xnnpack: work around Zephyr clamp macro collision

Zephyr's <zephyr/sys/util.h> unconditionally defines a function-like
macro named "clamp" in the global namespace (in C mode) since its
Sept 2025 rename of Z_CLAMP -> clamp.  XNNPACK uses "clamp" as an
identifier for a file-local static helper in
src/operators/fully-connected-nc.c:

    static float clamp(const float value, const float minimum,
                       const float maximum) { ... }

The Zephyr macro expands over this function definition and breaks
the XNNPACK build.

Add a small prelude header (zephyr/include/xnnpack_zephyr_prelude.h)
that force-includes util.h and immediately #undef's clamp.  The
util.h include guard then prevents any later transitive include
(via pthread.h etc.) from re-defining the macro.

The prelude is wired via -include on the xnnpack-base INTERFACE
target, propagating to all XNNPACK component targets.

Only clamp is undef'd; min/max have C++ guards upstream and do not
collide with XNNPACK's current C code.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>

Author: npitre

zephyr/CMakeLists.txt

@@ -188,6 +188,15 @@ if(CONFIG_EXECUTORCH)
         add_dependencies(${_xnn_dep} zephyr_generated_headers)
       endif()
     endforeach()
+    # Force-include a prelude that undefines Zephyr's global-namespace
+    # "clamp" macro, which collides with XNNPACK's file-local clamp()
+    # helper (see include/xnnpack_zephyr_prelude.h).  Applied via the
+    # xnnpack-base INTERFACE target which is linked by all XNNPACK
+    # component targets.
+    if(TARGET xnnpack-base)
+      target_compile_options(xnnpack-base INTERFACE
+        -include ${CMAKE_CURRENT_LIST_DIR}/include/xnnpack_zephyr_prelude.h)
+    endif()
     # pthreadpool uses cpuinfo for core count detection when available,
     # avoiding the need for sysconf(_SC_NPROCESSORS_ONLN).
     if(TARGET pthreadpool)

zephyr/include/xnnpack_zephyr_prelude.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2026 BayLibre SAS
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Prelude header force-included into every XNNPACK source file when
+ * building for Zephyr.
+ *
+ * Zephyr's <zephyr/sys/util.h> defines a function-like macro named
+ * "clamp" in the global namespace (since commit 37717b229f5, Sept 2025,
+ * which renamed Z_CLAMP -> clamp). XNNPACK uses "clamp" as an identifier
+ * for a file-local static helper function (src/operators/fully-
+ * connected-nc.c:45), which causes the macro to expand over the
+ * function definition and break the build.
+ *
+ * By including util.h here and immediately undef'ing clamp, the
+ * include guard on util.h prevents any later transitive include from
+ * re-defining the macro, and XNNPACK's clamp() function compiles
+ * cleanly.
+ *
+ * Only clamp is undef'd; min/max are left alone since they have C++
+ * guards upstream and currently do not collide with XNNPACK's C code.
+ */
+
+#ifdef __ZEPHYR__
+#include <zephyr/sys/util.h>
+#undef clamp
+#endif