Matlab Review Fi Object Code

fi Best Practices Review

Reviews MATLAB code for fixed-point (fi) best practices and suggests improvements for performance, code generation efficiency, and correctness.

When to Use

• Reviewing MATLAB code that uses fi, fimath, numerictype, or quantizenumeric
• Optimizing fixed-point simulation speed
• Preparing fixed-point code for C or hardware code generation

When Not to Use

• Code using only built-in integer types (int8, uint16, etc.) without fi
• Pure floating-point algorithms with no fixed-point intent
• Simulink-only workflows where fixed-point is configured through block dialogs (use Fixed-Point Tool instead)

Checklist

When reviewing code, check for ALL of the following:

1. Vectorize fi() Calls

Problem: Scalar fi() in a loop is slow due to per-element object construction overhead.

Fix: Pass entire arrays to fi() at once.

% BAD — slow: per-element fi object construction
for k = 1:N
    x_fi(k) = fi(x(k), 1, 18, 16, F);
end

% GOOD — fast: single vectorized call, bit-true identical result
x_fi = fi(x, 1, 18, 16, F);

2. Separate Data Types from Algorithm

Problem: Hardcoding fi types inside algorithm code makes it impossible to switch between float/fixed or compare configurations.

Fix: Use a types table with empty prototypes and cast(...,'like',...).

% Types table (separate function)
function T = mytypes(dt)
  switch dt
    case 'double'
      T.b = double([]);  T.x = double([]);  T.y = double([]);
    case 'single'
      T.b = single([]);  T.x = single([]);  T.y = single([]);
    case 'fixed16'
      F = fimath('RoundingMethod','Floor','OverflowAction','Wrap', ...
                 'ProductMode','KeepLSB','ProductWordLength',32, ...
                 'SumMode','KeepLSB','SumWordLength',32);
      T.b = fi([], 1, 16, 15, F);
      T.x = fi([], 1, 16, 15, F);
      T.y = fi([], 1, 16, 14, F);
  end
end

% Algorithm — no hardcoded types
function [y,z] = myfilter(b, x, z, T)
  y = zeros(size(x), 'like', T.y);
  for n = 1:length(x)
    z(:) = [x(n); z(1:end-1)];
    y(n) = b * z;
  end
end

% Entrypoint — wraps types + cast + algorithm
function [y,z] = entrypoint(dt, b, x)
  T = mytypes(dt);
  b = cast(b, 'like', T.b);
  x = cast(x, 'like', T.x);
  z = zeros(size(b'), 'like', T.x);
  [y,z] = myfilter(b, x, z, T);
end

Validation: Run with 'double' first, then 'single' (catches single-precision issues early — important for embedded targets where double is unavailable or slow), then 'fixed16'.

3. Prevent Bit Growth with Subscripted Assignment

Problem: acc = acc + x(n) overwrites acc with a new fi object whose type may change due to FullPrecision word growth.

Fix: Use acc(:) = acc + x(n) to retain the original data type.

% BAD — acc type may grow each iteration
acc = fi(0, 1, 32, 16);
for n = 1:numel(x)
    acc = acc + x(n);
end

% GOOD — preserves acc's declared type
acc = fi(0, 1, 32, 16);
for n = 1:numel(x)
    acc(:) = acc + x(n);
end

4. Configure fimath for Your Target

Problem: Default fimath (Nearest rounding, Saturate overflow, FullPrecision) generates bloated code. A simple a + b can produce many lines of C with sign-extension and overflow checks.

Fix: Choose fimath settings based on your code generation target.

% For C targets (MATLAB Coder) — models integer truncation behavior
F_c = fimath('RoundingMethod','Floor', 'OverflowAction','Wrap', ...
             'ProductMode','KeepLSB', 'ProductWordLength',32, ...
             'SumMode','KeepLSB', 'SumWordLength',32);

% For DSP processor targets — models shift-right behavior
F_dsp = fimath('RoundingMethod','Floor', 'OverflowAction','Wrap', ...
               'ProductMode','KeepMSB', 'ProductWordLength',32, ...
               'SumMode','KeepMSB', 'SumWordLength',32);

% For FPGA/hardware targets — use the built-in helper
% hdlfimath = Floor/Wrap/FullPrecision (hardware coder manages bit widths internally)
F_hw = hdlfimath;
x_fi = fi(x, 1, 18, 16, F_hw);

Product/Sum mode selection:

Note: hdlfimath returns Floor/Wrap/FullPrecision. The hardware coder manages bit widths through its own pipeline — do not use KeepLSB or KeepMSB with it unless explicitly required by your design constraints.

