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64bit-type-support.md

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Slang 64-bit Type Support

Summary

  • Not all targets support 64 bit types, or all 64 bit types
    • 64 bit integers generally require later APIs/shader models
  • When specifying 64 bit literals always use the type suffixes (ie L, ULL, LL)
  • GPU target/s generally do not support all double intrinsics
    • Typically missing are trascendentals (sin, cos etc), logarithm and exponental functions
    • CUDA is the exception supporting nearly all double intrinsics
  • D3D
    • D3D targets appear to support double intrinsics (like sin, cos, log etc), but behind the scenes they are actually being converted to float
    • When using D3D12, it is best to use DXIL if you use double because there are some serious issues around double and DXBC
  • VK will produce an error in validation if a double intrinsic is used it does support (which is most of them)
  • Vector and Matrix types have even spottier than scalar intrinsic support across targets

Overview

The Slang language supports 64 bit built in types. Such as

  • double
  • uint64_t
  • int64_t

This also applies to vector and matrix versions of these types.

Unfortunately if a specific target supports the type or the typical HLSL instrinsic functions (such as sin/cos/max/min etc) depends very much on the target.

Special attention has to be made with respect to literal 64 bit types. By default float and integer literals if they do not have an explicit suffix are assumed to be 32 bit. There is a variety of reasons for this design choice - the main one being around by default behavior of getting good performance. The suffixes required for 64 bit types are as follows

// double - 'l' or 'L'

double a = 1.34e-200L;
// WRONG!: This is the same as b = double(float(1.34e-200)) which will be 0. Will produce a warning.
double b = 1.34e-200; 

// int64_t - 'll' or 'LL' (or combination of upper/lower)

int64_t c = -5436365345345234ll;
// WRONG!: This is the same as d = int64_t(int32_t(-5436365345345234)) which means d ! = -5436365345345234LL. 
// Will produce a warning.
int64_t d = -5436365345345234;      

int64_t e = ~0LL;       // Same as 0xffffffffffffffff
// Does produce the same result as 'e' because equivalent int64_t(~int32_t(0))
int64_t f = ~0;         

// uint64_t - 'ull' or 'ULL' (or combination of upper/lower)

uint64_t g = 0x8000000000000000ull; 
// WRONG!: This is the same as h = uint64_t(uint32_t(0x8000000000000000)) which means h = 0
// Will produce a warning.
uint64_t h = 0x8000000000000000u;   

uint64_t i = ~0ull;       // Same as 0xffffffffffffffff
uint64_t j = ~0;          // Equivalent to 'i' because uint64_t(int64_t(~int32_t(0)));

These issues are discussed more on issue #1185

Double support

Target Compiler/Binary Double Type Intrinsics Notes
CPU Yes Yes 1
CUDA Nvrtx/PTX Yes Yes 1
D3D12 DXC/DXIL Yes Small Subset 4
Vulkan GlSlang/Spir-V Yes Partial 2
D3D11 FXC/DXBC Yes Small Subset 4
D3D12 FXC/DXBC Yes Small Subset 3, 4
  1. CUDA and CPU support most intrinsics, with the notable exception currently of matrix invert
  2. In terms of lack of general intrinsic support, the restriction is described in https://www.khronos.org/registry/spir-v/specs/1.0/GLSL.std.450.html

The following intrinsics are available for Vulkan

fmod (as %), rcp, sign, saturate, sqrt, rsqrt, frac, ceil, floor, trunc, abs, min, max, smoothstep, lerp, clamp, step and asuint.

These are tested in the test tests/hlsl-intrinsic/scalar-double-vk-intrinsic.slang.

What is missing are transedentals, expX, logX.

Note that GlSlang does produce Spir-V that contains double intrinsic calls for the missing intrinsics, the failure happens when validating the Spir-V

Validation: error 0:  [ UNASSIGNED-CoreValidation-Shader-InconsistentSpirv ] Object: VK_NULL_HANDLE (Type = 0) | SPIR-V module not valid: GLSL.std.450 Sin: expected Result Type to be a 16 or 32-bit scalar or vector float type
  %57 = OpExtInst %double %1 Sin %56

  1. That if a RWStructuredBuffer is used on D3D12 with DXBC, and a double is written, it can lead to incorrect behavior. Thus it is recommended not to use double with dxbc, but to use dxil to keep things simple. A test showing this problem is tests/bugs/dxbc-double-problem.slang. The test tests/hlsl-intrinsic/scalar-double-simple.slang shows not using a double resource, doubles do appear to work on D3D12 DXBC.

  2. If you compile code using double and intrinsics through Slang at first blush it will seem to work. Assuming there are no errors in your code, your code will even typically appear to work correctly. Unfortunately what is really happening is the backend compiler (fxc or dxc) compiler is narrowing double to float and then using float intrinsics. It typically generates a warning when this happens, but unless there is an error in your code you will not see these warnings because dxc doesn't appear to have a mechanism to return warnings if there isn't an error. This is why everything appears to work - but actually any intrinsic call is losing precision silently.

Note on dxc by default Slang disables warnings - warnings need to be enabled to see the narrowing warnings.

There is another exception around the use of % - if you do this with double it will return an error saying on float is supported.

It appears that no intrinsics are available for double with fxc.

On dxc the following intrinsics are available with double::

rcp, sign, saturate, abs, min, max, clamp, asuint.

These are tested in the test tests/hlsl-intrinsic/scalar-double-d3d-intrinsic.slang.

There is no suport for transcendentals (sin, cos etc) or log/exp. More surprising is thatsqrt, rsqrt, frac, ceil, floor, trunc, step, lerp, smoothstep are also not supported.

uint64_t and int64_t Support

Target Compiler/Binary u/int64_t Type Intrinsic support Notes
CPU Yes Yes
CUDA Nvrtx/PTX Yes Yes
Vulkan GlSlang/Spir-V Yes Yes
D3D12 DXC/DXIL Yes Yes 1
D3D11 FXC/DXBC No No 2
D3D12 FXC/DXBC No No 2

  1. The sm6.0 docs describe only supporting uint64_t, but dxc says int64_t is supported in HLSL 2016. Tests show that this is indeed the case.

  2. uint64_t support requires https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/hlsl-shader-model-6-0-features-for-direct3d-12, so DXBC is not a target.

The intrinsics available on uint64_t type are abs, min, max, clamp and countbits. The intrinsics available on uint64_t type are abs, min, max and clamp.

GLSL

GLSL/Spir-v based targets do not support 'generated' intrinsics on matrix types. For example 'sin(mat)' will not work on GLSL/Spir-v.