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SPIRVVersionsAndExtensionsHandling.rst

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SPIR-V versions and extensions handling

Overview

This document describes how the translator makes decisions about using instructions from different version of the SPIR-V core and extension specifications.

Being able to control the resulting SPIR-V version is important: the target consumer might be quite old, without support for new SPIR-V versions and there must be the possibility to control which version of the SPIR-V specification that will be used during translation.

SPIR-V extensions is another thing which must be controllable. Extensions can update and re-define semantics and validation rules for existing SPIR-V entries and it is important to ensure that the translator is able to generate valid SPIR-V according to the core spec, without uses of any extensions if such SPIR-V was requested by user.

For example, without such infrastructure it is impossible to disable use of SPV_KHR_no_integer_wrap_decoration - it will be always generated if corresponding LLVM IR counterparts are encountered in input module.

It is worth mentioning that SPIR-V versions and extensions the handling of SPIR-V versions and extension is mostly important for the SPIR-V generation step. On the consumer side it is the responsibility of the consumer to analyze the incoming SPIR-V file and reject it if it contains something that is not supported by the consumer.

However, translator simplifies this step for downstream users by checking version and extensions in SPIR-V module during readSpirv/readSpirvModule phases.

SPIR-V Versions

SPIR-V Generation step

By default translator selects version of generated SPIR-V file based on features used in this file. For example, if it contains dereferencable LLVM IR attribute, MaxByteOffset decoration will be generated and resulting SPIR-V version will be raised to 1.1.

Note

There is no documentation about which exact features from newest SPIR-V spec versions will be used by the translator. If you are interested when or why a particular SPIR-V instruction is generated, please check this in the source code. Consider this as an implementation detail and if you disagree with something, you can always open an issue or submit pull request - contributions are welcome!

There is one option to control the behavior of the translator with respect to the version of the SPIR-V file which is being generated/consumed.

  • --spirv-max-version= - instructs the translator to generate SPIR-V file corresponding to any spec version which is less than or equal to the specified one. Behavior of the translator is the same as by default with only one exception: resulting SPIR-V version cannot be raised higher than specified by this option.

Allowed values are 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 and 1.6.

Warning

These two options are mutually exclusive and cannot be specified at the same time.

If the translator encounters something that cannot be represented by set of allowed SPIR-V versions (which might contain only one version), it does one of the following things:

  • ignores LLVM IR entity in the input file.

    For example, dereferencable LLVM IR attribute can be ignored if it is not allowed to generate SPIR-V 1.1 and higher.

  • tries to represent LLVM IR entity with allowed instructions.

    For example, OpPtrEqual can be used if SPIR-V 1.4 is not allowed and can be emulated via OpConvertPtrToU + OpIEqual sequence.

  • emits error if LLVM IR entity cannot be ignored and cannot be emulated using available instructions.

    For example, if global constructors/destructors (represented by @llvm.global_ctors/@llvm.global_dtors) are present in a module then the translator should emit error if it cannot use SPIR-V 1.1 and higher where Initializer and Finalizer execution modes are described.

SPIR-V Consumption step

By default, translator consumes SPIR-V of any version which is supported.

This behavior, however, can be controlled via the same switches described in the previous section.

If one of the switches present and translator encountered SPIR-V file corresponding to a spec version which is not included into set of allowed SPIR-V versions, translator emits error.

SPIR-V Extensions

SPIR-V Generation step

By default, translator doesn't use any extensions. If it required to enable certain extension, the following command line option can be used:

  • --spirv-ext= - allows to control list of allowed/disallowed extensions.

Valid value for this option is comma-separated list of extension names prefixed with + or - - plus means allow to use extension, minus means disallow to use extension. There is one more special value which can be used as extension name in this option: all - it affects all extension which are known to the translator.

If --spirv-ext contains name of extension which is not know for the translator, it will emit error.

Examples:

  • --spirv-ext=+SPV_KHR_no_integer_wrap_decoration,+SPV_INTEL_subgroups
  • --spirv-ext=+all,-SPV_INTEL_fpga_loop_controls

Warning

Extension name cannot be allowed and disallowed at the same time: for inputs like --spirv-ext=+SPV_INTEL_subgroups,-SPV_INTEL_subgroups translator will emit error about invalid arguments.

Note

Since by default during SPIR-V generation all extensions are disabled, this means that -all, is implicitly added at the beggining of the -spirv-ext value.

If the translator encounters something that cannot be represented by set of allowed SPIR-V extensions (which might be empty), it does one of the following things:

  • ignores LLVM IR entity in the input file.

    For example, nsw/nuw LLVM IR attributes can be ignored if it is not allowed to generate SPIR-V 1.4 and SPV_KHR_no_integer_wrap_decoration extension is disallowed.

  • tries to represent LLVM IR entity with allowed instructions.

    Translator could translate calls to a new built-in functions defined by some extensions as usual call instructions without using special SPIR-V instructions.

    However, this could result in a strange SPIR-V and most likely will lead to errors during consumption. Having that, translator should emit errors if it encounters a call to a built-in function from an extension which must be represented as a special SPIR-V instruction from extension which wasn't allowed to be used. I.e. if translator knows that this certain LLVM IR entity belongs to an extension functionality and this extension is disallowed, it should emit error rather than emulating it.

  • emits error if LLVM IR entity cannot be ignored and cannot be emulated using available instructions.

    For example, new built-in types defined by cl_intel_device_side_avc_motion_estimation cannot be represented in SPIR-V if SPV_INTEL_device_side_avc_motion_estimation is disallowed.

SPIR-V Consumption step

By default, translator consumes SPIR-V regardless of list extensions which are used by the input file, i.e. all extensions are allowed by default during consumption step.

Note

This is opposite to the generation step and this is done on purpose: to not broke workflows of existing users of the translator.

Note

Since by default during SPIR-V consumption all extensions are enabled, this means that +all, is implicitly added at the beggining of the -spirv-ext value.

This behavior, however, can be controlled via the same switches described in the previous section.

If --spirv-ext switch presents, translator will emit error if it finds out that input SPIR-V file uses disallowed extension.

Note

If the translator encounters unknown extension in the input SPIR-V file, it will emit error regardless of -spirv-ext option value.

If one of the switches present and translator encountered SPIR-V file corresponding to a spec version which is not included into set of allowed SPIR-V versions, translator emits error.

How to control translator behavior when using it as library

When using translator as library it can be controlled via bunch of alternative APIs that have additional argument: TranslatorOpts object which encapsulates information about available SPIR-V versions and extensions.

List of new APIs is: readSpirvModule, writeSpirv and readSpirv.

Note

See LLVMSPIRVOpts.h for more details.

How to get TranslatorOpts object

  1. Default constructor. Equal to:

    --spirv-max-version=MaxKnownVersion --spirv-ext=-all

    Note

    There is method TranslatorOpts::enableAllExtensions() that allows you to quickly enable all known extensions if it is needed.

  2. Constructor which accepts all parameters

    Consumes both max SPIR-V version and optional map with extensions status (i.e. which one is allowed and which one is disallowed)

Extensions status map

This map is defined as std::map<ExtensionID, bool> and it is intended to show which extension is allowed to be used (true as value) and which is not (false as value).

Note

If certain ExtensionID value is missed in the map, it automatically means that extension is not allowed to be used.

This implies that by default, all extensions are disallowed.