# SPIR-V Dialect to LLVM Dialect conversion manual This manual describes the conversion from [SPIR-V Dialect](Dialects/SPIR-V.md) to [LLVM Dialect](Dialects/LLVM.md). It assumes familiarity with both, and describes the design choices behind the modelling of SPIR-V concepts in LLVM Dialect. The conversion is an ongoing work, and is expected to grow as more features are implemented. Conversion can be performed by invoking an appropriate conversion pass: ```shell mlir-opt -convert-spirv-to-llvm ``` This pass performs type and operation conversions for SPIR-V operations as described in this document. [TOC] ## Type Conversion This section describes how SPIR-V Dialect types are mapped to LLVM Dialect. ### Scalar types SPIR-V Dialect | LLVM Dialect :------------: | :-----------------: `i` | `!llvm.i` `si` | `!llvm.i` `ui` | `!llvm.i` `f16` | `f16` `f32` | `f32` `f64` | `f64` ### Vector types SPIR-V Dialect | LLVM Dialect :-------------------------------: | :-------------------------------: `vector< x >` | `vector< x >` ### Pointer types A SPIR-V pointer also takes a Storage Class. At the moment, conversion does **not** take it into account. SPIR-V Dialect | LLVM Dialect :-------------------------------------------: | :-------------------------: `!spv.ptr< , >` | `!llvm.ptr<>` ### Array types SPIR-V distinguishes between array type and run-time array type, the length of which is not known at compile time. In LLVM, it is possible to index beyond the end of the array. Therefore, runtime array can be implemented as a zero length array type. Moreover, SPIR-V supports the notion of array stride. Currently only natural strides (based on [`VulkanLayoutUtils`](VulkanLayoutUtils)) are supported. They are also mapped to LLVM array. SPIR-V Dialect | LLVM Dialect :-----------------------------------: | :-----------------------------------: `!spv.array< x >`| `!llvm.array< x >` `!spv.rtarray< >` | `!llvm.array<0 x >` ### Struct types Members of SPIR-V struct types may have decorations and offset information. Currently, there is **no** support of member decorations conversion for structs. For more information see section on [Decorations](#Decorations-conversion). Usually we expect that each struct member has a natural size and alignment. However, there are cases (*e.g.* in graphics) where one would place struct members explicitly at particular offsets. This case is **not** supported at the moment. Hence, we adhere to the following mapping: * Structs with no offset are modelled as LLVM packed structures. * Structs with natural offset (*i.e.* offset that equals to cumulative size of the previous struct elements or is a natural alignment) are mapped to naturally padded structs. * Structs with unnatural offset (*i.e.* offset that is not equal to cumulative size of the previous struct elements) are **not** supported. In this case, offsets can be emulated with padding fields (*e.g.* integers). However, such a design would require index recalculation in the conversion of ops that involve memory addressing. Examples of SPIR-V struct conversion are: ```mlir !spv.struct => !llvm.struct !spv.struct => !llvm.struct<(i8, i32)> // error !spv.struct ``` ### Not implemented types The rest of the types not mentioned explicitly above are not supported by the conversion. This includes `ImageType` and `MatrixType`. ## Operation Conversion This section describes how SPIR-V Dialect operations are converted to LLVM Dialect. It lists already working conversion patterns, as well as those that are an ongoing work. There are also multiple ops for which there is no clear mapping in LLVM. Conversion for those have to be discussed within the community on the case-by-case basis. ### Arithmetic ops SPIR-V arithmetic ops mostly have a direct equivalent in LLVM Dialect. Such exceptions as `spv.SMod` and `spv.FMod` are rare. SPIR-V Dialect op | LLVM Dialect op :-----------------------------------: | :-----------------------------------: `spv.FAdd` | `llvm.fadd` `spv.FDiv` | `llvm.fdiv` `spv.FNegate` | `llvm.fneg` `spv.FMul` | `llvm.fmul` `spv.FRem` | `llvm.frem` `spv.FSub` | `llvm.fsub` `spv.IAdd` | `llvm.add` `spv.IMul` | `llvm.mul` `spv.ISub` | `llvm.sub` `spv.SDiv` | `llvm.sdiv` `spv.SRem` | `llvm.srem` `spv.UDiv` | `llvm.udiv` `spv.UMod` | `llvm.urem` ### Bitwise ops SPIR-V has a range of bit ops that are mapped to LLVM dialect ops, intrinsics or may have a specific conversion pattern. #### Direct conversion As with arithmetic ops, most of bitwise ops have a semantically equivalent op in LLVM: SPIR-V Dialect op | LLVM Dialect op :-----------------------------------: | :-----------------------------------: `spv.BitwiseAnd` | `llvm.and` `spv.BitwiseOr` | `llvm.or` `spv.BitwiseXor` | `llvm.xor` Also, some of bitwise ops can be modelled with LLVM intrinsics: SPIR-V Dialect op | LLVM Dialect intrinsic :-----------------------------------: | :-----------------------------------: `spv.BitCount` | `llvm.intr.ctpop` `spv.BitReverse` | `llvm.intr.bitreverse` #### `spv.Not` `spv.Not` is modelled with a `xor` operation with a mask with all bits set. ```mlir %mask = llvm.mlir.constant(-1 : i32) : i32 %0 = spv.Not %op : i32 => %0 = llvm.xor %op, %mask : i32 ``` #### Bitfield ops SPIR-V dialect has three bitfield ops: `spv.BitFieldInsert`, `spv.BitFieldSExtract` and `spv.BitFieldUExtract`. This section will first outline the general design of conversion patterns for this ops, and then describe each of them. All of these ops take `base`, `offset` and `count` (`insert` for `spv.BitFieldInsert`) as arguments. There are two important things to note: * `offset` and `count` are always scalar. This means that we can have the following case: ```mlir %0 = spv.BitFieldSExtract %base, %offset, %count : vector<2xi32>, i8, i8 ``` To be able to proceed with conversion algorithms described below, all operands have to be of the same type and bitwidth. This requires broadcasting of `offset` and `count` to vectors, for example for the case above it gives: ```mlir // Broadcasting offset %offset0 = llvm.mlir.undef : vector<2xi8> %zero = llvm.mlir.constant(0 : i32) : i32 %offset1 = llvm.insertelement %offset, %offset0[%zero : i32] : vector<2xi8> %one = llvm.mlir.constant(1 : i32) : i32 %vec_offset = llvm.insertelement %offset, %offset1[%one : i32] : vector<2xi8> // Broadcasting count // ... ``` * `offset` and `count` may have different bitwidths from `base`. In this case, both of these operands have to be zero extended (since they are treated as unsigned by the specification) or truncated. For the above example it would be: ```mlir // Zero extending offset after broadcasting %res_offset = llvm.zext %vec_offset: vector<2xi8> to vector<2xi32> ``` Also, note that if the bitwidth of `offset` or `count` is greater than the bitwidth of `base`, truncation is still permitted. This is because the ops have a defined behaviour with `offset` and `count` being less than the size of `base`. It creates a natural upper bound on what values `offset` and `count` can take, which is 64. This can be expressed in less than 8 bits. Now, having these two cases in mind, we can proceed with conversion for the ops and their operands. ##### `spv.BitFieldInsert` This operation is implemented as a series of LLVM Dialect operations. First step would be to create a mask with bits set outside [`offset`, `offset` + `count` - 1]. Then, unchanged bits are extracted from `base` that are outside of [`offset`, `offset` + `count` - 1]. The result is `or`ed with shifted `insert`. ```mlir // Create mask // %minus_one = llvm.mlir.constant(-1 : i32) : i32 // %t0 = llvm.shl %minus_one, %count : i32 // %t1 = llvm.xor %t0, %minus_one : i32 // %t2 = llvm.shl %t1, %offset : i32 // %mask = llvm.xor %t2, %minus_one : i32 // Extract unchanged bits from the Base // %new_base = llvm.and %base, %mask : i32 // Insert new bits // %sh_insert = llvm.shl %insert, %offset : i32 // %res = llvm.or %new_base, %sh_insert : i32 %res = spv.BitFieldInsert %base, %insert, %offset, %count : i32, i32, i32 ``` ##### `spv.BitFieldSExtract` To implement `spv.BitFieldSExtract`, `base` is shifted left by [sizeof(`base`) - (`count` + `offset`)], so that the bit at `offset` + `count` - 1 is the most significant bit. After, the result is shifted right, filling the bits with the sign bit. ```mlir // Calculate the amount to shift left. // %size = llvm.mlir.constant(32 : i32) : i32 // %t0 = llvm.add %count, %offset : i32 // %t1 = llvm.sub %size, %t0 : i32 // Shift left and then right to extract the bits // %sh_left = llvm.shl %base, %t1 : i32 // %t2 = llvm.add %offset, %t1 : i32 // %res = llvm.ashr %sh_left, %t2 : i32 %res = spv.BitFieldSExtract %base, %offset, %count : i32, i32, i32 ``` ##### `spv.BitFieldUExtract` For this op a similar pattern as for `spv.BitFieldInsert` is used. First, a mask with bits set at [0, `count` - 1] is created. Then `base` is shifted by `offset` and the mask is applied. ```mlir // Create a mask // %minus_one = llvm.mlir.constant(-1 : i32) : i32 // %t0 = llvm.shl %minus_one, %count : i32 // mask = llvm.xor %t0, %minus_one : i32 // Shift Base and apply mask // %sh_base = llvm.lshr %base, %offset : i32 // %res = llvm.and %sh_base, %mask : i32 %res = spv.BitFieldUExtract %base, %offset, %count : i32, i32, i32 ``` ### Cast ops #### Direct conversions SPIR-V Dialect op | LLVM Dialect op :-----------------------------------: | :-----------------------------------: `spv.ConvertFToS` | `llvm.fptosi` `spv.ConvertFToU` | `llvm.fptoui` `spv.ConvertSToF` | `llvm.sitofp` `spv.ConvertUToF` | `llvm.uitofp` #### spv.Bitcast This operation has a direct counterpart in LLVM: `llvm.bitcast`. It is treated separately since it also supports pointer to pointer bit pattern-preserving type conversion, apart from regular scalar or vector of numerical type. #### Special cases Special cases include `spv.FConvert`, `spv.SConvert` and `spv.UConvert`. These operations are either a truncate or extend. Let's denote the operand component width as A, and result component width as R. Then, the following mappings are used: ##### `spv.FConvert` Case | LLVM Dialect op :-------------: | :-----------------------------------: A < R | `llvm.fpext` A > R | `llvm.fptrunc` ##### `spv.SConvert` Case | LLVM Dialect op :-------------: | :-----------------------------------: A < R | `llvm.sext` A > R | `llvm.trunc` ##### `spv.UConvert` Case | LLVM Dialect op :-------------: | :-----------------------------------: A < R | `llvm.zext` A > R | `llvm.trunc` The case when A = R is not possible, based on SPIR-V Dialect specification: > The component width cannot equal the component width in Result Type. ### Comparison ops SPIR-V comparison ops are mapped to LLVM `icmp` and `fcmp` operations. SPIR-V Dialect op | LLVM Dialect op :-----------------------------------: | :-----------------------------------: `spv.IEqual` | `llvm.icmp "eq"` `spv.INotEqual` | `llvm.icmp "ne"` `spv.FOrdEqual` | `llvm.fcmp "oeq"` `spv.FOrdGreaterThan` | `llvm.fcmp "ogt"` `spv.FOrdGreaterThanEqual` | `llvm.fcmp "oge"` `spv.FOrdLessThan` | `llvm.fcmp "olt"` `spv.FOrdLessThanEqual` | `llvm.fcmp "ole"` `spv.FOrdNotEqual` | `llvm.fcmp "one"` `spv.FUnordEqual` | `llvm.fcmp "ueq"` `spv.FUnordGreaterThan` | `llvm.fcmp "ugt"` `spv.FUnordGreaterThanEqual` | `llvm.fcmp "uge"` `spv.FUnordLessThan` | `llvm.fcmp "ult"` `spv.FUnordLessThanEqual` | `llvm.fcmp "ule"` `spv.FUnordNotEqual` | `llvm.fcmp "une"` `spv.SGreaterThan` | `llvm.icmp "sgt"` `spv.SGreaterThanEqual` | `llvm.icmp "sge"` `spv.SLessThan` | `llvm.icmp "slt"` `spv.SLessThanEqual` | `llvm.icmp "sle"` `spv.UGreaterThan` | `llvm.icmp "ugt"` `spv.UGreaterThanEqual` | `llvm.icmp "uge"` `spv.ULessThan` | `llvm.icmp "ult"` `spv.ULessThanEqual` | `llvm.icmp "ule"` ### Composite ops Currently, conversion supports rewrite patterns for `spv.CompositeExtract` and `spv.CompositeInsert`. We distinguish two cases for these operations: when the composite object is a vector, and when the composite object is of a non-vector type (*i.e.