[toolchain] allow zero-sized arrays

This is a GNU extension, which we disallow by default. However in many
cases special-casing 0 makes the code more verbose without any gain, so
just enable this extension.

The behaviour of this extension is well-specified and consistent across
different Unix compilers, and is already used by a lot of projects, so
is safe to enable.

Signed-off-by: Gary Guo <[email protected]>

Author: nbdd0121

sw/device/tests/sleep_pin_mio_dio_val_test.c

@@ -35,14 +35,12 @@ OTTF_DEFINE_TEST_CONFIG();
  */
 
 #if defined(OPENTITAN_IS_EARLGREY)
-enum { kNumOptOut = 2 };
 static const dt_pad_t kOptOut[] = {
     kDtPadSpiDeviceSck,
     kDtPadSpiDeviceCsb,
 };
 #elif defined(OPENTITAN_IS_DARJEELING)
-enum { kNumOptOut = 0 };
-static const dt_pad_t kOptOut[1] = {0};
+static const dt_pad_t kOptOut[] = {};
 #else
 #error Unsupported top
 #endif
@@ -143,7 +141,7 @@ bool lowpower_prep(dif_pwrmgr_t *pwrmgr, dif_pinmux_t *pinmux, bool deepsleep) {
 
   LOG_INFO("Selecting PADs retention modes...");
 
-  draw_pinmux_ret(kDtPadCount, kPads, kOptOut, kNumOptOut);
+  draw_pinmux_ret(kDtPadCount, kPads, kOptOut, ARRAYSIZE(kOptOut));
 
   print_chosen_values();
 

toolchain/BUILD

@@ -308,6 +308,8 @@ cc_args(
         "-Wno-missing-field-initializers",
         "-Wno-sign-compare",
         "-Wno-unused-parameter",
+        "-Wno-gnu-empty-initializer",
+        "-Wno-zero-length-array",
         "-Wstrict-prototypes",
         "-Wswitch-default",
         "-Wtype-limits",