> There are several deep challenges in making TGSI <-> LLVM IR translation
> lossless -- I'm sure we'll get around to overcome them -- but I don't
> think that using LLVM is a requirement for this module. Having a shared
> IR for simple TGSI optimization module would go a long way by itself.
What are these challenges?
If you keep vectors and don't scalarize, I don't see why it shouldn't
just work, especially if you just roundtrip without running any
passes.
The DAG instruction matcher should be able to match writemasks,
swizzles, etc. fine.
Control flow may not be exactly reconstructed, but I think LLVM has
control flow canonicalization that should allow to reconstruct a
loop/if control flow structure of equivalent efficiency.
Using LLVM has the obvious advantage that all optimizations have
already been written and tested.
And for complex shaders, you may really need a good full optimizer
(that can do inter-basic-block and interprocedural optimizations,
alias analysis, advanced loop optmizations, and so on), especially if
we start supporting OpenCL over TGSI.
There is also the option of having the driver directly consume the
LLVM IR, and the frontend directly produce it (e.g. clang supports
OpenCL -> LLVM).
Some things, like inlining, are easy to do directly in TGSI (but only
because all regs are global).
However, even determining the minimum number of loop iterations for
loop unrolling is very hard to do without a full compiler.
For instance, consider code like this:
if(foo >= 6)
{
if(foo == 1)
iters = foo + 3;
else if(bar == 1)
iters = foo + 5 + bar;
else
iters = foo + 7;
for(i = 0; i < iters; ++i) LOOP_BODY;
}
You need a non-trivial optimizer (with control flow support, value
range propagation, and constant folding) to find out that the loop
always executes at least 12 iterations, which you need to know to
unroll it optimally.
More complex examples are possible.
It general, anything that requires (approximately) determining any
property of the program potentially benefits from having the most
complex and powerful optimizer available.
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