Add a warning for large unrolled loops

Author: lwaern-intel

RELEASENOTES.docu

@@ -1748,4 +1748,7 @@ extern typedef struct { } my_type_t;</pre> </add-note></build-id>
       are themselves constant inlined method parameters (i.e. an inline method
       call propagates an inline parameter to an inline method call of its own.)
   </add-note></build-id>
+  <build-id value="next"><add-note> Added the warning `WBIGUNROLL` which is
+      emitted when a <tt>#select</tt> or <tt>#foreach</tt> statement is compiled
+      to a large unrolled loop.</add-note></build-id>
 </rn>

py/dml/codegen.py

@@ -3048,12 +3048,19 @@ def stmt_select(stmt, location, scope):
                     else_dead = True
                     break
 
+            iterations = len(clauses)
             if else_dead:
                 (last_cond, last_stmt) = clauses.pop(-1)
                 assert last_cond.constant and last_cond.value
                 if_chain = last_stmt
             else:
                 if_chain = codegen_statement(else_ast, location, scope)
+            if iterations > 50 and isinstance(lst, List):
+                report(WBIGUNROLL(stmt.site,
+                                  '#'*(stmt.site.dml_version() != (1, 2))
+                                  + 'select',
+                                  iterations))
+
             for (cond, stmt) in reversed(clauses):
                 if_chain = mkIf(cond.site, cond, stmt, if_chain)
             return [if_chain]
@@ -3179,6 +3186,10 @@ def foreach_constant_list(site, itername, lst, statement, location, scope):
                     stmt)
             spec.append(mkCompound(site, decls + [stmt]))
 
+        if len(spec) > 50 and isinstance(lst, List):
+            report(WBIGUNROLL(site,
+                              '#'*(site.dml_version() != (1, 2)) + 'foreach',
+                              len(spec)))
         return [mkUnrolledLoop(site, spec,
                                context.label if context.used else None)]
 

py/dml/messages.py

@@ -2011,6 +2011,28 @@ class WHOOKSEND(DMLWarning):
            + "Declarations section in the DML 1.4 reference manual for "
            + "information about the differences between 'send' and 'send_now'")
 
+class WBIGUNROLL(DMLWarning):
+    """
+    A `#select` or `#foreach` statement was specified which caused DMLC to
+    study the body and generate duplicated C code for it a large number of
+    times (more than 50 times.) This can dramatically increase both DMLC and
+    GCC compile times and code size, if not crash DMLC outright.
+
+    To address this, you have two options:
+    * Ensure most iterations can be entirely eliminated at compile-time by
+      DMLC. For `#select`, this can be done by ensuring that the `where` check
+      will be a constant (typically a constant equality) for most if not all
+      items of the specified list. For `#foreach`, encase the body in an `#if`
+      check that is false for most items of the specified list.
+    * Don't use `#select` or `#foreach`. Represent the specified compile-time
+      list instead as a `session` array or an `extern`ed C array, and iterate
+      through it using traditional `for` loops. For more information, see [the
+      various answers to this Stack Overflow question.](https://stackoverflow.com/questions/75073681/cannot-use-variable-index-in-a-constant-list)
+    """
+    fmt = ("This '%s' statement compiles to an unrolled loop where the loop "
+           + "body is studied and generated %s times. This may dramatically "
+           + "increase both DMLC and GCC compilation times, if not crash DMLC "
+           + "outright!")
 
 class PSHA1(PortingMessage):
     """The `port-dml` script requires that the DML file has not been

test/1.2/errors/T_WBIGUNROLL.dml

@@ -0,0 +1,80 @@
+/*
+  © 2023 Intel Corporation
+  SPDX-License-Identifier: MPL-2.0
+*/
+dml 1.2;
+
+device test;
+
+/// COMPILE-ONLY
+/// NO-CC
+
+data int zero = 0;
+
+parameter zeroes_50 = [$zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero];
+
+// Fiftieth zero is constant
+parameter zeroes_51 = [$zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, 0, $zero];
+
+bank b {
+    register regs[i in 0..50] size 4 @ undefined;
+}
+
+method init {
+    local int i;
+
+    // no warning
+    foreach x in ($zeroes_50) assert true;
+    // The else branch is not considered an iteration
+    select x in ($zeroes_50) where (x == i) {
+        assert true;
+    } else assert true;
+
+    /// WARNING WBIGUNROLL
+    foreach x in ($zeroes_51) assert true;
+    /// WARNING WBIGUNROLL
+    select x in ($zeroes_51) where (x == i) {
+        assert true;
+    } else assert true;
+
+    // no warning
+    foreach x in ($zeroes_51) {
+        // As fiftieth bit is constant 0, DML 1.2 semantics eliminates this if
+        // entirely, and subsequently causes that iteration to be omitted from
+        // codegen entirely; reducing the total count to 50
+        if (x != 0) assert true;
+    }
+
+    // As fiftieth bit is constant 0, select can cut short at it, reducing the
+    // total number of iterations to 50
+    select x in ($zeroes_50) where (x == 0) {
+        assert true;
+    } else assert true;
+
+    // As fiftieth bit is constant 0, select can omit the check and thus
+    // iteration for it, reducing the total number of iterations to 50
+    select x in ($zeroes_50) where (x == 1) {
+        assert true;
+    } else assert true;
+
+
+    // WBIGUNROLL is only reported for compile-time lists defined via list
+    // syntax, not the object lists generated by DMLC
+    // no warning
+    foreach x in ($b.unmapped_registers) assert true;
+    select x in ($b.unmapped_registers) where (i == 0) {
+        assert true;
+    } else assert true;
+}