Add doc for packoffset. (#207)

* Add doc for packoffset.

* Add section for layout struct.

proposals/infra/INF-0003-packoffset.md

@@ -0,0 +1,171 @@
+<!-- {% raw %} -->
+
+# packoffset attribute
+
+* Proposal: [INF-0003](INF-0003-packoffset.md)
+* Author(s): [Xiang Li](https://github.com/python3kgae)
+* Sponsor: 
+* Status: **Under Consideration**
+* Planned Version: 
+
+## Introduction
+
+packoffset attribute is used to change the layout of a cbuffer.
+
+```
+packoffset( c[Subcomponent][.component] )
+```
+
+It will overwrite the layout of a cbuffer with the packoffset attributes on
+the constants.
+
+Here are several examples of manually packing shader constants.
+
+```
+cbuffer MyBuffer
+{
+    float4 Element1 : packoffset(c0);
+    float1 Element2 : packoffset(c12);
+    float1 Element3 : packoffset(c1.y);
+}
+```
+without the packoffset, Element2 will be in c1.x.
+
+
+### Limitations
+
+There're some limitations when apply packoffset on a constant.
+
+1. packoffset is only allowed in a constant buffer.
+```
+float c : packoffset(c0); // invalid for apply packoffset on a global constant.
+```
+
+2. cannot mix packoffset elements with nonpackoffset elements in a cbuffer.
+```
+cbuffer A {
+  float a : packoffset(c0);
+  float b;  // invalid for mix packoffset elements and nonpackoffset elements.
+}
+```
+3. Pack subcomponents of vectors and scalars whose size is large enough to prevent crossing register boundaries.
+```
+cbuffer A {
+  float2 a : packoffset(c0.w); // invalid for a.x in c0.w and a.y in c1.x 
+                               // which crossed register boundary.
+}
+```
+4. struct, matrix, array cannot have component.
+```
+struct S {
+  float a;
+};
+
+cbuffer A{
+  S s : packoffset(c2.y); // invalid for offset to component y
+  int a[2] : packoffset(c3.z); // invalid for offset to component z
+  float2x2 m : packoffset(c6.z); // invalid for offset to component z
+  S s2 : packoffset(c12); // valid
+  int a2[2] : packoffset(c13); // valid
+  float2x2 m2 : packoffset(c16); // valid
+}
+```
+
+## Motivation
+
+To support all HLSL features available in compute profile.
+
+## Proposed solution
+
+### AST
+
+A new attribute HLSLPackOffsetAttr will be created with:
+```
+def HLSLPackOffset: HLSLAnnotationAttr {
+  let Spellings = [HLSLAnnotation<"packoffset">];
+  let LangOpts = [HLSL]
+  let Args = [IntArgument<"RegNum">, IntArgument<"Component", /*optional*/ 1>]
+}
+```
+
+The AST for MyBuffer example will be
+```
+HLSLBufferDecl cbuffer MyBuffer
+  VarDecl Element1 float4
+    HLSLPackOffsetAttr   0 0
+  VarDecl Element2 float
+    HLSLPackOffsetAttr  12 0
+  VarDecl Element3 float
+    HLSLPackOffsetAttr   1 1
+  
+```
+
+### llvm IR
+
+#### Layout struct for cbuffer
+In HLSL, constants inside a cbuffer have the cbuffer as their declaration 
+context. 
+However, the variable scope of these constants is similar to a regular 
+global variable. For instance, in the MyBuffer example, the constants are 
+accessed as Element1 instead of MyBuffer::Element1. 
+These constants are treated as global variables in clang AST.
+At the end of Clang’s code generation, cbuffer will be translated into a 
+global variable layout the cbuffer as a struct and replace the use of all 
+the constants with fields for the global variable.
+If translate into C,
+ ```
+ cbuffer A {
+   float a;
+   float b;
+ }
+ float foo() { return a + b; }
+```
+ will be translated into
+```
+ struct A {
+   float a;
+   float b;
+ } cbuffer_A;
+ float foo() { return cbuffer_A.a + cbuffer_A.b; }
+```
+struct A will created as the layout struct.
+The use of a and b will be replaced with cbuffer_A.a and cbuffer_A.b.
+
+#### packoffset on layout struct
+To apply the packoffset to a cbuffer, the layout of the struct constructed for 
+the cbuffer requires reordering of its fields, and padding should be introduced
+if any gaps exist between the fields.
+```
+cbuffer CB {
+  float a : packoffset(c2);
+  float b : packoffset(c4);
+  float2 c : packoffset(c2.z);
+}
+```
+will get layout struct like
+```
+struct CBLayout {
+  float4 padding0[2]; // c0/c1
+  float a;         // c2.x
+  float padding1;  // c2.y
+  float2 c;        // c2.zw
+  float4 padding;  // c3
+  float b;         // c4.x
+}
+```
+
+cbuffer llvm IR is not finalized yet.
+llvm IR example will be added once cbuffer llvm IR is ready.
+
+### metadata for reflection
+
+Reflection reqire cbuffer offer the name, type, offset for each constant inside it.
+This will be send to DirectX backend with metadata.
+The reflection data is tracked with https://github.com/llvm/llvm-project/issues/89292.
+Once that issue is resolved, packoffset only need to update the offset part of the 
+metadata for cbuffer.
+
+## Unresolved issue
+
+How to support 16bit types.
+The offset like c0.y is 32bit.