rtlil: Speeds up string decoding by 30%

This change represents about a 2% speed up of Yosys as a
whole.

Written by @rmlarsen

Co-authored-by: Rasmus Larsen <[email protected]>
Signed-off-by: Ethan Mahintorabi <[email protected]>

kernel/rtlil.cc

@@ -27,6 +27,7 @@
 
 #include <string.h>
 #include <algorithm>
+#include <array>
 
 YOSYS_NAMESPACE_BEGIN
 
@@ -200,17 +201,30 @@ const pool<IdString> &RTLIL::builtin_ff_cell_types() {
 	return res;
 }
 
-RTLIL::Const::Const(const std::string &str)
-{
-	flags = RTLIL::CONST_FLAG_STRING;
-	bits.reserve(str.size() * 8);
-	for (int i = str.size()-1; i >= 0; i--) {
-		unsigned char ch = str[i];
-		for (int j = 0; j < 8; j++) {
-			bits.push_back((ch & 1) != 0 ? State::S1 : State::S0);
-			ch = ch >> 1;
-		}
-	}
+PRIVATE_NAMESPACE_BEGIN
+
+using CharToBitsLUT = std::array<std::array<RTLIL::State, 8>, 256>;
+
+const CharToBitsLUT build_CharToBitsLUT() {
+        CharToBitsLUT LUT;
+        for (int i = 0; i < 256; ++i) {
+          unsigned char ch = i;
+          for (int j = 0; j < 8; j++) {
+            LUT[i][j] = (ch & 1) != 0 ? State::S1 : State::S0;
+            ch = ch >> 1;
+          }
+        }
+        return LUT;
+}
+PRIVATE_NAMESPACE_END
+
+RTLIL::Const::Const(const std::string &str) : flags(RTLIL::CONST_FLAG_STRING) {
+        static const CharToBitsLUT LUT = build_CharToBitsLUT();
+        bits.reserve(str.size() * 8);
+        for (int i = str.size() - 1; i >= 0; --i) {
+          const unsigned char ch = str[i];
+          bits.insert(bits.end(), LUT[ch].begin(), LUT[ch].end());
+        }
 }
 
 RTLIL::Const::Const(int val, int width)
@@ -4040,7 +4054,7 @@ void RTLIL::SigSpec::replace(const RTLIL::SigSpec &pattern, const RTLIL::SigSpec
 			}
 		}
 	}
-
+	
 	other->check();
 }