, provided it is expressed in IF/THEN/ELSE, LOOP/BREAK/CONTINUE/ENDLOOP type instructions. Even CALL/RETURN. The only thing it can't do is execute something like "GOTO" or "BRANCH" dynamically.
Does this make sense?
Yes, but at slight cross-purposes.
There are no cases where compilation should fail to produce a hardware executable result, within the constraints of the high-level language we are compiling. Dynamic branches and looping are entirely within the capability of the hardware, provided they are expressed in terms of the hardware IF/THEN/ELSE, LOOP/ENDLOOP, etc, opcodes.
But it seems like my initial understanding of the intermediate representation within llvm is incorrect & I probably should just dive into the source to figure out what's going on.
My concern was that llva throws away the information that I'd probably need to reconstruct these high-level opcodes required by the hardware - if the code generator can come in at a higher level while that information still exists, then a lot of things get easier.
Keith
