Add design doc for xlscc's scoped conditional activation barriers.

PiperOrigin-RevId: 913908403

Author: Sean Purser-Haskell

docs_src/design_docs/xlscc_activation_barriers.md

@@ -0,0 +1,293 @@
+# XLS[cc] Scoped Conditional Activation Barriers.
+
+[TOC]
+
+## Basic Principle
+
+XLS[cc] has the concept of “activation barriers”, which solve mutual exclusion
+errors by putting IO ops into a new activation after the
+barrier. In order to avoid cycles, the condition of ops after the barrier must
+not depend directly on the data received from ops before it. This seems
+infeasible in the general case:
+
+
+```c++
+int x = in.read();
+if (x > 10) {
+	__xlscc_activation_barrier</*conditional=*/true>();
+}
+// ...
+if (x == 20) {
+	int y = in.read();
+	// ...
+}
+```
+
+## Solution: Scoped barriers
+
+It can be made feasible for many useful cases by taking advantage of C++ scopes:
+
+```c++
+int x = in.read();
+if (x > 10) {
+	__xlscc_activation_barrier</*conditional=*/true>();
+	int y = in.read();
+	// ...
+}
+```
+
+In this situation, we know that y = in.read() cannot be active in the same
+activation as x = in.read(), because the barrier appears before y = in.read() in
+the same scope.
+
+However, “!(x > 10)” cannot be directly added to the condition for y =
+in.read(), because there would still be a data dependency.
+
+
+## Implementation of scoped mutual exclusion
+
+There are two areas to address:
+1.  How does the FSM determine which slices are active in a given activation
+without referencing any side-effecting values across the conditional barrier?
+    - For unconditional barriers, this is trivial: nothing after the next
+    barrier can be active, and this is known just from this activation’s
+    starting slice index
+2. How does the FSM ensure that the function slices do not reference any
+side-effecting values across the conditional barrier?
+    - For unconditional barriers, this is trivial: every continuation input can
+    be reset to directly reference state after the barrier
+
+### Slice activation calculation
+
+Determining when the scoped slices are active is feasible based on the principle
+that slices in the conditional scope cannot be active in the same activation as
+the barrier (start) slice. Either the condition is:
+
+- True: In which case they will be active in the next activation
+- False: In which they will never be active this Run() iteration
+
+If the activation starts within the scoped slices, then they can be active.
+
+Slices after the scope must be inactive if the barrier is active this iteration.
+This is achieved using the direct, side-effecting value referencing condition of
+the barrier. Each barrier resets this after-barrier condition to literal 0, so
+adding another barrier can remove this direct dependency.
+
+This rule works with nested scopes because in order to reach a barrier in an
+inner scope, all the outer scopes’ barriers’ conditions must also have been
+true. Therefore, if the outer most scope’s condition was:
+
+- True: Then it must be true that slices after the scope are after an active barrier
+- False: Then it must be false that slices after the scope are after an active barrier
+
+### Example A, data_in receives values 1, 1, 4:
+```c++
+//   Inactive this activation
+//** Inactive this activation via direct reference to received value(s)
+```
+
+```c++
+    const int a = data_in.read();
+    if (a == 1) {
+      __xlscc_activation_barrier</*conditional=*/true>();
+//    const int b = data_in.read();
+//    if (b == 1) {
+//      __xlscc_activation_barrier</*conditional=*/true>();
+//      const int c = data_in.read();
+//      data_out.write(a + b + c);
+//    }
+//    ctrl_a.write(100);
+    }
+    ctrl_b.write(2);
+```
+
+
+```c++
+//  const int a = data_in.read();
+//  if (a == 1) {
+//    __xlscc_activation_barrier</*conditional=*/true>();
+      const int b = data_in.read();
+      if (b == 1) {
+        __xlscc_activation_barrier</*conditional=*/true>();
+//      const int c = data_in.read();
+//      data_out.write(a + b + c);
+//    }
+//**  ctrl_a.write(100);
+    }
+//**ctrl_b.write(2);
+```
+
+
+```c++
+//  const int a = data_in.read();
+//  if (a == 1) {
+//    __xlscc_activation_barrier</*conditional=*/true>();
+//    const int b = data_in.read();
+//    if (b == 1) {
+//      __xlscc_activation_barrier</*conditional=*/true>();
+        const int c = data_in.read();
+        data_out.write(a + b + c);
+      }
+      ctrl_a.write(100);
+  }
+  ctrl_b.write(2);
+```
+
+### Example A, data_in receives values 1, 4:
+```c++
+//   Inactive this activation
+//** Inactive this activation via direct reference to received value(s)
+```
+Activation 0:
+
+```c++
+    const int a = data_in.read();
+    if (a == 1) {
+      __xlscc_activation_barrier</*conditional=*/true>();
+//    const int b = data_in.read();
+//    if (b == 1) {
+//      __xlscc_activation_barrier</*conditional=*/true>();
+//      const int c = data_in.read();
+//      data_out.write(a + b + c);
+//    }
+//    ctrl_a.write(100);
+    }
+//**ctrl_b.write(2);
+```
+
+Activation 1:
+
+
+```c++
+//  const int a = data_in.read();
+//  if (a == 1) {
+//    __xlscc_activation_barrier</*conditional=*/true>();
+      const int b = data_in.read();
+      if (b == 1) {
+        __xlscc_activation_barrier</*conditional=*/true>();
+//      const int c = data_in.read();
+//      data_out.write(a + b + c);
+      }
+      ctrl_a.write(100);
+    }
+    ctrl_b.write(2);
+```
+### Example A, data_in receives value 4:
+Activation 0:
+
+
+```c++
+    const int a = data_in.read();
+    if (a == 1) {
+      __xlscc_activation_barrier</*conditional=*/true>();
+//    const int b = data_in.read();
+//    if (b == 1) {
+//      __xlscc_activation_barrier</*conditional=*/true>();
+//      const int c = data_in.read();
+//      data_out.write(a + b + c);
+//    }
+//    ctrl_a.write(100);
+    }
+    ctrl_b.write(2);
+```
+
+### Example B, data_in receives values 11, 10, 1:
+```c++
+//   Inactive this activation
+//** Inactive this activation via direct reference to received value(s)
+```
+
+Activation 0:
+
+```c++
+    const int a = data_in.read();
+    int b = 0;
+    if (a > 10) {
+      __xlscc_activation_barrier</*conditional=*/true>();
+//    b = data_in.read();
+//  }
+
+//**__xlscc_activation_barrier</*conditional=*/true>();
+//**int c = data_in.read();
+//**data_out.write(a + b + c);
+```
+
+Activation 1:
+
+```c++
+//  const int a = data_in.read();
+//  int b = 0;
+//  if (a > 10) {
+//    __xlscc_activation_barrier</*conditional=*/true>();
+      b = data_in.read();
+    }
+
+    __xlscc_activation_barrier</*conditional=*/false>();  // conditional=true is equivalent
+//  int c = data_in.read();
+//  data_out.write(a + b + c);
+```
+
+Activation 2:
+
+```c++
+//  const int a = data_in.read();
+//  int b = 0;
+//  if (a > 10) {
+//    __xlscc_activation_barrier</*conditional=*/true>();
+//    b = data_in.read();
+//  }
+
+//  __xlscc_activation_barrier</*conditional=*/false>();  // conditional=true is equivalent
+    int c = data_in.read();
+    data_out.write(a + b + c);
+```
+
+
+## Continuation value handling
+
+Another source of cycles can be direct references from after the barrier to
+side-effecting values from before it. Section 1 only handles the implicit slice
+activity condition. Although operations before and after the barrier are
+mutually exclusive, according to the slice activity, XLS will still see a cycle
+and be unable to merge the operations.
+
+For example:
+
+```c++
+const int a = data_in.read();
+if (a <= 5) {
+  __xlscc_activation_barrier</*conditional=*/true>();
+  int b = 0;
+  // Introduces data dependency
+  if (a == 1) {
+    b = data_in.read();
+  }
+  data_out.write(a + b);
+}
+```
+
+With unconditional barriers, it is safe to simply remap every continuation value
+input* after the barrier to reference the continuation value’s state element:
+that is, to reference the value from the last activation. This is safe because
+an unconditional barrier will always proceed to the next activation, and so
+nothing after it can be active to use the value before it is loaded into the
+state element. For conditional barriers, it is possible that the condition will
+be false, in which case the FSM will fall through the barrier within the same
+activation, and the state elements will not have been updated. The solution to
+this is, again, scoping, this time of continuation input references.
+
+Each conditional barrier’s scope maintains its own map of continuation value
+input references, much like the variable maps in the TranslationContext. When a
+new scope is started, the parent’s map is copied, and all applicable
+continuation inputs are reset to use state elements. When the scope ends,
+propagation down to the parent scope is done, so that direct references are
+referenced for any continuation values produced by slices inside the scope. This
+is because it is possible to fall out of the scope within a single activation
+(see section 1).
+
+When the conditional barrier’s condition is: True: Continuation values produced
+by the scoped slices are fully valid, being computed from state elements that
+were updated during the activation transition. False: Continuation values
+produced by the scoped slices are invalid, referencing state values from a
+previous activation. However, they should never be used, since the select phis
+generated in the slice after the end of the scope will always ignore them.

mkdocs.yml

@@ -82,6 +82,7 @@ nav:
       - Optimizations:
         - Value Set Simplification:
           - Division: 'design_docs/optimizations/value_set_simp/division_simplification_design.md'
+      - XLS[cc] Activation Barriers: 'design_docs/xlscc_activation_barriers.md'
     - Releasing: 'releasing.md'
   - NoC:
     - Overview: 'noc/xls_noc_readme.md'