Lecture Notes in Computer Science 4917

388 Y. Ben Asher et al.
Table 1. Dynamic branch predictors on leading high-end embedded processors
Processor BHT BTB Return Stack L1 I-cache
AMCC 440GX 1024 16 – 32KB, 32B, 64 ways
Broadcom BCM1250 1024 64 16 32KB, 32B, 4 ways
Cavium Octeon 256 – 4 32KB, 32B, 4 ways
IBM 750GX 512 64 – 32KB, 32B, 8 ways
FreeScale MPC7447A 2048 2 – 32KB, 32B, 8 ways
FreeScale MPC8560 512 512 – 32KB, 32B, 8 ways
PMC-Sierra RM9000x2GL 256 – 4 32KB, 32B, 4 ways
and 16-entry return address cache. On the other hand their L1 I-caches are in
par with the aforementioned class of processors. The numbers demonstrate the
importance of aggressive inlining techniques for embedded systems.
2 ILB Based Aggressive Function Inlining
Inlining decisions are based on the Call Graph (CG) of the program. Each edge
of the graph is assigned a weight: the frequency of each function call according to
the profile information collected. The Average Heat ratio (AvgHeat) is the sum
of all the frequencies of all the executed (dynamic) instructions (DI) gathered
during the profiling stage, divided by the total number of static instructions (SI)
in the program:
�DI i freqi
AvgHeat =
SI
Cold edges are defined as any edge in the extended CFG whose weight is lower
than some threshold (e.g., 10%) of AvgHeat. The algorithm implementing the
ILB method is:
1. Based on edge profile information, create the call graph CG for the given
program and attach a weight to each edge. The weight is its execution frequency.
2. Traverse CG and remove all cold edges. Section 2.2 elaborates on this.
3. Remove cyclic paths from CG by finding the smallest weighted set of edges
in CG using the algorithm of Eades et. al. [6] for solving the feedback edge
set problem. Section 3 elaborates on this.
4. Let EG be the extended control flow graph containing the control flow graph
of each function and direct edges for call and return instructions. For each
function f in the call graph CG, which is a candidate for inlining:
(a) For every two incoming edges e1 ande2 tof in CG, letcaller1 bethe
caller basic block ending with e1 andletcaller2 be the caller basic block
ending with e2 inEG. fallthru1 andfallthru2 arethebasicblocks
following caller1 andcaller2 inEG respectively.
(b) Traverse EG and search for directed paths from fallthru1 tocaller2
and from fallthru2 tocaller1.
(c) If both paths exist, remove the e1 ande2 edgesfromCG.