Support for `[[vk::spirv_instruction(op)]]` (#2242)

* #include an absolute path didn't work - because paths were taken to always be relative.

* Add extension required by SPIRVOpDecoration into part of emit (could be a prior pass).

* Add [[vk::spirv_instruction]] attribute

* Add documentation for [[vk::spirv_instruction].

* Update 08-attributes.md

* Update 08-attributes.md

1 files changed, 26 insertions, 0 deletions

diff --git a/docs/language-reference/08-attributes.md b/docs/language-reference/08-attributes.md
index 4098303cf..76a739315 100644
--- a/docs/language-reference/08-attributes.md
+++ b/docs/language-reference/08-attributes.md
@@ -2,3 +2,29 @@ Attributes
 ==========
 
 > Note: This section is not yet complete.
+
+## [[vk::spirv_instruction]]
+
+** SPIR-V only **
+
+This attribute is only available for Vulkan SPIR-V output via GLSLANG. In the future it could be supported via the `direct-spirv` option. The attribute will be ignored for any other target. This attribute requires the `GL_EXT_spirv_intrinsics` GLSL extension.
+
+The attibute allows access to SPIR-V intrinsics, by supplying a function declaration with the appropriate signature for the SPIR-V op and no body. The intrinsic takes a single parameter which is the integer value for the SPIR-V op. 
+
+In the example below the add function, uses the mechanism to directly use the SPIR-V integer add 'op' which is 128 in this case.
+
+```HLSL
+// 128 is OpIAdd in SPIR-V
+[[vk::spirv_instruction(128)]]
+uint add(uint a, uint b);
+
+RWStructuredBuffer<uint> resultBuffer;
+
+[numthreads(4,1,1)]
+void computeMain(uint3 dispatchThreadID : SV_DispatchThreadID)
+{
+    uint threadId = dispatchThreadID.x;
+    resultBuffer[threadId] = add(threadId, threadId);
+}
+```
+