| Commit message (Collapse) | Author | Age |
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The main practical change here is that things that used to be `IRValue`s, like literals, are now being expressed as instructions in the global scope.
In order to validate that things are actually being handled correctly, this change introduces an explicit "validation" pass that can be run on the IR to check for different invariants (although it doesn't check many of the important ones right now). I've left the validation pass turned off by default, but with a command-line flag to enable it. We may want to make it be on by default in debug builds, just to keep us honest. The main invariant for the moment is that when on IR instruction is used as an operand to another, it had better come from the same IR module.
Some of the existing passes were violating this rule, in particular when it came to cloning of witness tables related to global generic parameter substitution. Those features can in theory be handled better now by allowing `specialize` instructions at other scopes, but I didn't want to over-complicate this change, so I make just enough fixes to ensure that these steps always clone witness tables they get from the "symbols" on an IR specialization context. In order for this to work when recursively specializing, I had to ensure that the logic for generic specialization had a notion of a "parent" specialization context that it would fall back to to perform cloning when necessary.
This change keeps the logic that was caching and re-using the instructions for literal values within a module, but adds some logic that isn't really being tested right now for picking the right parent instruction to insert a constant instruction into. This logic doesn't trigger right now because all of the cases we are using it on have zero operands (and so they always get "hoisted" to the global scope), but eventually for things like types we want to be able to support instructions with operands (e.g., `vector<float, 4>`) and handle the case where some of those operands come from different scopes (e.g., when nested inside a generic).
The final change here is mostly cosmetic: the `IRBuilder` is now more abstract about where insertion occurs: it tracks a single `IRParentInst` to insert into, and then an optional `IRInst` to insert before. In the common case, that parent is an `IRBlock`, but it could conceivably also be the global scope, or a witness table, etc. Use sites where we used to change those fields directly now use distinct methods `setInsertInto(parent)` and `setInsertBefore(inst)` which capture the two cases we care about. Accessors are also defined to extract the current block (if the current parent is a block), and the current "function" (global value with code, if the current parent is a global value with code, or a block inside one).
With this work in place, it should be possible for a follow-on change to start putting `specialize` instructions at the global scope and thus clean up some of the on-the-fly specialization work. This work should also help with some of the requirements around a distinct IR-level type system and more explicit generics.
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* Initial work on validating "constexpr"-ness in IR
The underlying issue here is that certain operations in the target shading languages constrain their operands to be compile-time constants. A notable example is the optional texel offset parameter to the `Texture2D.Sample` operation.
When calling these operations in GLSL, the user is required to pass a "constant expression," and any variables in that expression must therefore be marked with the `const` qualifier (and themselves be initialized with constant expressions). Any GLSL output we generate must of course respect these rules.
When calling these operations in HLSL, the user is not so constrained. Instead, they can pass an arbitrary expression, which may involve ordinary variables with no particular markup, and then the compiler is responsible for determining if the actual value after simplification works out to be a constant. In some cases, the requirement that a value be constant might actually trigger things like loop unrolling. Also, it is okay to use a function parameter to determine such a constant expression, as long as the argument turns out to be a constant at all call sites.
The way we have decided to tackle these challenges in Slang is that we we propagate a notion of `constexpr`-ness through the IR. This is currently being tackled in `ir-constexpr.cpp` with a combination of forward and backward iterative dataflow:
* When the operands to an instruction are all `constexpr`, and the opcode is one we believe can be constant-folded, then we infer that the instruction *can* be evaluated as `constexpr`
* When instruction is required to be `constexpr`, then we infer that all of its operands are also required to be `constexpr`.
If this process ever infers that a function parameter is required to be `constexpr`, then we might have to continue propagation at all the call sites to that function.
If after all the propagation is done, there are any cases where an instruction is *required* to be `constexpr`, but it *can't* be `constexpr` (we weren't able to infer `constexpr`-ness for its operands), then we issue an error.
This implementation encodes the idea of `constexpr`-ness in the IR as part of the type system, using a simplified notion of rates. This change adds a `RateQualifiedType` that can represent `@R T`, and then introduces a `ConstExprRate` that can be used for `R`. Many accessors for the type information on IR nodes were updated to distinguish when one wants the "full" type of an IR value (which might include rate information) vs. just the "data" type.