* struct, array or runtime array). Composite type | SPIR-V Dialect op | LLVM Dialect op :-------------: | :--------------------: | :--------------------: vector | `spv.CompositeExtract` | `llvm.extractelement` vector | `spv.CompositeInsert` | `llvm.insertelement` non-vector | `spv.CompositeExtract` | `llvm.extractvalue` non-vector | `spv.CompositeInsert` | `llvm.insertvalue` ### `spv.EntryPoint` and `spv.ExecutionMode` First of all, it is important to note that there is no direct representation of entry points in LLVM. At the moment, we use the following approach: * `spv.EntryPoint` is simply removed. * In contrast, `spv.ExecutionMode` may contain important information about the entry point. For example, `LocalSize` provides information about the work-group size that can be reused. In order to preserve this information, `spv.ExecutionMode` is converted to a struct global variable that stores the execution mode id and any variables associated with it. In C, the struct has the structure shown below. ```C // No values are associated // There are values that are associated // with this entry point. // with this entry point. struct { struct { int32_t executionMode; int32_t executionMode; }; int32_t values[]; }; ``` ```mlir // spv.ExecutionMode @empty "ContractionOff" llvm.mlir.global external constant @{{.*}}() : !llvm.struct<(i32)> { %0 = llvm.mlir.undef : !llvm.struct<(i32)> %1 = llvm.mlir.constant(31 : i32) : i32 %ret = llvm.insertvalue %1, %0[0 : i32] : !llvm.struct<(i32)> llvm.return %ret : !llvm.struct<(i32)> } ``` ### Logical ops Logical ops follow a similar pattern as bitwise ops, with the difference that they operate on `i1` or vector of `i1` values. The following mapping is used to emulate SPIR-V ops behaviour: SPIR-V Dialect op | LLVM Dialect op :-----------------------------------: | :-----------------------------------: `spv.LogicalAnd` | `llvm.and` `spv.LogicalOr` | `llvm.or` `spv.LogicalEqual` | `llvm.icmp "eq"` `spv.LogicalNotEqual` | `llvm.icmp "ne"` `spv.LogicalNot` has the same conversion pattern as bitwise `spv.Not`. It is modelled with `xor` operation with a mask with all bits set. ```mlir %mask = llvm.mlir.constant(-1 : i1) : i1 %0 = spv.LogicalNot %op : i1 => %0 = llvm.xor %op, %mask : i1 ``` ### Memory ops This section describes the conversion patterns for SPIR-V dialect operations that concern memory. #### `spv.AccessChain` `spv.AccessChain` is mapped to `llvm.getelementptr` op. In order to create a valid LLVM op, we also add a 0 index to the `spv.AccessChain`'s indices list in order to go through the pointer. ```mlir // Access the 1st element of the array %i = spv.constant 1: i32 %var = spv.Variable : !spv.ptr>, Function> %el = spv.AccessChain %var[%i, %i] : !spv.ptr>, Function>, i32, i32 // Corresponding LLVM dialect code %i = ... %var = ... %0 = llvm.mlir.constant(0 : i32) : i32 %el = llvm.getelementptr %var[%0, %i, %i] : (!llvm.ptr)>>, i32, i32, i32) ``` #### `spv.Load` and `spv.Store` These ops are converted to their LLVM counterparts: `llvm.load` and `llvm.store`. If the op has a memory access attribute, then there are the following cases, based on the value of the attribute: * **Aligned**: alignment is passed on to LLVM op builder, for example: `mlir // llvm.store %ptr, %val {alignment = 4 : i64} : !llvm.ptr spv.Store "Function" %ptr, %val ["Aligned", 4] : f32` * **None**: same case as if there is no memory access attribute. * **Nontemporal**: set `nontemporal` flag, for example: `mlir // %res = llvm.load %ptr {nontemporal} : !llvm.ptr %res = spv.Load "Function" %ptr ["Nontemporal"] : f32` * **Volatile**: mark the op as `volatile`, for example: `mlir // %res = llvm.load volatile %ptr : !llvm.ptr %res = spv.Load "Function" %ptr ["Volatile"] : f32` Otherwise the conversion fails as other cases (`MakePointerAvailable`, `MakePointerVisible`, `NonPrivatePointer`) are not supported yet. #### `spv.globalVariable` and `spv.mlir.addressof` `spv.globalVariable` is modelled with `llvm.mlir.global` op. However, there is a difference that has to be pointed out. In SPIR-V dialect, the global variable returns a pointer, whereas in LLVM dialect the global holds an actual value. This difference is handled by `spv.mlir.addressof` and `llvm.mlir.addressof` ops that both return a pointer and are used to reference the global. ```mlir // Original SPIR-V module spv.module Logical GLSL450 { spv.globalVariable @struct : !spv.ptr>, Private> spv.func @func() -> () "None" { %0 = spv.mlir.addressof @struct : !spv.ptr>, Private> spv.Return } } // Converted result module { llvm.mlir.global private @struct() : !llvm.struct llvm.func @func() { %0 = llvm.mlir.addressof @struct : !llvm.ptr> llvm.return } } ``` The SPIR-V to LLVM conversion does not involve modelling of workgroups. Hence, we say that only current invocation is in conversion's scope. This means that global variables with pointers of `Input`, `Output`, and `Private` storage classes are supported. Also, `StorageBuffer` storage class is allowed for executing [`mlir-spirv-cpu-runner`](#`mlir-spirv-cpu-runner`). Moreover, `bind` that specifies the descriptor set and the binding number and `built_in` that specifies SPIR-V `BuiltIn` decoration have no conversion into LLVM dialect. Currently `llvm.mlir.global`s are created with `private` linkage for `Private` storage class and `External` for other storage classes, based on SPIR-V spec: > By default, functions and global variables are private to a module and cannot be accessed by other modules. However, a module may be written to export or import functions and global (module scope) variables. If the global variable's pointer has `Input` storage class, then a `constant` flag is added to LLVM op: ```mlir spv.globalVariable @var : !spv.ptr => llvm.mlir.global external constant @var() : f32 ``` #### `spv.Variable` Per SPIR-V dialect spec, `spv.Variable` allocates an object in memory, resulting in a pointer to it, which can be used with `spv.Load` and `spv.Store`. It is also a function-level variable. `spv.Variable` is modelled as `llvm.alloca` op. If initialized, an additional store instruction is used. Note that there is no initialization for arrays and structs since constants of these types are not supported in LLVM dialect (TODO). Also, at the moment initialization is only possible via `spv.constant`. ```mlir // Conversion of VariableOp without initialization %size = llvm.mlir.constant(1 : i32) : i32 %res = spv.Variable : !spv.ptr, Function> => %res = llvm.alloca %size x vector<3xf32> : (i32) -> !llvm.ptr> // Conversion of VariableOp with initialization %c = llvm.mlir.constant(0 : i64) : i64 %c = spv.constant 0 : i64 %size = llvm.mlir.constant(1 : i32) : i32 %res = spv.Variable init(%c) : !spv.ptr => %res = llvm.alloca %[[SIZE]] x i64 : (i32) -> !llvm.ptr llvm.store %c, %res : !llvm.ptr ``` Note that simple conversion to `alloca` may not be sufficient if the code has some scoping. For example, if converting ops executed in a loop into `alloca`s, a stack overflow may occur. For this case, `stacksave`/`stackrestore` pair can be used (TODO). ### Miscellaneous