[SYCL][DOC] Add spec and design for "if_device"

sycl/ReleaseNotes.md

@@ -163,7 +163,7 @@ plugin. [b1533c5e]
 - Deprecated `group::get_global_range()`. [95338719]
 
 ### Documentation
-- Updated the [`sycl_ext_oneapi_device_if`](doc/extensions/proposed/sycl_ext_oneapi_device_if.asciidoc)
+- Updated the [`sycl_ext_oneapi_device_if`](doc/extensions/proposed/sycl_ext_oneapi_if_device_has.asciidoc)
 extension proposal to allow chaining `if_device_has`, `else_if_device_has` and
 `else_device` calls. [7f2b17ed]
 - Updated the [`sycl_ext_intel_fpga_device_selector`](doc/extensions/supported/sycl_ext_intel_fpga_device_selector.asciidoc)
@@ -1725,7 +1725,7 @@ Release notes for commit range 6a49170027fb..962909fe9e78
    [Uniform](doc/extensions/proposed/sycl_ext_oneapi_uniform.asciidoc) extensions [72e1611]
  - Added [Matrix Programming Extension for DPC++ document](doc/extensions/experimental/sycl_ext_oneapi_matrix/sycl_ext_oneapi_matrix.asciidoc) [ace4c733]
  - Implemented SYCL 2020 `sycl::span` [9356d53]
- - Added [device-if](doc/extensions/proposed/sycl_ext_oneapi_device_if.asciidoc) extension
+ - Added [device-if](doc/extensions/proposed/sycl_ext_oneapi_if_device_has.asciidoc) extension
    [4fb95fc]
  - Added a [programming guide](doc/MultiTileCardWithLevelZero.md) for
    multi-tile and multi-card under Level Zero backend [d581178a]

sycl/doc/design/DeviceAspectTraitDesign.md

@@ -10,14 +10,14 @@ corresponding compilation environment can guarantee that any and all the
 supported devices support the `aspect`.
 
 The design of these traits is inspired by the implementation of the
-[sycl\_ext\_oneapi\_device\_if][2] and
+[sycl\_ext\_oneapi\_if\_device\_has][2] and
 [sycl\_ext\_oneapi\_device\_architecture][3] extensions as described in
-[DeviceIf.md][4]. Additionally, it leverages part of the design for optional
+[IfDeviceHas.md][4]. Additionally, it leverages part of the design for optional
 kernel features, as described in [OptionalDeviceFeatures.md][5].
 
 ## Changes to the compiler driver
 
-Using the `-fsycl-targets` options introduced in [DeviceIf.md][4] and the
+Using the `-fsycl-targets` options introduced in [IfDeviceHas.md][4] and the
 configuration file introduced in [OptionalDeviceFeatures.md][5], the compiler
 driver finds the set of all aspects supported by each specified target. Note
 that in this section we refer to aspects as their integral representation as
@@ -124,7 +124,7 @@ This relies on the fact that unspecialized variants of `any_device_has` and
 `all_devices_have` are undefined.
 
 [1]: <https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:device-aspects>
-[2]: <../extensions/proposed/sycl_ext_oneapi_device_if.asciidoc>
+[2]: <../extensions/proposed/sycl_ext_oneapi_if_device_has.asciidoc>
 [3]: <../extensions/proposed/sycl_ext_oneapi_device_architecture.asciidoc>
-[4]: <DeviceIf.md>
+[4]: <IfDeviceHas.md>
 [5]: <OptionalDeviceFeatures.md>

sycl/doc/design/ESIMDDesignNotes.md

@@ -242,7 +242,7 @@ This section lists current major ESIMD gaps/TODOs.
    [design](https://github.com/intel/llvm/blob/sycl/sycl/doc/design/OptionalDeviceFeatures.md).
    This might require splitting implementations into per-architecture variants.
    `if_device_has`
-   [feature](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/proposed/sycl_ext_oneapi_device_if.asciidoc)
+   [feature](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/proposed/sycl_ext_oneapi_if_device_has.asciidoc)
    may help avoid duplication of common parts and dispatch to
    architecture-dependent code at fine-grained level from within a function.
 1. As VC BE moves away from `genx.*` intrinsics replacing them with `__spirv_*`

