Add a proposal on how to handle the loop unroll factor in HLSL 202x (#…

…276)

* Add a proposal on how to handle the loop unroll factor in HLSL 202x

* update proposal

proposals/0020-hlsl-202x-202y.md

@@ -57,6 +57,7 @@ DXC and it is expected to cause potential disruption.
 Existing proposals that fall into this category are:
 * [Numeric Constants](/proposals/0003-numeric-constants.md)
 * [Conforming Literals](/proposals/0017-conforming-literals.md)
+* [Loop Unroll Factor](/proposals/0022-hlsl-loop-unroll-factor.md)
 
 HLSL 202x will not include features for all differences between DXC and Clang.
 Some of the

proposals/0022-hlsl-loop-unroll-factor.md

@@ -0,0 +1,40 @@
+# HLSL Loop Unroll Factor
+
+* Proposal: [0022](0022-hlsl-loop-unroll-factor.md)
+* Author(s): [Farzon Lotfi](https://github.com/farzonl)
+* Sponsor: [Farzon Lotfi](https://github.com/farzonl)
+* Status: **Accepted**
+* Impacted Projects: DXC & Clang
+
+## Introduction
+
+This proposal seeks to unify the diverging behaviors between the agreed upon 
+[spec for HLSL loop unroll](https://github.com/microsoft/hlsl-specs/pull/263)
+and the current behavior of DXC.
+
+The new spec wants to treat the unroll factor as a hint to the compiler for 
+partial loop unroll. The current behavior in dxc is that the unroll factor 
+specifies the maximum number of times the loop is to execute. 
+
+The DXC behavior was determined to violate user expectations by overriding 
+the existing loop termination conditions. Further the DXC behavior diverges 
+from how both clang and openCL treat the loop unroll factor which would have 
+made our port of HLSL loop unroll a bunch special cases instead of just 
+syntactic sugar that could sit on top of the existing loop unroll 
+implementations that exist in LLVM.
+
+## Motivation
+The HLSL compilers transition to clang has resulted in a compat break between 
+the [HLSL loop unroll implementation in clang](https://github.com/llvm/llvm-project/pull/93879)
+and the one in DXC. While there is an expectation that the new compiler will 
+not be fully compatible with the previous compilers, These compat breaks should
+ be minimized when possible.
+
+ ## Proposed solution
+In [HLSL 202X](0020-hlsl-202x-202y.md) the DXC implementation of the loop 
+unroll factor should no longer alter the  number of iteration executions. 
+Ideally the loop unroll factor will also become a compiler hint for partial 
+loop unroll. If this can not be done because of concerns of invalid DXIL 
+generation then the loop unroll factor as a feature should be removed from DXC.
+Removal in this case means ignoring the unroll factor and issuing a warning to 
+inform the user.