A `constexpr` qualifier was added in the front-end, and is being used to decorate the texel offset parameter for `Texture2D.Sample`. Lowering from AST to IR looks for this qalifier and infers when a function parameter must be typed as `@ConstExpr T` instead of just `T`.
There are lots of limitations and gotchas in the implementation so far:
* The `@ConstExpr` rate is the only one added in this change, but it seems clear that the conceptual `ThreadGroup` rate that was added to represent `groupshared` should probably get folded into the representation.
* I'm not 100% pleased with how many places in the IR I have to special-case for rate-qualified types. At the same type, pulling out rate as a distinct field on `IRValue` would probably require that we pay attention to rate everywhere.
* I've added a test case to show that we can issue errors when users fail to provide a constant expression for the texel offset, but the actual error message isn't great because it doesn't indicate *why* a constant expression was required. Realistically the "initial IR" should contain a few more decorations we can use to relate error conditions back to the original code (even if this is in a side-band structure).
* I've added a test case that is supposed to show that we can back-propagate `constexpr`-ness to local variables, and I've manually confirmed that it works for Vulkan/SPIR-V output, but the level of Vulkan support in `render_test` today means I can't enable the test for check-in.
* While I'm attempting to propagate `@ConstExpr` information from callees to callers, I haven't implemented any logic to specialize callee functions based on values at call sites.
* In a similar vein, there is no handling of control-flow dependence in the current code. If we infer that a phi (block parameter) needs to be `@ConstExpr`, then it isn't actually enough to require that the inputs to the phi (arguments from predecessor blocks) are all `@ConstExpr` because we also need any control-flow decisions that pick which incoming edge we take to be `@ConstExpr` as well.
* As a practical matter, implicit propagation of `@ConstExpr` from a function body to a function parameter should only be allowed for functions that are "local" to a module. Any function that might be accessed from outside of a module should really have had its `@ConstExpr` parameter marked manually, and our pass should validate that they follow their own rules. Right now we have no kind of visibility (`public` vs `private`) system, so I'm kind of ignoring this issue.
While that is a lot of gaps, this is also just enough code to get the Falcor MultiPassPostProcess example working, so I'm inclined to get it checked in.
* Fixup: missing expected output for test
* Fixup: disable test that relies on [unroll] for now
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* Re-define deprecated compile flags
By including these flags in the header file, with a value of zero, we can allow some existing code to compile even after the major changes to the implementation.
* The `SLANG_COMPILE_FLAG_NO_CHECKING` option will effectively be ignored, since checking is always enabled.
* The `SLANG_COMPILE_FLAG_SPLIT_MIXED_TYPES` option will now act as if it is always enabled (and indeed some of the code has been relying on this flag being set always).
* Make subscript operators writable for writable textures
This even had a `TODO` comment saying that we needed to fix it, and now I'm seeing semantic checking failures because we didn't define these and so we find assignment to non l-values.
* Fix definitions of any() and all() intrinsics
These should always return a scalar `bool` value, but they were being defined wrong in two ways:
1. They were using their generic type parameter `T` in the return type
2. They were returning a vector in the vector case, and a matrix in the matrix case.
This change just alters the return type to be `bool` in all cases.
* Fix bug in SSA construction
When eliminating a trivial phi node, it is possible that the phi is still recorded as the "latest" value for a local variable in its block.
When later code queries that value from the block (which can happen whenever another block looks up a variable in its predecessors), it would get the old phi and not the replacement value.
I simply added a loop that checks if the value we look up is a phi that got replaced, and then continues with the replacement value (which might itself be a phi...). A more advanced solution might try to get clever and have the map itself hold `IRUse` values so that we can replace them seamlessly.
* Simplify IR control flow representation
This change gets rid of various special-case operations for conditional and unconditional branches, and instead requires emit logic to recognize when a direct branch is targetting a `break` or `continue` label.
The new approach here isn't perfect, but it seems beter than what we had before, because it can actually work in the presence of control-flow optimizations (including our current critical-edge-splitting step).
