backend: dwarfdbginf: Use LEB128 encoding for lines and columns (#13246)

Using variable length form for lines and columns in debug info can decrease
binary size from 1.04% for 16-bit and 3.06% for 32-bit according to my tests.
These numbers were based on 100 000 functions with 4 variables. Increasing the
line count makes less difference, increasing the variable count makes more
difference. Overall it reduces the binary size on all tests.

This adds the LEB128 encoding overhead however a SIMD implementation can be
taken to consideration since the current implementation uses manual for loop.

Signed-off-by: Luís Ferreira <contact@lsferreira.net>

src/dmd/backend/dwarfdbginf.d

@@ -1842,8 +1842,8 @@ static if (1)
                 DW_AT_type,        DW_FORM_ref4,
                 DW_AT_artificial,  DW_FORM_flag,
                 DW_AT_decl_file,   DW_FORM_data1,
-                DW_AT_decl_line,   DW_FORM_data2,
-                DW_AT_decl_column, DW_FORM_data2,
+                DW_AT_decl_line,   DW_FORM_udata,
+                DW_AT_decl_column, DW_FORM_udata,
                 DW_AT_location,    DW_FORM_block1,
                 0,                0,
             ];
@@ -1892,8 +1892,8 @@ static if (1)
         abuf.writeByte(DW_FORM_string);
 
         abuf.writeByte(DW_AT_decl_file); abuf.writeByte(DW_FORM_data1);
-        abuf.writeByte(DW_AT_decl_line); abuf.writeByte(DW_FORM_data2);
-        abuf.writeByte(DW_AT_decl_column); abuf.writeByte(DW_FORM_data2);
+        abuf.writeByte(DW_AT_decl_line); abuf.writeByte(DW_FORM_udata);
+        abuf.writeByte(DW_AT_decl_column); abuf.writeByte(DW_FORM_udata);
         if (ret_type)
         {
             abuf.writeByte(DW_AT_type);  abuf.writeByte(DW_FORM_ref4);
@@ -1960,8 +1960,8 @@ static if (1)
         debug_info.buf.writeString(name);             // DW_AT_name
         debug_info.buf.writeString(sfunc.Sident.ptr);    // DW_AT_MIPS_linkage_name
         debug_info.buf.writeByte(filenum);            // DW_AT_decl_file
-        debug_info.buf.write16(sfunc.Sfunc.Fstartline.Slinnum);   // DW_AT_decl_line
-        debug_info.buf.write16(sfunc.Sfunc.Fstartline.Scharnum);   // DW_AT_decl_column
+        debug_info.buf.writeuLEB128(sfunc.Sfunc.Fstartline.Slinnum);   // DW_AT_decl_line
+        debug_info.buf.writeuLEB128(sfunc.Sfunc.Fstartline.Scharnum);   // DW_AT_decl_column
         if (ret_type)
             debug_info.buf.write32(ret_type);         // DW_AT_type
 
@@ -2020,8 +2020,8 @@ static if (1)
                         debug_info.buf.write32(tidx);                 // DW_AT_type
                         debug_info.buf.writeByte(sa.Sflags & SFLartifical ? 1 : 0); // DW_FORM_tag
                         debug_info.buf.writeByte(filenum);            // DW_AT_decl_file
-                        debug_info.buf.write16(sa.lposscopestart.Slinnum);   // DW_AT_decl_line
-                        debug_info.buf.write16(sa.lposscopestart.Scharnum);   // DW_AT_decl_column
+                        debug_info.buf.writeuLEB128(sa.lposscopestart.Slinnum);   // DW_AT_decl_line
+                        debug_info.buf.writeuLEB128(sa.lposscopestart.Scharnum);   // DW_AT_decl_column
                         soffset = cast(uint)debug_info.buf.length();
                         debug_info.buf.writeByte(2);                  // DW_FORM_block1
                         if (sa.Sfl == FLreg || sa.Sclass == SCpseudo)