document UOps.IF [run_process_replay] (#6374)

* document UOps.IF [run_process_replay]

* this will be a block of STOREs after merge_gates

* now i can enable the assert

* more docs

* raw code block

* cname

* cleanup

Revert "cname"

This reverts commit d823f87561.

tinygrad/ops.py

@@ -270,7 +270,7 @@ class UOps(Enum):
       - Buffer UOp `UOps.DEFINE_GLOBAL` or `UOps.DEFINE_LOCAL`.
       - Indexing UOp, can only return `dtypes.int32`.
       - Value to store.
-      - Gate UOp, can only return `dtypes.bool`.
+      - Gate UOp, can only return `dtypes.bool`. We rewrite this to an IF block in the end.
   - **`arg`**: `None`
   """
   PHI = auto()
@@ -284,6 +284,37 @@ class UOps(Enum):
   - **`arg`**: `None`
   """
   IF = auto()
+  """
+  Gates a single STORE to global memory. The IF block could also contain additional UOps the STORE depends on.
+
+  - **`dtype`**: `None`
+  - **`src`**:
+    `Tuple[UOp, UOp]`
+      - Gate UOp, can only return `dtypes.bool`
+      - The second UOp starts the gate block; All of its children are gated until the final STORE.
+  - **`arg`**: `None`
+
+  For example, a local reduce must only run on one thread.
+
+  The STORE's IF gate:
+  ```
+  UOp(UOps.IF, src=(
+    UOp(UOps.ALU, dtypes.bool, (...), BinaryOps.CMPNE),
+    UOp(UOps.BARRIER, None, (...))))
+  ```
+  The kernel:
+  ```
+  barrier(CLK_LOCAL_MEM_FENCE);
+  if (lidx0!=1) {
+    int acc1 = 0;
+    for (int ridx1 = 0; ridx1 < 16; ridx1++) {
+      int val1 = temp1[ridx1];
+      acc1 = (acc1+val1);
+    }
+    data0[0] = acc1;
+  }
+  ```
+  """
   RANGE = auto()
   # ops that are not graph nodes
   ENDRANGE = auto()
@@ -634,6 +665,7 @@ def type_verify(uops):
       assert all(dtype == x.dtype.vec(len(src)) for x in src), f"{dtype=} must be {src[0].dtype.vec(len(src))}"
     if uop is UOps.LOAD and len(src) > 3 and src[3].op is UOps.ALU: assert src[3].dtype == dtypes.bool and src[2].dtype == dtype
     if uop is UOps.GEP: assert dtype == src[0].dtype.scalar(), f"GEP of {src[0].dtype=} should be {src[0].dtype.scalar()} != {dtype}"
+    if uop is UOps.IF: assert dtype is None and len(src) == 2 and src[0].dtype == dtypes.bool
     if uop is UOps.STORE:
       assert dtype is None, f"{uop} dtype must be None, got {dtype}"
       if len(src) == 4: