ec/common: Introduce IS_ENABLED to check config options

This is copied from coreboot with added support for empty defines. We
should favor using this macro instead of using #ifdef. The macro will
evaluate to 0 if the option is not defined. This allows all the code to
be compiled and then the optimizer will remove the sections of code that
won't ever run. This way we don't end up with #ifdef sections with
invalid syntax because no one ever tests that specific permutation.

e.g.,

    if (IS_ENABLED(CONFIG_USBC_SS_MUX)) {
        ...
    }

There are currently spots where #ifdefs are nested 3 levels deep. This
makes it very hard to follow the code.

BUG=none
TEST=Added some code that uses the macro and verified it executes when
the config value is defined, and doesn't when it's not.

Change-Id: I796b899f7cbbd3067ea3a4d52527d980c68935c9
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1553573
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>

1 files changed, 27 insertions, 0 deletions

diff --git a/include/common.h b/include/common.h
index 56ba15a8a4..da8b4b73e2 100644
--- a/include/common.h
+++ b/include/common.h
@@ -234,4 +234,31 @@ enum ec_error_list {
 /* find the most significant bit. Not defined in n == 0. */
 #define __fls(n) (31 - __builtin_clz(n))
 
+/*
+ * Getting something that works in C and CPP for an arg that may or may
+ * not be defined is tricky.  Here, if we have "#define CONFIG_BOOGER"
+ * we match on the placeholder define, insert the "0," for arg1 and generate
+ * the triplet (0, 1, 0).  Then the last step cherry picks the 2nd arg (a one).
+ * When CONFIG_BOOGER is not defined, we generate a (... 1, 0) pair, and when
+ * the last step cherry picks the 2nd arg, we get a zero.
+ */
+#define __ARG_PLACEHOLDER_ 0,
+#define config_enabled(cfg) _config_enabled(cfg)
+#define _config_enabled(value) __config_enabled(__ARG_PLACEHOLDER_##value)
+#define __config_enabled(arg1_or_junk) ___config_enabled(arg1_or_junk 1, 0, 0)
+#define ___config_enabled(__ignored, val, ...) val
+
+/**
+ * Checks if a config option is defined to an empty value.
+ *
+ * IS_ENABLED(CONFIG_MY_OPTION) will return 1 in the following case:
+ * #define CONFIG_MY_OPTION
+ *
+ * Otherwise if the option has not been defined or defined with a value, it will
+ * return 0.
+ *
+ * @param CONFIG_OPTION
+ */
+#define IS_ENABLED(option) config_enabled(option)
+
 #endif  /* __CROS_EC_COMMON_H */