* Load from groupshared isn't groupshared
When loading from a `groupshared` variable, the resulting temporary shouldn't have the `groupshared` qualifier on it.
This might eventually need to generalize to a better understanding of storage modifiers in the IR, but I don't really want to deal with that right now.
* Don't emit references to typedefs in output code
Now that we are using the IR for all codegen, we shouldn't be dealing with surface-level things like `typedef` declarations in the output code; just use the type that was being referred to in the first place.
* Fix floating-point literal printing for IR
The IR was calling `emit()` instead of `Emit()` (we really need to normalize our convention here), and was implicitly invoking a default constructor on `String` that takes a `double` (that constructor should really be marked `explicit`), and which doesn't meet our requirements for printing floating-point values.
* Fix error when importing module that doesn't parse
We already added a case to bail out if semantic checking fails, but neglected to add a case if there is an error during parsing of a module to be imported.
Note: this logic doesn't correctly register the module as being loaded (but still in error), so users could see multiple error messages if there are multiple `import`s for the same module.
* Improve error message for overload resolution failure
- Drop debugging info from the candidate printing
- Add cases to print `double` and `half` types properly
* Fixup: switch loopTest to ifElse in expected IR output
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* Generate SSA form for IR functions
The basic idea here is simple: in the front-end after we have lowered the AST to initial IR we will apply a set of "mandatory" optimization passes. The first of these is to attempt to translate the all functions into SSA form so that they are amenable to subsequent dataflow optimizations. Eventually, the mandatory optimization passes would include diagnostic passes that make sure variables aren't used when undefined, etc.
Just doing basic SSA generation already cleans up a lot of the messiness in our IR today, because constructs that used to involve many local variables can now be handled via SSA temporaries.
The implementation of SSA generation is in `ir-ssa.cpp`, and it follows the approach of Braun et al.'s "Simple and Efficient Construction of Static Single Assignment Form." I used this instead of the more well-known Cytron et al. algorithm because Braun's algorith mis very simple to code, and does not require auxiliary analyses to generate the dominance frontier.
The main wrinkle in our SSA representation right now is that instead of using ordinary phi nodes, we instead allow basic blocks to have parameters, where predecessor blocks pass in different parameter values. This encodes information equivalent to traditional phi nodes, but has two (small) benefits:
1. There is no fixed relationship between the order of phi operands and predecessor blocks, so we don't have to worry about breaking the phis when we alter the order in which predecessors are stored. This is important for us because predecessors are being stored implicitly.
2. It is easy to operationalize a "branch with arguments" either when lowering to other languages, or when interpreting the IR. A branch with arguments is implemented as a sequence of stores from the arguments to the parameters of the target block (very similar to a call), followed by a jump to the block.
Relevant to the above, this change also adds an interface for enumerating the predecessors or successors of a block in our CFG. Rather than use an auxliary structure, we directly use the information already encoded in the IR:
* The sucessors of a block are the target label operands of its terminator instruction. In our IR this is a contiguous range of `IRUse`s, possible with a stride (to account for the way `switch` interleaves values and blocks).
* The predecessors of a block are a subset of the uses of the block's value. Specifically, they are any uses that are on a terminator instruction, and within the range of values that represent the successor list of that instruction.
One important limitation of the "blocks with arguments" model for handling phis is that it is really only convenient to stash extra arguments on an unconditional terminator instruction. This change works around this prob lem by breaking any "critical edges" - edges between a block with multiple successors and one with multiple predecessors. We assume that "phi" nodes will only ever be needed on a block with multiple predecessors, and because critical edges are broken, each of these predecessors will then have only a single successor, so its branch instruction can handle the extra arguments.
This change introduces a notion of an "undefined" instruction in the IR. This is handled as an instruction rather than a value because I anticipate that we will want to distinguish different undefined values when it comes time to start issuing error messages (those messages will need to point to the variable that was used when undefined).
* Fix expected test output.
Another change was merged that enabled the `glsl-parameter-blocks` test, and its output is affected by our IR optimization work.
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* IR: fixes for subscript accessors
Fixes #320
This is a bunch of fixes for handling of `__subscript` operations on builtin types (notably `RWStructuredBuffer` and `StructuredBuffer` at this point).
