Add a warning for large unrolled loops

RELEASENOTES.docu

@@ -1795,4 +1795,11 @@ extern typedef struct { } my_type_t;</pre> </add-note></build-id>
   <build-id _6="next" _7="next"><add-note>Multiple `extern` declarations for the
       same identifier are now allowed as long as they all declare the same type
       for the identifier <bug number="SIMICS-22472"/>.</add-note></build-id>
+  <build-id _6="next" _7="next"><add-note> Added the warning <tt>WBIGUNROLL</tt>
+      which is emitted when a <tt>#select</tt> or <tt>#foreach</tt> statement is
+      compiled to a large unrolled loop.
+      This warning is only enabled by default with Simics API version 8 or
+      above. With version 7 and below it must be explicitly enabledby passing
+      either <tt>--no-compat=suppress_WBIGUNROLL</tt> or
+      <tt>--warn=WBIGUNROLL</tt> to DMLC.</add-note></build-id>
 </rn>

lib/1.2/dml-builtins.dml

@@ -215,6 +215,7 @@ template device {
     parameter _compat_io_memory auto;
     parameter _compat_shared_logs_on_device auto;
     parameter _compat_suppress_WLOGMIXUP auto;
+    parameter _compat_suppress_WBIGUNROLL auto;
     parameter _compat_legacy_attributes auto;
     parameter _compat_lenient_typechecking auto;
     parameter _compat_dml12_inline auto;

lib/1.4/dml-builtins.dml

@@ -561,6 +561,7 @@ template device {
     param _compat_io_memory auto;
     param _compat_shared_logs_on_device auto;
     param _compat_suppress_WLOGMIXUP auto;
+    param _compat_suppress_WBIGUNROLL auto;
     param _compat_legacy_attributes auto;
     param _compat_lenient_typechecking auto;
     param _compat_dml12_inline auto;

py/dml/codegen.py

@@ -3031,12 +3031,19 @@ 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 > WBIGUNROLL.limit 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]
@@ -3162,6 +3169,10 @@ def foreach_constant_list(site, itername, lst, statement, location, scope):
                     stmt)
             spec.append(mkCompound(site, decls + [stmt]))
 
+        if len(spec) > WBIGUNROLL.limit 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/compat.py

@@ -198,6 +198,30 @@ class suppress_WLOGMIXUP(CompatFeature):
     short = "Suppress the warning 'WLOGMIXUP' by default"
     last_api_version = api_6
 
+@feature
+class suppress_WBIGUNROLL(CompatFeature):
+    '''This compatibility feature makes it so the warning `WBIGUNROLL` is
+    suppressed by default. `WBIGUNROLL` warns about `#foreach` and `#select`
+    statements that compile to large unrolled loops — for more
+    information, see the documentation of `WBIGUNROLL` in the
+    [Messages](messages.html) section.
+
+    `WBIGUNROLL` is suppressed by default below Simics API version 8 in order
+    to avoid overwhelming users with warnings. Addressing occurences of large
+    unrolled loops should be done before or as part of migration to Simics API
+    version 8.
+
+    Passing `--no-compat=suppress_WBUGUNROLL` to DMLC has almost the same effect
+    as passing `--warn=WBIGUNROLL`; either will cause DMLC to report the warning
+    even when the Simics API version in use is below 8. The only difference
+    between these two options is that if `--no-compat=suppress_WBIGUNROLL` is
+    used (and `--warn=WBIGUNROLL` is not), then `WBIGUNROLL` may still be
+    explicitly suppressed via `--no-warn=WBIGUNROLL`. In contrast,
+    `--warn=WBIGUNROLL` doesn't allow for `WBIGUNROLL` to be suppressed at
+    all.'''
+    short = "Suppress the warning 'WBIGUNROLL' by default"
+    last_api_version = api_7
+
 @feature
 class legacy_attributes(CompatFeature):
     '''This compatibility feature makes DMLC register all attributes using the

py/dml/dmlc.py

@@ -599,6 +599,8 @@ def main(argv):
 
     if compat.suppress_WLOGMIXUP in dml.globals.enabled_compat:
         ignore_warning('WLOGMIXUP')
+    if compat.suppress_WBIGUNROLL in dml.globals.enabled_compat:
+        ignore_warning('WBIGUNROLL')
 
     for w in options.disabled_warnings:
         if not is_warning_tag(w):

py/dml/messages.py

@@ -2291,6 +2291,32 @@ 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):
+    limit = 64
+    __doc__ = f"""
+    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 {limit} 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 value (e.g. 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)
+
+    This warning is only enabled by default with Simics API version 8 or above
+    (due to the compatibility feature `suppress_WBIGUNROLL`.)
+    """
+    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/legacy/T_suppress_WBIGUNROLL_disabled.dml

@@ -0,0 +1,15 @@
+/*
+  © 2024 Intel Corporation
+  SPDX-License-Identifier: MPL-2.0
+*/
+dml 1.2;
+
+device test;
+
+/// COMPILE-ONLY
+/// NO-CC
+
+/// DMLC-FLAG --no-compat=suppress_WBIGUNROLL
+
+/// SCAN-FOR-TAGS suppress_WBIGUNROLL.dml
+import "suppress_WBIGUNROLL.dml";

test/1.2/legacy/T_suppress_WBIGUNROLL_enabled.dml

@@ -0,0 +1,14 @@
+/*
+  © 2024 Intel Corporation
+  SPDX-License-Identifier: MPL-2.0
+*/
+dml 1.2;
+
+device test;
+
+/// COMPILE-ONLY
+/// NO-CC
+
+/// DMLC-FLAG --simics-api=7
+
+import "suppress_WBIGUNROLL.dml";

test/1.2/legacy/suppress_WBIGUNROLL.dml

@@ -0,0 +1,80 @@
+/*
+  © 2024 Intel Corporation
+  SPDX-License-Identifier: MPL-2.0
+*/
+dml 1.2;
+
+// expectations in this file are selectively enabled using SCAN-FOR-TAGS
+
+data int zero = 0;
+
+parameter zeroes_64 = [$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, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, $zero];
+
+// Sixtyfourth zero is constant
+parameter zeroes_65 = [$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, $zero, $zero, $zero, $zero, $zero, $zero,
+                       $zero, $zero, $zero, $zero, $zero, $zero, $zero, 0,
+                       $zero];
+
+bank b {
+    register regs[i in 0..64] size 4 @ undefined;
+}
+
+method init {
+    local int nonconstant;
+
+    // no warning
+    foreach x in ($zeroes_64) assert true;
+    // The else branch is not considered an iteration
+    select x in ($zeroes_64) where (x == nonconstant) {
+        assert true;
+    } else assert true;
+
+    /// WARNING WBIGUNROLL
+    foreach x in ($zeroes_65) assert true;
+    /// WARNING WBIGUNROLL
+    select x in ($zeroes_65) where (x == nonconstant) {
+        assert true;
+    } else assert true;
+
+    // no warning
+    foreach x in ($zeroes_65) {
+        // As sixtyfourth bit is constant 0, DML 1.2 semantics eliminates this
+        // if, and subsequently causes that iteration to be omitted from
+        // codegen entirely; reducing the total count to 64
+        if (x != 0) assert true;
+    }
+
+    // As sixtyfourth bit is constant 0, select can cut short at it, reducing
+    // the total number of iterations to 64
+    select x in ($zeroes_65) where (x == 0) {
+        assert true;
+    } else assert true;
+
+    // As sixtyfourth bit is constant 0, select can omit the check and thus
+    // iteration for it, reducing the total number of iterations to 64
+    select x in ($zeroes_65) where (x != 0) {
+        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 (x == nonconstant) {
+        assert true;
+    } else assert true;
+}