Optimize ceil_log2 function

2 files changed, 5 insertions, 3 deletions

diff --git a/kernel/yosys.cc b/kernel/yosys.cc
index 20d972150..377572fc2 100644
--- a/kernel/yosys.cc
+++ b/kernel/yosys.cc
@@ -151,14 +151,16 @@ void yosys_banner()
 
 int ceil_log2(int x)
 {
+#if defined(__GNUC__)
+        return x > 1 ? (8*sizeof(int)) - __builtin_clz(x-1) : 0;
+#else
 	if (x <= 0)
 		return 0;
-
 	for (int i = 0; i < 32; i++)
 		if (((x-1) >> i) == 0)
 			return i;
-
 	log_abort();
+#endif
 }
 
 std::string stringf(const char *fmt, ...)
diff --git a/kernel/yosys.h b/kernel/yosys.h
index 82eb069ab..c7b671724 100644
--- a/kernel/yosys.h
+++ b/kernel/yosys.h
@@ -244,7 +244,7 @@ extern bool memhasher_active;
 inline void memhasher() { if (memhasher_active) memhasher_do(); }
 
 void yosys_banner();
-int ceil_log2(int x);
+int ceil_log2(int x) YS_ATTRIBUTE(const);
 std::string stringf(const char *fmt, ...) YS_ATTRIBUTE(format(printf, 1, 2));
 std::string vstringf(const char *fmt, va_list ap);
 int readsome(std::istream &f, char *s, int n);