- Automatically add a `GetterDecl` to any subscript decalratio was declithout any accessors. This avoids hitting a null- dereference in the emit logic.
- Add a notion of a `RefAccessor` (declared with `ref`) as a peer to getters and setters. The idea is that a `ref` accessor returns a pointer to the element data, so that it can be used for both getting and setting values. This is closer to the behavior of `RWStructuredBuffer` element access in HLSL.
- Fixes for dealing with "access chains" where there might be a combination of a subscript (where the is a `get` and `set` but no `ref`) and member access, so that we have to read the base value into a temp, modify it, and then write it back.
- This logic is still a bit of a mess, so we will eventually want to take a more consistent pass over this to deal with how we "materialize" values for setters.
- Update `RWStructuredBuffer` to have a `ref` accessor, and then fix up the IR tests to handle the new opcode that I added for it.
- Note: I didn't handle this as an intrinsic simply because the `tests/ir/*` tests aren't really set up to handle builtins with ugly mangled names.
* Fixup: type error in VM for buffer element ref
I was using the result type of the op as the element type for computing the element address, but the result type is a pointer to the real element type.
This caused test failures on 64-bit platforms, where the stride of the buffer in the `ir/factorial` test needs to be 4.
The fix is to assume the result type is a pointer, and extract the pointed-to type out of that.
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- Change function mangling so we use `p<parameterCount>p` instead of just `p<parameterCount>` to avoid the parameter count running into digits at the start of a mangled type name and tripping up the un-mangling logic.
- We really need to step back at some point and define our mangling scheme a bit more carefully, especially if we are going to keep going down this road where un-mangling things is important for generating HLSL output.
- Also allow the unmangling logic to unmangle a few more cases of generic parameters, so that it can skip over them to get to the parameter count of the underlying function.
- Add a notion of an `unreachable` instruction to the IR, and emit it as the terminator (if needed) at the end of the last block for a function with a non-void return type.
- This does *not* implement any logic to emit a diagnostic if the `unreachable` turns out to be potentially reachable
- Fix a bug in IR specialization of generics where we can't create two different specializations of the same function, because both get registered in the same hash map
With all these fixes, testing in Falcor modified to use the full Slang compiler and IR for all HLSL/Slang:
- The UI and text rendering shaders yield HLSL that compiles without error; no idea if they actually *work*
- The ModelViewer shaders yield HLSL, but there are some issues (looks like type legalization isn't applying to stuff inside constant buffers)
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The test case had previously been calling `GroupBarrierWithGroupSync` as if it was a special-cased instruction, but now it is just calling it as an ordinary (intrinsic) function.
I haven't removed the now-useless instruction, but it would be a good cleanup to go through and eliminate all the instruction cases we aren't using in the near future.
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This change includes a lot of infrastructure work, but the main point is to allow code like the following:
```
// define an interface
interface Helper { float help(); }
// define a generic function that uses the interface
float test<T : Helper>( T t ) { return t.help(); }
// define a type that implements the interface
struct A : Helper { float help() { return 1.0 } }
// define an ordinary function that calls the
// generic function with a concrete type:
float doIt()
{
A a;
return test<A>(a);
}
```
Getting this to generate valid code involves a lot of steps. This change includes the initial version of all of these steps, but leaves a lot of gaps where more complete implementation is required.
The changes include:
- Member lookup on types has been centralized, and now handles the case where the type we are looking for a member in is a generic parameter (e.g., given `t.help()` we can now look up `help` in `Helper` by knowing that `t` is a `T` and `T` conforms to `Helper`).
- There is an obvious cleanup still to be done here where the same exact logic should be used to look up available "constructor" declarations inside a type when the type is used like a function.
- Add a notion of subtype constraint "wittnesses" to the type system. When a generic is declared as taking `<T : Helper>` it really takes two generic parameters: the type `T` and a proof that `T` conforms to `Helper`. The actual arguments to a generic will then include both the type argument and a suitable witness argument (both type-level values).
- As it stands right now, a witness wraps a `DeclRef` to the declaration that represents the appropriate subtype relationship. So if we have `struct A : Helper`, that `: Helper` part turns into an `InheritanceDecl` member, and a reference to that member can serve as a witness to the fact that `A` conforms to `Helper`.
