Emit VHDL hex constants for some unsigned bits strings

This patch changes the output of VHDL unsigned bit strings
which are 4, 8, 16, 32, or 64 bits to use VHDL hex string
constants.

So the following:
  "00000001"
Becomes
  X"01"

Which is much easier to read

tgt-vhdl/cast.cc

@@ -221,13 +221,13 @@ vhdl_expr *vhdl_const_int::to_vector(vhdl_type_name_t name, int w)
       return vhdl_expr::to_vector(name, w);
 }
 
-int vhdl_const_bits::bits_to_int() const
+int64_t vhdl_const_bits::bits_to_int() const
 {
    char msb = value_[value_.size() - 1];
-   int result = 0, bit;
-   for (int i = sizeof(int)*8 - 1; i >= 0; i--) {
+   int64_t result = 0, bit;
+   for (int i = sizeof(int64_t)*8 - 1; i >= 0; i--) {
       if (i > (int)value_.size() - 1)
-         bit = msb == '1' ? 1 : 0;
+         bit = (msb == '1' && signed_) ? 1 : 0;
       else
          bit = value_[i] == '1' ? 1 : 0;
       result = (result << 1) | bit;

tgt-vhdl/vhdl_syntax.cc

@@ -26,6 +26,9 @@
 #include <iostream>
 #include <typeinfo>
 #include <algorithm>
+#include <iomanip>
+
+using namespace std;
 
 vhdl_scope::vhdl_scope()
    : parent_(NULL), init_(false), sig_assign_(true)
@@ -564,17 +567,36 @@ vhdl_const_bits::vhdl_const_bits(const char *value, int width, bool issigned)
       value_.push_back(*value++);
 }
 
+// True if char is not '1' or '0'
+static bool is_meta_bit(char c)
+{
+   return c != '1' && c != '0';
+}
+
+// True if the bit strings contains characters other than '1' and '0'
+bool vhdl_const_bits::has_meta_bits() const
+{
+   return find_if(value_.begin(), value_.end(), is_meta_bit) != value_.end();
+}
+
 void vhdl_const_bits::emit(std::ostream &of, int level) const
 {
    if (qualified_)
-      of << (signed_ ? "signed" : "unsigned") << "'(\"";
-   else
-      of << "\"";
+      of << (signed_ ? "signed" : "unsigned") << "'(";
 
-   // The bits appear to be in reverse order
-   std::string::const_reverse_iterator it;
-   for (it = value_.rbegin(); it != value_.rend(); ++it)
-      of << vl_to_vhdl_bit(*it);
+   // If it's a width we can write in hex, prefer that over binary
+   size_t bits = value_.size();
+   if (!signed_ && !has_meta_bits() && bits <= 64 && bits % 4 == 0) {
+      int64_t ival = bits_to_int();
+      of << "X\"" << hex << setfill('0') << setw(bits / 4) << ival;
+   }
+   else { 
+      of << "\"";
+      
+      std::string::const_reverse_iterator it;
+      for (it = value_.rbegin(); it != value_.rend(); ++it)
+         of << vl_to_vhdl_bit(*it);
+   }
 
    of << (qualified_ ? "\")" : "\"");
 }
@@ -586,7 +608,7 @@ void vhdl_const_bit::emit(std::ostream &of, int level) const
 
 void vhdl_const_int::emit(std::ostream &of, int level) const
 {
-   of << value_;
+   of << dec << value_;
    // We need to find a way to display a comment, since $time, etc. add one.
 }
 
@@ -597,7 +619,7 @@ void vhdl_const_bool::emit(std::ostream &of, int level) const
 
 void vhdl_const_time::emit(std::ostream &of, int level) const
 {
-   of << value_;
+   of << dec << value_;
    switch (units_) {
    case TIME_UNIT_NS:
       of << " ns";

tgt-vhdl/vhdl_syntax.hh

@@ -181,8 +181,9 @@ public:
    vhdl_expr *to_vector(vhdl_type_name_t name, int w);
    vhdl_expr *resize(int w);
 private:
-   int bits_to_int() const;
+   int64_t bits_to_int() const;
    char sign_bit() const;
+   bool has_meta_bits() const;
    
    std::string value_;
    bool qualified_, signed_;