Architecture of Computing Systems (Lecture Notes in Computer ...

Compiler-Directed Performance Model Construction 191
Source Code
GCC-PME
instrumented executable
MA library
performance trace files
validation
MATLAB
Compiletime
Run-time
Post Processing
Tools
Fig. 1. Developed tool chain to create the performance model
time. To benefit from this fact, the results of GCC’s loop analysis are incorporated
in the PME process. This includes the results of scalar evolutions (SCEV)
and induction variables analysis (IV). In case these analysis passes fail to provide
a loop bound, we fall back to a user-specified value: The ma-loop switch defines
the default value that is assumed for loops whose number of iterations was not
derivable from static analysis. In order to avoid over instrumentation, steer the
instrumentation process and, hence, decrease time and space overhead, the user
may set a threshold value (called ma-barrier). The estimated amount of operations
is divided by the total estimated amount of operations and compared to
the threshold value. Every function or basic block must exceed the threshhold to
qualify for annotation. GCC’s infrastructure delivers information from the added
passes to the user. Specific switches trigger the writing of the internal program
representation into a file. These files contain the modifications carried out by the
corresponding passes, enabling the user to examine the results of the instrumentation
process. Taking a look at these files, the user will quickly and easily learn
to adjust the additional compiler parameters to gain decent instrumentation.
Loop Annotation. The first PME pass annotates every loop with a call to the MA
library. Herewith the identification of the loop nesting level and the frequently
executed parts of an application at run time is straightforward. The arrangement
of loop components in GCC’s intermediate representation is shown in Figure 2.
The preheader edge leads to the loop header and is mandatory for every loop.
Loops are strongly connected parts of the control flow graph, which have exactly
one entry block (header) [15]. The loop header holds the condition deciding to
enter the loop body. After executing the loop body the latch edge is taken
to the loop header. If the loop has multiple exits, the loop body may be left
through an additional exit edge as well. The annotation of the loop adds the
ma_loop_start call at the preheader edge and the ma_loop_end call to all exit
edges. The start call takes three parameters: an unique identifier, variable name
of the loop bound (if derivable), expected loop count (passing the corresponding
variable). To complement this information the end call takes two arguments: