ACCEPT

An approximate compiler for C and C++ programs based on Clang.

Source

Think of it as your assistant in breaking your program in small ways to trade off correctness for performance.

Let's Get Started

(From here)

Add Source Files

First, make a new directory in apps/ with the name of your program. We'll use apps/foo in this example.

Put your C/C++ sources in apps/foo. By default, The build system will assume
all .c and .cpp files are to be built and linked together. (You can
customize this later if need be.)

Makefile

Next, add a Makefile for your experiment. Your Makefile should include at least these incantations to make everything work:

APP_MK := ../app.mk
include $(APP_MK)

For simple programs, this is all you need; if you need more options (like specifying the source files to compile or the arguments to use during execution), see the Makefile section of the tool documentation.

Try Building

You can now use the ACCEPT toolchain to try building your application. Just type:

$ accept -f build

(The -f flag avoids memoization—see the tool documentation.) This command shows the output of the build process, including any errors emitted by the compiler.

Like most accept commands, accept build uses the application in the working directory by default. You can specify a path as an argument to build something else.

Source Setup

You will need to make a few small changes to your application's source code to make it fit the ACCEPT workflow:

Annotate

Your next task is to actually annotate the application to enable approximation.
Insert APPROX type qualifiers and ENDORSE() casts into your code as appropriate.

The ACCEPT paper contains details on the annotation language. (See tech report UW-CSE-15-01-1.) This guide should eventually contain a summary of the annotation features, but for now, take a look at the paper.

You might find it helpful to repeatedly run accept -f build during annotation to see type errors and guide your placement of qualifiers.

Remember that you will need to use #include <enerc.h> in files where use annotations.

See Optimizations

Now that you have an annotated application, you can ask ACCEPT to analyze the program for optimization opportunities. Type:

$ accept log

(Remember to add -f if you make any changes to your source files.) This will spit out a log of places where ACCEPT looked for—and found—possible relaxations. It will attempt to point you toward source locations that, given a bit more attention, could unlock to more opportunities. Again, the ACCEPT paper describes the purpose of this feedback log.

Write a Quality Metric

The dynamic feedback loop component of ACCEPT relies on a function that assesses the quality of a relaxed program's output. You write this function in a Python script that accompanies the source code of your program.

To write your application's quality metric, create a file called eval.py alongside your source files. There, you'll define two Python functions: load and score. You never have to worry about calling these functions—the ACCEPT driver itself invokes them during the auto-tuning process.

Load function. The load function takes no arguments. It loads and parses the output of one execution of the program and returns a data structure representing it. For example, you might parse a CSV file to get a list of floating-point numbers and return that.

Score function. The score function takes two arguments, which are both outputs returned by previous invocations of load: the first is the output of a precise execution and the second is the output from some relaxed execution. The scoring function should compute the "difference" (defined in a domain-specific way) between the two and return a value between 0.0 and 1.0, where 0.0 is perfectly correct and 1.0 is completely wrong.

It bears repeating that both of these functions are application-specific: there is no "standard" implementation of either load or score. Both functions convey unique information about your program to the ACCEPT system. This means that your program can have any output format as long as the output is written to a file (or even multiple files); you write load to explain your chosen format. Similarly, you get to decide what "quality" means for your program; you write score to mechanise your chosen notion of quality.

Here's an example eval.py written for a notional program whose output consists of a text file, my_output.txt, containing a list of numbers. The load function here strips off some additional (irrelevant) text on each line and returns the parsed data as a list of floats. The score function takes the mean absolute difference, capped at 1, between the two lists of numbers:

def load():
    out = []
    with open('my_output.txt') as f:
        for line in f:
            first_num, _ = line.split(None, 1)
            out.append(float(first_num))
    return out

def score(orig, relaxed):
    total = 0.0
    for a, b in zip(orig, relaxed):
        total += min(abs(a - b), 1.0)
    return total / len(orig)

Add Arguments

Because ACCEPT is a profiling compiler, it needs to know how to execute your program. You need to provide the command-line arguments for your program in the Makefile using the RUNARGS variable. Add a line like this to your project's Makefile:

RUNARGS := --foo input.txt

indicating how you want your program to be executed. You can also specify a different invocation using the TESTARGS variable for a separate, final performance evaluation. See the Makefile section for details.

Run the Toolchain

Once you're happy with your annotations, you can run the full toolchain to optimize your program. Run this command:

$ accept run

from the directory containing your application (and its Makefile).

Experiment Workflow

Here's what happens when you execute accept run.

The program is first run once without approximation. This run without approximation is known as the precise run.

The program is then run several times with approximation. These runs with approximation are known as the approximate runs. The approximate runs differ slightly in their methods of relaxation, but each approximate run is evaluated in the same manner.

Options

The reference page has more detail about how to invoke the accept run command. Here are a few options you'll want to be aware of:

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Last modified 07 October 2024