Add functions to compute minimum number of bits required to store int…

…eger values

Separate functions are implemented for signed and unsigned integers.

Note that the current use of ceil_log2(x) for x >= 0 may be incorrect in
some cases; the minimum number of bits required to store x is really
ceil_log2(x + 1).

kernel/yosys.cc

@@ -73,6 +73,10 @@
 #include <limits.h>
 #include <errno.h>
 
+#if __cplusplus >= 202002L
+#  include <bit>
+#endif
+
 #include "libs/json11/json11.hpp"
 
 YOSYS_NAMESPACE_BEGIN
@@ -175,6 +179,40 @@ int ceil_log2(int x)
 #endif
 }
 
+// Calculate the minimum number of bits required to store an unsigned integer value.
+// Note that min_bit_width(0u) == 0.
+int min_bit_width(unsigned x)
+{
+#if __cplusplus >= 202002L
+	return std::bit_width(x);
+#else
+	int width = 0;
+	while (x) {
+		width += 1;
+		x >>= 1;
+	}
+	return width;
+#endif
+}
+
+// Calculate the minimum number of bits required to store a signed integer value,
+// using two's complement representation for negative values.
+// Note that the sign bit is included in the width for all values except 0.
+int min_bit_width(int x)
+{
+	if (x < 0) {
+		// Compute two's complement of negative value.
+		// Negative numbers are represented in two's complement form
+		// in SystemVerilog, and this is also guaranteed by C++20.
+		// C++20 also makes integral conversion of unsigned integers
+		// to signed integers well defined.
+		x = -(unsigned)x;
+	}
+	// Add a bit for sign, except for 0 and the most negative number, whose
+	// two's complement is equal to itself, i.e. the sign bit is still set.
+	return (x > 0) + min_bit_width((unsigned)x);
+}
+
 int readsome(std::istream &f, char *s, int n)
 {
 	int rc = int(f.readsome(s, n));

kernel/yosys.h

@@ -274,6 +274,8 @@ inline void memhasher() { if (memhasher_active) memhasher_do(); }
 
 void yosys_banner();
 int ceil_log2(int x) YS_ATTRIBUTE(const);
+int min_bit_width(unsigned x);
+int min_bit_width(int x);
 
 inline std::string vstringf(const char *fmt, va_list ap)
 {