- Make explicit generic application `G<A,B>` synthesize the additional arguments that represent conformances required by the generic.
- This does *not* yet deal with the case where a generic is implicitly specialized as part of an ordinary call `G(a,b)`
- A bug fix to not auto-specialize generics during lookup. The problem here was related to an attempted fix of an earlier issue.
During checking of a method nested in a generic type, we were running into problems where `DeclRefType::create()` was getting called on an un-specialized reference to `vector`, and this was leading to a crash when the code looked for the arguments for the generic. This was worked around by having name lookup automatically specialize any generics it runs into while going through lookup contexts.
That choice creates the problem that in a generic method like this:
```
void test<T>(T val) { ... }
```
any reference to `val` inside the body of `test` will end up getting specialized so that it is effectively `test<T>::val`, when that isn't really needed.
- Add front-end logic to check that when a type claims to conform to an interface it actually must provide the methods required by the interface. The checking process goes ahead and builds a front-end "witness table" that maps declarations in the interface being conformed to over to their concrete implementations for the type.
- At the moment the checking is completely broken and bad: it assumes that *any* member with the right name is an appropriate declaration to satisfy a requirement. That obviously needs to be fixed.
- Add an explicit operation to the IR for lookup of methods: `lookup_interface_method(w, r)` where `w` is a reference to the "witness" value and `r` is an `IRDeclRef` for the member we want to look up.
- Add an explicit notion of witness tables to the IR. These end up being the IR representation of an `InheritanceDecl` in a type, and they are generated by enumerating the members that satisfy the interface requirements (which were handily already enumerated by the front-end checking). The witness table is an explicit IR value, and so it will be referenced/used at the site where conformance is being exploited (e.g., as part of a `specialize` call), so it should be safe to eliminate witness tables that are unused (since they represent conformances that aren't actually exploited). Similarly, the entries in a witness table are uses of the functions that implement interface methods, and so keep those live.
- In order to implement the above, I did a bit of a cleanup pass on the IR representation so that there is an `IRUser` base that `IRInst` inherits from, so that we can have users of values that aren't instructions.
- One annoying thing is that because of how types and generics are handled in the IR, we needed a way to have a type-level `Val` that wraps an IR-level value: e.g., to allow an IR-level witness table to be used as one of the arguments for specialization of a generic. The design I chose here is to have a "proxy" `Val` subclass (`IRProxyVal`) that wraps an `IRValue*`. These should only ever appear as part of types and `DeclRef`s that are used by the IR.
- One annoying bit here is that an IR value might then have a use that is not manifest in the set of IR instructions, and instead only appears as part of a type somewhere.
- I'm not 100% happy with this design, but it seems like we'd have to tackle similar issues if/when we eventually allow functions to have `constexpr` or `@Constant` parameters
- Make generic specialization also propagate witness table arguments through to their use sites (this is mostly just the existing substitution machinery, once we have `IRProxyVal`), and then include logic to specialize `lookup_interface_method` instructions when their first operand is a concrete witness table.
All of this work allows a single limited test using generics with constraints to pass, but more work is needed to make the solution robust.
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There are two big changes here:
- Add logic during the initial IR cloning pass for an entry point + target that tries to pick the best possible version of any target-overloaded function. This allows us to pick the intrinsic version of `saturate()` when compiling for HLSL output, but then pick the non-intrinsic version (that is implemented in terms of `clamp()`) when targetting GLSL.
- Add an initial specialization pass that tries to deal with generics. This required some fixing work to IR generation, so that we correctly generate explicit operations to specialize a generic for specific types (this is currently implemented as a `specialize` instruction that takes the generic to specialize plus a declaration-reference that represents the specialized form). With that work in place, we can scan for `specialize` instructions inside of non-generic functions, and use them to trigger generation of specialized code. We rely on the name-mangling scheme to help us find pre-existing specializations when possible.
