hello-wcet

A Patmos helloworld example including steps for WCET analysis.

Prerequisites

We assume to have the Patmos tools (patmos-clang, pasim, platin) installed in our PATH.

All major steps below can be conveniently reproduced via the Makefile.

Compiling and Running

We start by compiling and linking the helloworld program like this:

patmos-clang -o hello.elf hello.c

Executing the binary using the Patmos simulator pasim does what we expect it to do:

pasim hello.elf   # (make run)
Hello world.

To make helloworld interesting for WCET analysis, the number of characters we output from the "Hello world." string depends on a (loop) count defined here:

volatile int count = 12;

We can change the loop count and rerun pasim to see the effect immediately.

WCET Analysis

In order to perform WCET analysis, we compile the application again, this time without printing characters to output, as we are not interested in analyzing the stdio functions. (Instead characters are written to an output register.) Defining NOPRINT will exclude the stdio functions; at the same time we export meta-information from the compiler to a PML file:

patmos-clang -DNOPRINT -o hello.noprint.elf -mserialize=hello.noprint.pml hello.c   # (make hello.noprint.elf)

Running the noprint version of helloworld will no longer give us any output on the console, but we can still see what is happening by e.g. tracing function calls in the simulator:

pasim --gtime 0 --debug --debug-fmt calls hello.noprint.elf
<...>
00020350      4786 call from <_start> to <main> args: r3 = 00000000, r4 = 00000000, r5 = f0000100, r6 = 00000040, r7 = 00000040, r8 = 0002f100
000207c0      5015 return from <main> to <_start> retval: r1 = 00000000, r2 = 4ec4ec4f
<...>

We can now perform WCET analysis (of function main) using platin's internal analysis (--enable-wca); results will be stored in the file hello.wca:

platin wcet --enable-wca --disable-ait --stats --verbose -i hello.noprint.pml --binary hello.noprint.elf -o hello.wca   # (make hello.wca)
<...>
Cycles: 229

Success! We can see that the cycles computed as the WCET bound for our helloworld program matches the runtime we measured using the simulator above.

Loop bounds

WCET analysis for our helloworld example depends on a loopbound that is provided as a source code annotation:

  _Pragma("loopbound min 0 max 12")
  for (i = 0; i < count; ++i) {

Changing the upper bound will directly affect the WCET bound, while removing it and rerunning the platin analysis will result in an 'unbounded' error:

[platin] WARNING: LPSolve: Unbounded loops: (main)4/2
[platin] WARNING: WCA: ILP failed: LPSolver Error: UNBOUNDED (E3)