Rounding efficiency (most to least efficient for codegen):

1. Floor — two's complement truncation, no extra logic
2. Zero — truncation toward zero
3. Nearest — ties to +inf (default)
4. Convergent — ties to nearest even
5. Round — ties away from zero (most expensive)

Overflow: Wrap (no logic) vs Saturate (requires comparison).

Slope-bias scaling: If your fi objects use slope-bias (non-power-of-two slope or non-zero bias):

• ProductMode and SumMode must be 'SpecifyPrecision' with CastBeforeSum set to true
• Hardware code generation and DSP System Toolbox do not support slope-bias — use binary-point for hardware targets
• Slope-bias maximizes accuracy per bit when values are bunched away from zero (e.g., sensor ranges like 273–283 K)
• Match net scaling so operations resolve to shifts; non-zero bias makes multiplication costlier, but zero-bias with non-power-of-two slope can still produce shift-only code

5. Preallocate fi Arrays

Problem: Growing fi arrays inside loops causes quadratic memory and time growth.

Fix: Preallocate using zeros(...,'like',...) with a prototype.

T = fi([], 1, 18, 16, F);       % empty prototype
Y = zeros(N, 1, 'like', T);     % preallocated output

for k = 1:N
    Y(k) = cast(x(k), 'like', T);
end

6. Avoid Division in Fixed-Point

Problem: Division is expensive in fixed-point hardware and generates complex code.

Fix: Replace with bit shifts (power-of-2) or inverse multiplication (constants).

% BAD
y = x / 8;
y = x / 5;

% GOOD — bit shift for power-of-2
y = bitsra(x, 3);  % x/8

% GOOD — multiply by precomputed inverse
inv5 = fi(0.2, 1, 16, 15);
y = x * inv5;      % x/5

7. Replace Expensive Functions with Lookup Tables or CORDIC

Functions like sin, cos, sqrt, exp, log generate inefficient code for fi inputs and may not be supported for C or hardware code generation.

Choose replacement strategy based on your target:

Recommended for C targets — FunctionApproximation.Problem (Fixed-Point Designer, R2018a+):

% 1. Define the problem: function, input range, input type
problem = FunctionApproximation.Problem('sin');
problem.InputTypes = numerictype(1, 16, 14);
problem.InputLowerBounds = -pi;
problem.InputUpperBounds = pi;

% 2. (Optional) Configure options
problem.Options.WordLengths = [8 16];            % allowed word lengths
problem.Options.ApproximateSolutionType = 'MATLAB'; % or 'Simulink'

% 3. Solve — finds optimal breakpoints and output type
solution = solve(problem);

% 4. Inspect — compare approximation accuracy
compare(solution);

% 5. Generate — produces a lookup table MATLAB function or Simulink block
approximate(solution, 'Name', 'mysin_lut');

This workflow automatically selects breakpoints, word lengths, and interpolation methods to meet accuracy requirements. It generates codegen-ready MATLAB functions or Simulink blocks.

Recommended for FPGA/hardware targets — CORDIC (iterative, no ROM, maps to shift-add logic):

y = cordicsin(theta, nIterations);
y = cordiccos(theta, nIterations);
y = cordicatan2(y_in, x_in, nIterations);

CORDIC is best suited to FPGA/hardware targets where iterative shift-add pipelines are cheap and ROM-based lookup tables are expensive. For C code targeting embedded processors, lookup tables are generally more efficient — CORDIC's iterative loops add cycle overhead that a direct table read avoids.

Last resort — cast to single, compute, cast back (loses fixed-point bit-trueness but stays efficient on embedded targets):

y = cast(sin(single(x)), 'like', T.y);

Do not cast to double for this purpose — single is sufficient for intermediate computation and is far more efficient on embedded processors (many MCUs/DSPs lack double-precision hardware, making double ops significantly slower).

Detection: Flag code that uses sin, cos, sqrt, exp, log, or similar transcendental functions on fi inputs, OR code that mentions using lookup tables without actually implementing them via FunctionApproximation.Problem or CORDIC.

8. fi Constructor Best Practices

Use positional or numerictype syntax — the name-value pair constructor is slower due to string parsing overhead.