ops with direct conversions There are multiple SPIR-V ops that do not fit in a particular group but can be converted directly to LLVM dialect. Their conversion is addressed in this section. SPIR-V Dialect op | LLVM Dialect op :-----------------------------------: | :-----------------------------------: `spv.Select` | `llvm.select` `spv.Undef` | `llvm.mlir.undef` ### Shift ops Shift operates on two operands: `shift` and `base`. In SPIR-V dialect, `shift` and `base` may have different bit width. On the contrary, in LLVM Dialect both `base` and `shift` have to be of the same bitwidth. This leads to the following conversions: * if `base` has the same bitwidth as `shift`, the conversion is straightforward. * if `base` has a greater bit width than `shift`, shift is sign or zero extended first. Then the extended value is passed to the shift. * otherwise, the conversion is considered to be illegal. ```mlir // Shift without extension %res0 = spv.ShiftRightArithmetic %0, %2 : i32, i32 => %res0 = llvm.ashr %0, %2 : i32 // Shift with extension %ext = llvm.sext %1 : i16 to i32 %res1 = spv.ShiftRightArithmetic %0, %1 : i32, i16 => %res1 = llvm.ashr %0, %ext: i32 ``` ### `spv.constant` At the moment `spv.constant` conversion supports scalar and vector constants **only**. #### Mapping `spv.constant` is mapped to `llvm.mlir.constant`. This is a straightforward conversion pattern with a special case when the argument is signed or unsigned. #### Special case SPIR-V constant can be a signed or unsigned integer. Since LLVM Dialect does not have signedness semantics, this case should be handled separately. The conversion casts constant value attribute to a signless integer or a vector of signless integers. This is correct because in SPIR-V, like in LLVM, how to interpret an integer number is also dictated by the opcode. However, in reality hardware implementation might show unexpected behavior. Therefore, it is better to handle it case-by-case, given that the purpose of the conversion is not to cover all possible corner cases. ```mlir // %0 = llvm.mlir.constant(0 : i8) : i8 %0 = spv.constant 0 : i8 // %1 = llvm.mlir.constant(dense<[2, 3, 4]> : vector<3xi32>) : vector<3xi32> %1 = spv.constant dense<[2, 3, 4]> : vector<3xui32> ``` ### Not implemented ops There is no support of the following ops: * All atomic ops * All group ops * All matrix ops * All OCL ops As well as: * spv.CompositeConstruct * spv.ControlBarrier * spv.CopyMemory * spv.FMod * spv.GLSL.Acos * spv.GLSL.Asin * spv.GLSL.Atan * spv.GLSL.Cosh * spv.GLSL.FSign * spv.GLSL.SAbs * spv.GLSL.Sinh * spv.GLSL.SSign * spv.MemoryBarrier * spv.mlir.referenceof * spv.SMod * spv.specConstant * spv.Unreachable * spv.VectorExtractDynamic ## Control flow conversion ### Branch ops `spv.Branch` and `spv.BranchConditional` are mapped to `llvm.br` and `llvm.cond_br`. Branch weights for `spv.BranchConditional` are mapped to corresponding `branch_weights` attribute of `llvm.cond_br`. When translated to proper LLVM, `branch_weights` are converted into LLVM metadata associated with the conditional branch. ### `spv.FunctionCall` `spv.FunctionCall` maps to `llvm.call`. For example: ```mlir %0 = spv.FunctionCall @foo() : () -> i32 => %0 = llvm.call @foo() : () -> f32 spv.FunctionCall @bar(%0) : (i32) -> () => llvm.call @bar(%0) : (f32) -> () ``` ### `spv.selection` and `spv.loop` Control flow within `spv.selection` and `spv.loop` is lowered directly to LLVM via branch ops. The conversion can only be applied to selection or loop with all blocks being reachable. Moreover, selection and loop control attributes (such as `Flatten` or `Unroll`) are not supported at the moment. ```mlir // Conversion of selection %cond = spv.constant true %cond = llvm.mlir.constant(true) : i1 spv.selection { spv.BranchConditional %cond, ^true, ^false