Dwarf 2

From Dmz-portal

Jump to: navigation, search

CFI support for GNU assembler (GAS)

Modern ABIs don't require frame pointers to be used in functions. However missing FPs bring difficulties when doing a backtrace. One solution is to provide Dwarf-2 CFI data for each such function. This can be easily done for example by GCC in it's output, but isn't that easy to write by hand for pure assembler functions.

With the help of these .cfi_* directives one can add appropriate unwind info into his asm source without too much trouble.

Directives implemented so far:

Use at the beginning of each function. It initializes some internal data structures and emits initial CFI instructions.
Opens .eh_frame, generates appropriate binary structures (CIE, FDE) and sets up relocation records.
.cfi_def_cfa reg,imm
Set a rule for computing CFA to: take content of register reg and add imm to it.
.cfi_def_cfa_register reg
Change rule for CFA to use reg. Offset remains the same.
.cfi_def_cfa_offset imm
Change rule for CFA to use offset imm. Register remains the same.
.cfi_adjust_cfa_offset imm
Like the previous one but imm is a relative value that will be added to current offset instead of an absolute value as in .cfi_def_cfa_offset.
.cfi_offset reg,imm
Generate a rule saying that register reg is saved at offset imm from CFA.
Related links:

Example file
example.s (Colorized) [1]
Dwarf-2 standard [2]

Personal tools