There are also a bunch of cleanups encountered along the way:
- Don't use the explicit `layout(offset=...)` for uniforms, because it isn't supported by all current drivers. For now we will just assume that our layout rules compute the same values that the driver would for un-marked-up code. We can come back later and try to implement a workaround in the cases where this doesn't apply (e.g., by re-running the layout logic as part of emission, and dropping layout modifiers from variables that don't need explicit layout).
- Fix some issues in IR dump printing so that we print function declarations more nicely.
- Testing: print out failing pixel when image-diff fails
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* Get rid of the `-slang-ir-asm` target
This is really only useful for debugging, so I've replaced the functionality with a `-dump-ir` command line option (which dump's the IR for an entry point before doing codegen).
* fixup: use HLSL target, not DXBC, so test can run on Linux
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* IR: overhaul IR design/implementation
Closes #192
Closes #188
This is a major overhaul of how the IR is implemented, with the primary goal of just using the AST-level type representation as the IR's type representation, rather than inventing an entire shadow set of types (as captured in issue #192).
One consequence of this choice is that types in the IR are no longer explicit "instructions" and are not represented as ordinary operands (so a bunch of `+ 1` cases end up going away when enumerating ordinary operands).
Along the way I also got rid of the embedded IDs in the IR (issue #188) because this wasn't too hard to deal with at the same time.
Another related change was to split the `IRValue` and `IRInst` cases, so that there are values that are not also instructions. Non-instruction values are now used to represent literals, references to declarations, and would eventually be used for an `undef` value if we need one. IR functions, global variables, and basic blocks are all values (because they can appear as operands), but not instructions.
The main benefit of this approach is that the top-level structure of a bytecode (BC) module is much simpler to understand and walk, and BC-level types are represented much more directly (such that we could conceivably use them for reflection soon).
* fixup: 64-bit build fix
* fixup: try to silence clang's pedantic dependent-type errors
* fixup: bug in VM loading of constants
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None of these changes are made "live" at the moment. I'm just trying to get them checked in to avoid divering too far from `master` at any point during development.
- Add basic emit logic to produce GLSL from the IR in a few cases (the existing IR emit logic was ad hoc and HLSL-specific)
- When lowering a function declaration, walk up its chain of parent declarations to collect additional parameters as needed
- When lowering a call, make sure to add generic arguments that come from the declaration reference being called
- Attach a "mangled name" to symbols when lowering, so that we can eventually use that name to resolve things for linkage.
- After the above work, I had to apply some fixups to make sure that generic arguments *don't* get added when the user is calling an `__intrinsic_op` function, since those should map 1-to-1 down to instructions with just their ordinary parameter list.
A big open question right now is whether I should continue to represent the generic arguments as just part of the ordinary argument list for a function, or split them out into separate `applyGeneric` and `apply` steps.
A strongly related question is whether a declaration with generic parameters should lower into a single declaration, or one declaration nested inside an outer generic declaration.
A good future step at this point would be to eliminate a lot of the `__intrinsic_op` stuff in favor of having the builtin functions include their own definitions, which might be in terms of a new expression-level construct for writing inline IR operations. This can't be done until the existing AST-to-AST path is no longer needed for cross-compilation purposes.
More immediate next steps here:
- We need a way to round-trip calls to external declaration that get handled by this mangled-name logic. Basically, if we are asked to output HLSL and we see a call to `_S...GetDimensions...(float4, t, a, ...)` we need to be able to walk the mangled name and get back to `t.getDimensions(a, ...)` without a whole lot of manual definitions to make things round-trip.
- In the other case, where a declaration isn't built-in for the chosen target, we need to be able to load a module of target-specific definitions (which will somehow map back to symbols with certain mangled names) and then look these up (by mangled name) and then load/link/inline them into the user's IR to satisfy requirements in their code.
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At a high level, this commit adds two things:
1. A "bytecode" format for serializing Slang IR instructions and related structure (functions, "registers")
2. A virtual machine that can load and then execute code in that bytecode format.
The reason for kicking off this work right now is that we *need* a way to run tests on Slang code generation that doesn't rely on having a GPU present (given that our CI runs on VM instances without GPUs), nor on textual comparison to the output of other compilers. With these features I've implemented a slapdash `slang-eval-test` test fixture that can run a (trivial) compute shader to very our compilation flow through to bytecode.
