[doc] Add some documentation around translations.
This CL adds the start of a document listing how things translate into various backends. Change-Id: Iebeb4488450b4f299e83d0de7ee124b441795e98 Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/31240 Reviewed-by: David Neto <dneto@google.com> Commit-Queue: dan sinclair <dsinclair@chromium.org>
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# Translations
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This document attempts to document how WGSL translates into the various backends
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for the cases where the translation is not a direct mapping.
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# Access Control
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## HLSL
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* ReadOnly -> `ByteAddressBuffer`
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* ReadWrite -> `RWByteAddressBuffer`
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## MSL
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* ReadOnly -> `const`
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## SPIR-V
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There are two ways this can be achieved in SPIR-V. Either the variable can be
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decorated with `NonWritable` or each member of the struct can be decorated with
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`NonWritable`. We chose to go the struct member route.
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* The read-only becomes part of the type in this case. Otherwise, you are
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treating the readonly type information as part of the variable which is
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confusing.
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* Treating the readonly as part of the variable means we should be
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deduplicating the types behind the access control, which causes confusing
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with the type_names and various tracking systems within Tint.
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# Builtin Decorations
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Name | SPIR-V | MSL | HLSL
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=====|========|=====|=====
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position | SpvBuiltInPosition |position | SV_Position
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vertex_idx | SpvBuiltInVertexIndex |vertex_id | SV_VertexID
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instance_idx | SpvBuiltInInstanceIndex | instance_id| SV_InstanceID
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front_facing | SpvBuiltInFrontFacing | front_facing | SV_IsFrontFacing
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frag_coord | SpvBuiltInFragCoord | position | SV_Position
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frag_depth | SpvBuiltInFragDepth | depth(any) | SV_Depth
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local_invocation_id | SpvBuiltInLocalInvocationId | thread_position_in_threadgroup | SV_GroupThreadID
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local_invocation_idx | SpvBuiltInLocalInvocationIndex | thread_index_in_threadgroup | SV_GroupIndex
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global_invocation_id | SpvBuiltInGlobalInvocationId | thread_position_in_grid | SV_DispatchThreadID
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# Builtins Methods
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Name | SPIR-V | MSL | HLSL
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=====|========|=====|======
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abs | GLSLstd450FAbs or GLSLstd450SAbs| fabs or abs | abs
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acos | GLSLstd450Acos | acos | acos
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all | SpvOpAll | all | all
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any | SpvOpAny | any | any
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arrayLength | SpvOpArrayLength | |
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asin | GLSLstd450Asin | asin | asin
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atan | GLSLstd450Atan | atan | atan
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atan2 | GLSLstd450Atan2| atan2 | atan2
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ceil | GLSLstd450Ceil| ceil | ceil
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clamp | GLSLstd450NClamp or GLSLstd450UClamp or GLSLstd450SClamp| clamp | clamp
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cos | GLSLstd450Cos | cos | cos
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cosh | GLSLstd450Cosh | cosh | cosh
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countOneBits | SpvOpBitCount | popcount | countbits
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cross | GLSLstd450Cross | cross | cross
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determinant | GLSLstd450Determinant | determinant | determinant
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distance | GLSLstd450Distance | distance | distance
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dot | SpOpDot | dot | dot
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dpdx | SpvOpDPdx | dpdx | ddx
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dpdxCoarse | SpvOpDPdxCoarse | dpdx | ddx_coarse
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dpdxFine | SpvOpDPdxFine | dpdx | ddx_fine
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dpdy | SpvOpDPdy | dpdy | ddy
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dpdyCoarse | SpvOpDPdyCoarse | dpdy | ddy_coarse
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dpdyFine | SpvOpDPdyFine | dpdy | ddy_fine
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exp | GLSLstd450Exp | exp | exp
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exp2 | GLSLstd450Exp2 | exp2 | exp2
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faceForward | GLSLstd450FaceForward | faceforward | faceforward
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floor | GLSLstd450Floor | floor | floor
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fma | GLSLstd450Fma | fma | fma
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fract | GLSLstd450Fract | fract | frac
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frexp | GLSLstd450Frexp | |
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fwidth | SpvOpFwidth | fwidth | fwidth
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fwidthCoarse | SpvOpFwidthCoarse | fwidth | fwidth
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fwidthFine | SpvOpFwidthFine | fwidth | fwidth
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inverseSqrt | GLSLstd450InverseSqrt | rsqrt | rsqrt
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isFinite | | isfinite | isfinite
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isInf | SpvOpIsInf | isinf | isinf
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isNan | SpvOpIsNan | isnan | isnan
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isNormal | | isnormal |
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ldexp | GLSLstd450Ldexp | |
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length | GLSLstd450Length | length | length
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log | GLSLstd450Log | log | log
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log2 | GLSLstd450Log2 | log2 | log2
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max | GLSLstd450NMax or GLSLstd450SMax or GLSLstd450UMax | fmax or max | max
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min | GLSLstd450NMin or GLSLstd450SMin or GLSLstd450UMin | fmin or min | min
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mix | GLSLstd450FMix | mix | mix
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modf | GLSLstd450Modf | |
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normalize | GLSLstd450Normalize | normalize | normalize
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outerProduct | SpvOpOuterProduct | |
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pow | GLSLstd450Pow | pow | pow
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reflect | GLSLstd450Reflect | reflect | reflect
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reverseBits | SpvOpBitReverse | reverse_bits | reversebits
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round | GLSLstd450Round | round | round
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select | SpvOpSelect | select |
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sign | GLSLstd450FSign | sign | sign
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sin | GLSLstd450Sin | sin | sin
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sinh | GLSLstd450Sinh | sinh | sinh
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smoothStep | GLSLstd450SmoothStep | smoothstep | smoothstep
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sqrt | GLSLstd450Sqrt | sqrt | sqrt
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step | GLSLstd450Step | step | step
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tan | GLSLstd450Tan | tan | tan
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tanh | GLSLstd450Tanh | tanh | tanh
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trunc | GLSLstd450Trunc | trunc | trunc
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# Short-circuting
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## HLSL
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TODO(dsinclair): Nested if's
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## SPIR-V
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TODO(dsinclair): Nested if's
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# Storage classes
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TODO(dsinclair): do ...
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# Storage buffers
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## HLSL
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TODO(dsinclair): Rewriting of accessors to loads
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# Loop blocks
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## HLSL
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TODO(dsinclair): Rewrite with bools
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## MSL
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TODO(dsinclair): Rewrite with bools
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# Input / Output storage class
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## HLSL
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TODO(dsinclair): Structs and params
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## MSL
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TODO(dsinclair): Structs and params
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# Discard
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## HLSL
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* `discard`
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## MSL
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* `discard_fragment()`
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# Specialization constants
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## HLSL
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```
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#ifndef WGSL_SPEC_CONSTANT_<id>
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-- if default provided
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#define WGSL_SPEC_CONSTANT_<id> default value
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-- else
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#error spec constant required for constant id
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--
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#endif
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static const <type> <name> = WGSL_SPEC_CONSTANT_<id>
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```
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## MSL
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`[[function_constant(<id>)]]`
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