% SLOW — name-value pairs
x = fi(v, 'Signed', 1, 'WordLength', 16, 'FractionLength', 14);

% FAST — positional (sign, wordLength, fractionLength)
x = fi(v, 1, 16, 14);

% FAST — pre-built numerictype (best for repeated use)
T = numerictype(1, 16, 14);
x = fi(v, T);

% FAST — with fimath
x = fi(v, T, F);

Additional rules:

• fi() always uses Nearest/Saturate for initial quantization regardless of globalfimath.
• Non-finite values (Inf, NaN) require fully specified numerictype.
• For codegen, numerictype properties must be compile-time constants.

9. Use quantizenumeric for Double-Based Quantization

Problem: Converting to fi objects just to model quantization effects adds unnecessary overhead when your algorithm otherwise stays in double.

Fix: Use quantizenumeric — quantizes values in-place, output remains double.

% Quantize to signed 16-bit, 13 fractional bits
y = quantizenumeric(x, 1, 16, 13);                    % nearest, saturate
y = quantizenumeric(x, 1, 16, 13, 'floor', 'wrap');   % floor + wrap

% Works on arrays — no loop needed
q_data = quantizenumeric(data, 1, 8, 6, 'floor', 'wrap');

When to use quantizenumeric vs fi:

Legacy alternative — manual floor-mode quantization (fastest, but limited):

% Only correct for floor rounding, no overflow handling
x_quantized = floor(x * 2^FL) * 2^-FL;

Prefer quantizenumeric (R2016a+) which handles all rounding/overflow modes.

10. Manage Floating-Point in Fixed-Point Algorithms

For efficient code generation, minimize floating-point variables in the algorithm body. However, not everything benefits from fixed-point conversion.

Convert to fixed-point when:

• Dynamic range is bounded and well-characterized
• The function has fi-compatible replacements
• Code generation requires it

Leave in floating-point when:

• Dynamic range exceeds 40 dB
• Code is sensitive to round-off errors
• No suitable fixed-point replacement exists

Pattern: Isolate floating-point sections with explicit casts. Prefer single over double for embedded targets (many MCUs/DSPs lack double-precision FPU):

function y = myAlgorithm(x_fi, T)
  % Fixed-point path
  filtered = myFIRFilter(x_fi, T);

  % Isolated floating-point section (high dynamic range)
  scaled = single(filtered);
  compressed = log2(abs(scaled) + 1);

  % Back to fixed-point
  y = cast(compressed, 'like', T.y);
end

Loop indices are exempt — MATLAB Coder automatically converts them to integers.

11. Profile and Accelerate fi Code

Common fi performance bottlenecks (in order of typical cost):

Acceleration options (from least to most effort):

1. Vectorize — eliminate scalar fi loops, significantly faster
2. fiaccel — compile to MEX, faster execution, no Coder license needed
3. quantizenumeric — stay in double, inject quantization only where needed (Check 9)
4. buildInstrumentedMex — MEX with logging for type proposals

% Benchmark with timeit
f_fi = @() myAlgorithm(x_fi, T);
f_dbl = @() myAlgorithm(x_dbl, T_dbl);
fprintf('fi/double ratio: %.1fx\n', timeit(f_fi) / timeit(f_dbl));

Output Format

When reviewing code, report findings using these categories:

[PERF] Line XX: <description> (Check N)
[CODEGEN] Line XX: <description> (Check N)
[CORRECTNESS] Line XX: <description> (Check N)
[PATTERN] Line XX: <description> (Check N)
[DEBUG] Line XX: <description> (Check N)

Examples:

[PERF] Line 42: Scalar fi() in loop — vectorize (Check 1)
[PERF] Line 88: Name-value fi constructor in hot path — use positional or numerictype (Check 8)
[CODEGEN] Line 15: Default fimath generates bloated C — use Floor/Wrap/KeepLSB (Check 4)
[CODEGEN] Line 67: Division in fixed-point — use bitsra or inverse multiply (Check 6)
[CORRECTNESS] Line 33: Missing subscripted assignment — accumulator type will grow (Check 3)
[PATTERN] Line 20: Hardcoded fi type — separate into types table (Check 2)
[PATTERN] Line 55: High dynamic range forced to fixed-point — isolate in floating-point (Check 10)
[DEBUG] Line 40: Output diverges from double — check for overflow

Report every applicable check. Omit checks with no findings.

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