Some key design constraints/challenges:
- The bytecode format should be "position independent" so that a user can just load a blob of data and then inspect it without having to deserialize into another format, allocate memory, etc. Eventually the bytecode format might be a replacement for out current reflection API (we used to base reflection off a similar format, but the cost/benefit wasn't there at the time and we switched to just using the AST).
- The VM should be able to execute bytecode functions without doing any per-operation translation, JIT, etc. (translation of more coarse-grained symbols is okay). For now the VM is just being used to run tests, but eventually I'd like it to be viable for:
- Running Slang-based code in the context of the compiler itself. This starts with stuff like constant-folding in the front-end, but could expand to more general metaprogramming features.
- Running Slang-based ocde within a runtime application (e.g., a game engine) that wants to be able to run things like "parameter shader" code, or even just evaluate compute-like code on CPU (e.g., when supporting particles on both CPU and GPU).
- Finally, the bytecode format should ideally be able to round-trip back to the IR without unacceptable loss of information. This requirement and the previous one play off of each other, because things like a traditional SSA phi operation is ugly when you have to actually *execute* it. This doesn't matter right now when we don't have SSA yet, but it might be part of the decision-making here.
The actual implementation is centralized in `bytecode.{h,cpp}` and `vm.{h.cpp}`.
Big picture notes:
- The space of opcodes is shared between IR and bytecode (BC), with the hope that this makes translation of operations between the two easy.
- The actual bytecode instruction stream relies on a variable-length encoding for integer values, including opcodes and operand numbers, so that the common case is single-byte encoding.
- In the long term I intend to have a rule that if you use a single-byte encoding for an opcode, then all operands are required to use single-byte encodings too. Operations that need multi-byte operands would then be forced to use a multi-byte encoding of the op, and would be sent down a slower path in the interpeter.
- The "bytecode"'s outer structure is based on ordinary data structures linked with pointers, but they are "relative pointers" so the actual structure is position-independent.
- There are two main kinds of operands: registers and "constants." An operand is a signed integer where non-negatie values indicate registers (with `index == operandVal`) and negative values indicate constants (with `index == ~operandVal`).
- Registers are stored in the "stack frame" for a VM function call, and each has a fixed offset based on the size of the type and those that come before it. Conceptually, registers are allowed to overlap if they aren't live at the same time, and we manage this with a simple stack model: every register is supposed to identify the register that comes directly before it (this isn't implemented yet).
- "Constants" are more realistically a representation of "captured" values, but they are currently also how constants come in. Basically we can use a compact range of indices in the bytecode for a function, and each of these indices indirectly refers to some value in the next outer scope.
- The actual encoding of bytecode instructions right now is largely ad-hoc and very wasteful (we encode the type on everything, and we also encode everything as if it had varargs).
- In some cases, an instruction needs to know the types of the values involved (e.g., because it needs to load an array element, which means copying a number of bytes based on the size). The way the VM works we have types attached to our registers, so we currently get sneaky and look at those types in some ops. Longer term is makes sense to encode the required type info directly in the BC.
- There's a whole lot of hand-waving going on with how the actual top-level bytecode module gets loaded, because of the way we currently treat the top-level module as an instruction stream in the IR. This means that we try to represent the loaded module as a "stack frame" for a call to the module as a function, but that approach as serious problems, and isn't realistically what we want to do.
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* IR: handle control flow constructs
This change includes a bunch of fixes and additions to the IR path:
- `slang-ir-assembly` is now a valid output target (so we can use it for testing)
- This uses what used to be the IR "dumping" logic, revamped to support much prettier output.
- A future change will need to add back support for less prettified output to use when actually debugging
- IR generation for `for` loops and `if` statements is supported
- HLSL output from the above control flow constructs is implemented
- Revamped the handling of l-values, and in particular work on compound ops like `+=`
- Add basic IR support for `groupshared` variables
- Add basic IR support for storing compute thread-group size
- Output semantics on entry point parameters
- This uses the AST structures to find semantics, so its still needs work
- Pass through loop unroll flags
- This is required to match `fxc` output, at least until we implement
unrolling ourselves.
* Fixup: 64-bit build issues.
* fixup for merge
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