Exception Handling ABI for the ARM Architecture

Exception handling ABI for the ARM architecture
13 APPENDIX C: UNWINDING INSTRUCTION ENCODING COSTS
In this analysis we assume the proportion of functions containing > 64KB of code is negligible. Such functions
require 32 bit offsets in the handling table and use long unwinding descriptions.
(Aside: We could introduce a fourth personality routine to handle the rare case of > 64KB functions that are
nonetheless easy to unwind, but today this does not seem to be good use of the encoding space. End aside).
We are principally concerned with the cost of unwinding C functions, where the cost is pure overhead. In C++,
additional functionality justifies the cost. Treating the ARM code-size database as C gives the following table.
Table 5, Cost of unwinding only (C-style)
ARM code-size database build option
RO size
(bytes)
Index table size
(including short unwinding)
Extra cost of
long unwinding
ARM-state, software floating-point, -O1 9,327,956 30033 * 8
(2.6%)
Thumb-state, software floating-point, -O1 6,275,440 30208 * 8
(3.9%)
ARM-state, VFP, -O1 8,776,180 30035 * 8
(2.7%)
210 * 8
(0.02%)
218 * 8
(.03%)
268 * 8
(0.02%)
Overheads –O2 are very similar (for example, 4% instead of 3.9% in Thumb-state).
In particular:
Only about 200 frames of more than 30,000 need more than 3 unwinding instructions, whether the code is
built to use software floating point (no floating-point unwinding) or built to use VFP.
No frame requires more than 7 unwinding instructions (so the long format consumes at most 1 extra word).
ARM IHI 0038A Copyright © 2002-2005, 2007 ARM Limited. All rights reserved. Page 50 of 50