LLVM: llvm::gsym::LineTable Class Reference (original) (raw)

LineTable class contains deserialized versions of line tables for each function's address ranges. More...

LineTable class contains deserialized versions of line tables for each function's address ranges.

When saved to disk, the line table is encoded using a modified version of the DWARF line tables that only tracks address to source file and line.

ENCODING

The line table starts with a small prolog that contains the following values:

ENCODING NAME DESCRIPTION ======== =========== ==================================================== SLEB MinDelta The min line delta for special opcodes that advance the address and line number. SLEB MaxDelta The max line delta for single byte opcodes that advance the address and line number. ULEB FirstLine The value of the first source line number to initialize the LineEntry with.

Once these prolog items are read, we initialize a LineEntry struct with the start address of the function from the FunctionInfo's address range, a default file index of 1, and the line number set to "FirstLine" from the prolog above:

LineEntry Row(BaseAddr, 1, FirstLine);

The line table state machine is now initialized and ready to be parsed. The stream that follows this encodes the line entries in a compact form. Some opcodes cause "Row" to be modified and some opcodes may also push "Row" onto the end of the "LineTable.Lines" vector. The end result is a vector of LineEntry structs that is sorted in ascending address order.

NORMAL OPCODES

The opcodes 0 through 3 are normal in opcodes. Their encoding and descriptions are listed below:

ENCODING ENUMERATION VALUE DESCRIPTION ======== ================ ===== ======================================== LTOC_EndSequence 0x00 Parsing is done. ULEB LTOC_SetFile 0x01 Row.File = ULEB ULEB LTOC_AdvancePC 0x02 Row.Addr += ULEB, push "Row". SLEB LTOC_AdvanceLine 0x03 Row.Line += SLEB LTOC_FirstSpecial 0x04 First special opcode (see SPECIAL OPCODES below).

SPECIAL OPCODES

Opcodes LTOC_FirstSpecial through 255 are special opcodes that always increment both the Row.Addr and Row.Line and push "Row" onto the LineEntry.Lines array. They do this by using some of the bits to increment/decrement the source line number, and some of the bits to increment the address. Line numbers can go up or down when making line tables, where addresses always only increase since line tables are sorted by address.

In order to calculate the amount to increment the line and address for these special opcodes, we calculate the number of values reserved for the line increment/decrement using the "MinDelta" and "MaxDelta" from the prolog:

const int64_t LineRange = MaxDelta - MinDelta + 1;

Then we can adjust the opcode to not include any of the normal opcodes:

const uint8_t AdjustedOp = Opcode - LTOC_FirstSpecial;

And we can calculate the line offset, and address offset:

const int64_t LineDelta = MinDelta + (AdjustedOp % LineRange); const uint64_t AddrDelta = (AdjustedOp / LineRange);

And use these to modify our "Row":

Row.Line += LineDelta; Row.Addr += AddrDelta;

And push a row onto the line table:

Lines.push_back(Row);

This is verify similar to the way that DWARF encodes its line tables. The only difference is the DWARF line tables have more normal opcodes and the "Row" contains more members, like source column number, bools for end of prologue, beginnging of epilogue, is statement and many others. There are also more complex rules that happen for the extra normal opcodes. By leaving these extra opcodes out, we leave more bits for the special opcodes that allows us to encode line tables in fewer bytes than standard DWARF encodings.

Opcodes that will push "Row" onto the LineEntry.Lines include the LTOC_AdvancePC opcode and all special opcodes. All other opcodes only modify the current "Row", or cause the line table to end.

Definition at line 119 of file LineTable.h.