sycl/doc/design/IfDevice.md

@@ -0,0 +1,175 @@
+# Implementation design for "if\_device"
+
+This document describes the design for the DPC++ implementation of the
+[sycl\_ext\_oneapi\_if\_device][1] extension.
+
+[1]: <../extensions/proposed/sycl_ext_oneapi_if_device.asciidoc>
+
+
+## Phased implementation
+
+This document presents two designs for the "if\_device" extension.  The first
+is a trivial implementation that works in a multi-pass compiler like DPC++.
+The second is a hypothetical implementation that could work in a 1-pass
+compiler.
+
+
+## Multi-pass compiler implementation
+
+This implementation requires changes only to the device headers.  The
+implementation is very trivial, leveraging the existing `__SYCL_DEVICE_ONLY__`
+macro which is defined differently in the host compiler pass vs. the device
+compiler passes.
+
+```
+namespace sycl::ext::oneapi::experimental {
+namespace detail {
+
+// Helper object used to implement "otherwise".  The "MakeCall" template
+// parameter tells whether the previous call to "if_device" or "if_host" called
+// its "fn".  When "MakeCall" is true, the previous call to "fn" did not
+// happen, so the "otherwise" should call "fn".
+template<bool MakeCall>
+class if_device_or_host_helper {
+ public:
+  template<typename T>
+  void otherwise(T &&fn) {
+    if constexpr (MakeCall) {
+      fn();
+    }
+  }
+};
+
+} // namespace detail
+
+template<typename T>
+static auto if_device(T &&fn) {
+#ifdef __SYCL_DEVICE_ONLY__
+  fn();
+  return detail::if_device_or_host_helper<false>{};
+#else
+  return detail::if_device_or_host_helper<true>{};
+#endif
+}
+
+template<typename T>
+static auto if_host(T &&fn) {
+#ifdef __SYCL_DEVICE_ONLY__
+  return detail::if_device_or_host_helper<true>{};
+#else
+  fn();
+  return detail::if_device_or_host_helper<false>{};
+#endif
+}
+
+} // namespace sycl::ext::oneapi::experimental
+```
+
+
+## Single-pass compiler implementation
+
+This implementation requires changes to the device headers, some changes to
+the error diagnostics in the front-end (CFE), and a new IR pass.
+
+### Device headers
+
+The device headers translate the API into calls to two functions that are
+decorated with attributes named "sycl-call-if-on-device" and
+"sycl-call-if-on-host".
+
+```
+namespace sycl::ext::oneapi::experimental {
+namespace detail {
+
+// Call the callable object "fn" only when this code runs on a device.
+//
+// IR passes recognize this function from the "sycl-call-if-on-device"
+// attribute.
+template<typename T>
+[[clang::noinline]]
+[[__sycl_detail__::add_ir_attributes_function("sycl-call-if-on-device", true)]]
+void call_if_on_device(T &&fn) {
+  fn();
+}
+
+// Call the callable object "fn" only when this code runs on the host.
+//
+// IR passes recognize this function from the "sycl-call-if-on-host" attribute.
+template<typename T>
+[[clang::noinline]]
+[[__sycl_detail__::add_ir_attributes_function("sycl-call-if-on-host", true)]]
+void call_if_on_host(T &&fn) {
+  fn();
+}
+
+class call_if_on_device_helper {
+ public:
+  template<typename T>
+  void otherwise(T &&fn) {
+    call_if_on_device(std::forward<T>(fn));
+  }
+};
+
+class call_if_on_host_helper {
+ public:
+  template<typename T>
+  void otherwise(T &&fn) {
+    call_if_on_host(std::forward<T>(fn));
+  }
+};
+
+} // namespace detail
+
+template<typename T>
+static auto if_device(T &&fn) {
+  detail::call_if_on_device(std::forward<T>(fn));
+  return detail::call_if_on_host_helper{};
+}
+
+template<typename T>
+static auto if_host(T &&fn) {
+  detail::call_if_on_host(std::forward<T>(fn));
+  return detail::call_if_on_device_helper{};
+}
+
+} // namespace sycl::ext::oneapi::experimental
+```
+
+Note the use of `[[clang::noinline]]`.  It is important that the bodies of
+these functions are not inlined until after the IR pass described below.
+
+### Changes to the front-end (CFE)
+
+The CFE currently diagnoses some errors that are specific to device code. To do
+this, the CFE must first traverse the static call tree to determine which
+functions are called from kernels.  This pass of the CFE must recognize the
+functions marked with the attribute "sycl-call-if-on-host" and skip the bodies
+of these functions when building the static call tree of the kernels.  As a
+result, the CFE will not emit any diagnostics that are specific to device code
+for the callable object that is passed to these functions.
+
+In a 1-pass compiler, we expect that the CFE will emit a single stream of
+LLVM IR for both host and device.  This IR retains any calls to the functions
+marked with "sycl-call-if-on-host" or "sycl-call-if-on-device" and retains the
+full bodies of those functions.  The filtering described above is used only to
+determine the functions that are checked for device-specific errors.
+
+### New IR pass
+
+The 1-pass compiler will eventually split the LLVM IR into two parts: one that
+contains the device code and one that contains the host code.  We expect that
+this pass will traverse the static call tree of the kernels to identify device
+code.  This pass also recognizes the functions marked with
+"sycl-call-if-on-host" and "sycl-call-if-on-device".  When generating the IR
+for the device code, the bodies of functions marked "sycl-call-if-on-host" are
+deleted, leaving empty functions.  When generating the IR for the host code,
+the bodies of functions marked "sycl-call-if-on-device" are deleted.
+
+Alternatively, the IR pass could use metadata from the CFE to identify host vs.
+device code, rather than repeating the static call tree traversal here.  These
+details could be resolved later as part of a 1-pass compiler design.
+
+Up until this point, it was important to prevent inlining of the functions
+marked "sycl-call-if-on-host" and "sycl-call-if-on-device".  Once the IR is
+split, inlining is permitted, so this IR pass also removes the LLVM IR
+`noinline` attributes from these functions.

