[DirectX][Docs] Add DXILIntrinsicExpansion Pass to DXILArchitecture.rst

[farzonl]

Completes #84839

[llvmbot]

@llvm/pr-subscribers-backend-directx

Author: Farzon Lotfi (farzonl)

Changes

for instruction expansion
Completes #84839

---

Full diff: https://github.com/llvm/llvm-project/pull/86198.diff

1 Files Affected:

• (modified) llvm/docs/DirectX/DXILArchitecture.rst (+13)

diff --git a/llvm/docs/DirectX/DXILArchitecture.rst b/llvm/docs/DirectX/DXILArchitecture.rst
index d6712bea4f7712..b3d29cba0a3c95 100644
--- a/llvm/docs/DirectX/DXILArchitecture.rst
+++ b/llvm/docs/DirectX/DXILArchitecture.rst
@@ -61,6 +61,19 @@ on the utilities described in "Common Code" above in order to share
 logic with both the DirectX backend and with Clang's codegen of HLSL
 support as much as possible.
 
+The DirectX Intrinsic Expansion Pass
+===================
+There are intrinsics that don't map directly to DXIL Ops. In some cases
+an intrinsic needs to be expanded to a set of LLVM IR instructions. In
+other cases an intrinsic needs modifications to the arguments or return
+values of a DXIL Op. This pass (`DXILIntrinsicExpansion.cpp`) handles all 
+the cases where our intrinsics don't have a one to one mapping. You should 
+also consider using this pass when the expansion is specific to DXIL so 
+as to keep implementation details out of CodeGen. Finally, there is an 
+expectation that we maintain vectors through this pass. Therefore, best 
+practice would be to avoid scalarization in this pass.
+
+
 The DirectX Backend
 ===================
 

[damyanp]

Is there anything we can do with the PR title so that the key bit of information (ie that it is about instruction expansion) doesn't fall off the title?