llvm.cond_br %cond, ^true, ^false ^true: ^true: // True block code // True block code spv.Branch ^merge => llvm.br ^merge ^false: ^false: // False block code // False block code spv.Branch ^merge llvm.br ^merge ^merge: ^merge: spv.mlir.merge llvm.br ^continue } // Remaining code ^continue: // Remaining code ``` ```mlir // Conversion of loop %cond = spv.constant true %cond = llvm.mlir.constant(true) : i1 spv.loop { spv.Branch ^header llvm.br ^header ^header: ^header: // Header code // Header code spv.BranchConditional %cond, ^body, ^merge => llvm.cond_br %cond, ^body, ^merge ^body: ^body: // Body code // Body code spv.Branch ^continue llvm.br ^continue ^continue: ^continue: // Continue code // Continue code spv.Branch ^header llvm.br ^header ^merge: ^merge: spv.mlir.merge llvm.br ^remaining } // Remaining code ^remaining: // Remaining code ``` ## Decorations conversion **Note: these conversions have not been implemented yet** ## GLSL extended instruction set This section describes how SPIR-V ops from GLSL extended instructions set are mapped to LLVM Dialect. ### Direct conversions SPIR-V Dialect op | LLVM Dialect op :-----------------------------------: | :-----------------------------------: `spv.GLSL.Ceil` | `llvm.intr.ceil` `spv.GLSL.Cos` | `llvm.intr.cos` `spv.GLSL.Exp` | `llvm.intr.exp` `spv.GLSL.FAbs` | `llvm.intr.fabs` `spv.GLSL.Floor` | `llvm.intr.floor` `spv.GLSL.FMax` | `llvm.intr.maxnum` `spv.GLSL.FMin` | `llvm.intr.minnum` `spv.GLSL.Log` | `llvm.intr.log` `spv.GLSL.Sin` | `llvm.intr.sin` `spv.GLSL.Sqrt` | `llvm.intr.sqrt` `spv.GLSL.SMax` | `llvm.intr.smax` `spv.GLSL.SMin` | `llvm.intr.smin` ### Special cases `spv.InverseSqrt` is mapped to: ```mlir %one = llvm.mlir.constant(1.0 : f32) : f32 %res = spv.InverseSqrt %arg : f32 => %sqrt = "llvm.intr.sqrt"(%arg) : (f32) -> f32 %res = fdiv %one, %sqrt : f32 ``` `spv.Tan` is mapped to: ```mlir %sin = "llvm.intr.sin"(%arg) : (f32) -> f32 %res = spv.Tan %arg : f32 => %cos = "llvm.intr.cos"(%arg) : (f32) -> f32 %res = fdiv %sin, %cos : f32 ``` `spv.Tanh` is modelled using the equality `tanh(x) = {exp(2x) - 1}/{exp(2x) + 1}`: ```mlir %two = llvm.mlir.constant(2.0: f32) : f32 %2xArg = llvm.fmul %two, %arg : f32 %exp = "llvm.intr.exp"(%2xArg) : (f32) -> f32 %res = spv.Tanh %arg : f32 => %one = llvm.mlir.constant(1.0 : f32) : f32 %num = llvm.fsub %exp, %one : f32 %den = llvm.fadd %exp, %one : f32 %res = llvm.fdiv %num, %den : f32 ``` ## Function conversion and related ops This section describes the conversion of function-related operations from SPIR-V to LLVM dialect. ### `spv.func` This op declares or defines a SPIR-V function and it is converted to `llvm.func`. This conversion handles signature conversion, and function control attributes remapping to LLVM dialect function [`passthrough` attribute](Dialects/LLVM.md#Attribute-pass-through). The following mapping is used to map [SPIR-V function control](SPIRVFunctionAttributes) to [LLVM function attributes](LLVMFunctionAttributes): SPIR-V Function Control Attributes | LLVM Function Attributes :-----------------------------------: | :-----------------------------------: None | No function attributes passed Inline | `alwaysinline` DontInline | `noinline` Pure | `readonly` Const | `readnone` ### `spv.Return` and `spv.ReturnValue` In LLVM IR, functions may return either 1 or 0 value. Hence, we map both ops to `llvm.return` with or without a return value. ## Module ops Module in SPIR-V has one region that contains one block. It is defined via `spv.module` op that also takes a range of attributes: * Addressing model * Memory model * Version-Capability-Extension attribute `spv.module` is converted into `ModuleOp`. This plays a role of enclosing scope to LLVM ops. At the moment, SPIR-V module attributes are ignored. `spv.mlir.endmodule` is mapped