sycl/doc/design/DeviceIf.md → sycl/doc/design/IfDeviceHas.md

@@ -1,11 +1,11 @@
-# Implementation design for "device\_if" and "device\_architecture"
+# Implementation design for "if\_device\_has" and "device\_architecture"
 
 This document describes the design for the DPC++ implementation of the
-[sycl\_ext\_oneapi\_device\_if][1] and
+[sycl\_ext\_oneapi\_if\_device\_has][1] and
 [sycl\_ext\_oneapi\_device\_architecture][2] extensions.
 
-[1]: <../extensions/proposed/sycl_ext_oneapi_device_if.asciidoc>
-[2]: <../extensions/proposed/sycl_ext_oneapi_device_architecture.asciidoc>
+[1]: <../extensions/proposed/sycl_ext_oneapi_if_device_has.asciidoc>
+[2]: <../extensions/experimental/sycl_ext_oneapi_device_architecture.asciidoc>
 
 
 ## Phased implementation

sycl/doc/extensions/experimental/sycl_ext_oneapi_device_architecture.asciidoc

@@ -61,7 +61,7 @@ limitations.
 
 This extension provides a way for device code to query the device architecture
 on which it is running.  This is similar to the
-link:../proposed/sycl_ext_oneapi_device_if.asciidoc[sycl_ext_oneapi_device_if]
+link:../proposed/sycl_ext_oneapi_if_device_has.asciidoc[sycl_ext_oneapi_if_device_has]
 extension except the comparison is for the device's architecture not the
 device's aspects.  In some cases, low-level application code can use special
 features or do specific optimizations depending on the device architecture, and
@@ -492,7 +492,7 @@ template<architecture ...Archs, typename T>
 ```
 
 This function operates exactly like `if_device_has` from the
-link:../proposed/sycl_ext_oneapi_device_if.asciidoc[sycl_ext_oneapi_device_if]
+link:../proposed/sycl_ext_oneapi_if_device_has.asciidoc[sycl_ext_oneapi_if_device_has]
 extension except that the condition gating execution of the callable function
 `fn` is determined by the `Archs` parameter pack.  This condition is `true` if
 the device which executes `if_architecture_is` matches **any** of the
@@ -513,7 +513,7 @@ class /* unspecified */ {
 ```
 
 The `otherwise` function behaves exactly like the `otherwise` function from the
-link:../proposed/sycl_ext_oneapi_device_if.asciidoc[sycl_ext_oneapi_device_if]
+link:../proposed/sycl_ext_oneapi_if_device_has.asciidoc[sycl_ext_oneapi_if_device_has]
 extension.  The `else_if_architecture_is` function behaves exactly like
 `else_if_device_has` from that extension except that the condition gating
 execution of the callable object `fn` is determined by the `Archs` parameter