to an equivalent terminator `ModuleTerminatorOp`. ## `mlir-spirv-cpu-runner` `mlir-spirv-cpu-runner` allows to execute `gpu` dialect kernel on the CPU via SPIR-V to LLVM dialect conversion. Currently, only single-threaded kernel is supported. To build the runner, add the following option to `cmake`: ```bash -DMLIR_SPIRV_CPU_RUNNER_ENABLED=1 ``` ### Pipeline The `gpu` module with the kernel and the host code undergo the following transformations: * Convert the `gpu` module into SPIR-V dialect, lower ABI attributes and update version, capability and extension. * Emulate the kernel call by converting the launching operation into a normal function call. The data from the host side to the device is passed via copying to global variables. These are created in both the host and the kernel code and later linked when nested modules are folded. * Convert SPIR-V dialect kernel to LLVM dialect via the new conversion path. After these passes, the IR transforms into a nested LLVM module - a main module representing the host code and a kernel module. These modules are linked and executed using `ExecutionEngine`. ### Walk-through This section gives a detailed overview of the IR changes while running `mlir-spirv-cpu-runner`. First, consider that we have the following IR. (For simplicity some type annotations and function implementations have been omitted). ```mlir gpu.module @foo { gpu.func @bar(%arg: memref<8xi32>) { // Kernel code. gpu.return } } func @main() { // Fill the buffer with some data %buffer = alloc : memref<8xi32> %data = ... call fillBuffer(%buffer, %data) "gpu.launch_func"(/*grid dimensions*/, %buffer) { kernel = @foo::bar } } ``` Lowering `gpu` dialect to SPIR-V dialect results in ```mlir spv.module @__spv__foo /*VCE triple and other metadata here*/ { spv.globalVariable @__spv__foo_arg bind(0,0) : ... spv.func @bar() { // Kernel code. } spv.EntryPoint @bar, ... } func @main() { // Fill the buffer with some data. %buffer = alloc : memref<8xi32> %data = ... call fillBuffer(%buffer, %data) "gpu.launch_func"(/*grid dimensions*/, %buffer) { kernel = @foo::bar } } ``` Then, the lowering from standard dialect to LLVM dialect is applied to the host code. ```mlir spv.module @__spv__foo /*VCE triple and other metadata here*/ { spv.globalVariable @__spv__foo_arg bind(0,0) : ... spv.func @bar() { // Kernel code. } spv.EntryPoint @bar, ... } // Kernel function declaration. llvm.func @__spv__foo_bar() : ... llvm.func @main() { // Fill the buffer with some data. llvm.call fillBuffer(%buffer, %data) // Copy data to the global variable, call kernel, and copy the data back. %addr = llvm.mlir.addressof @__spv__foo_arg_descriptor_set0_binding0 : ... "llvm.intr.memcpy"(%addr, %buffer) : ... llvm.call @__spv__foo_bar() "llvm.intr.memcpy"(%buffer, %addr) : ... llvm.return } ``` Finally, SPIR-V module is converted to LLVM and the symbol names are resolved for the linkage. ```mlir module @__spv__foo { llvm.mlir.global @__spv__foo_arg_descriptor_set0_binding0 : ... llvm.func @__spv__foo_bar() { // Kernel code. } } // Kernel function declaration. llvm.func @__spv__foo_bar() : ... llvm.func @main() { // Fill the buffer with some data. llvm.call fillBuffer(%buffer, %data) // Copy data to the global variable, call kernel, and copy the data back. %addr = llvm.mlir.addressof @__spv__foo_arg_descriptor_set0_binding0 : ... "llvm.intr.memcpy"(%addr, %buffer) : ... llvm.call @__spv__foo_bar() "llvm.intr.memcpy"(%buffer, %addr) : ... llvm.return } ``` [LLVMFunctionAttributes]: https://llvm.org/docs/LangRef.html#function-attributes [SPIRVFunctionAttributes]: https://www.khronos.org/registry/spir-v/specs/unified1/SPIRV.html#_a_id_function_control_a_function_control [VulkanLayoutUtils]: https://github.com/llvm/llvm-project/blob/master/mlir/include/mlir/Dialect/SPIRV/LayoutUtils.h