Mark Functions with __attribute__((hot)) and ((cold)) | Compiler Optimization Techniques | Performance & Optimization

Split Hot and Cold Code

Placing cold error-handling functions in separate translation units or marking them __attribute__((cold)) keeps icache filled with the fast path. LLVM outlines cold blocks from hot functions during optimization.

Attributes and Layout

hot encourages layout near callers; cold moves unlikely blocks away. -freorder-blocks-and-partition is a GCC knob for similar effect.

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Pair with unlikely() on branches into cold helpers
I-cache pressure shows up as frontend stalls in VTune
Do not mark everything hot; the attribute loses meaning
Measure icache misses if you suspect bloat

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

Trace first: walk one failing input step by step
Read the manual: skim the official doc for the mechanism named in this task
Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will mark functions hot/cold and inspect section placement or size changes. This exercise requires showing icache-friendly layout for the common case.

Demonstrate hot/cold function attributes with performance comparison.

Requirements:

Implement hot function (called in tight loop)
Implement cold function (error handling, rarely called)
Use attribute((hot)) for performance-critical code
Use attribute((cold)) for error handling
Compare assembly code with and without attributes
Show code layout difference
Measure performance improvement (5-15% speedup)
Demonstrate I-cache benefit
Print function placement in binary
Explain when to use each attribute

Split Hot and Cold Code

Attributes and Layout

hot encourages layout near callers; cold moves unlikely blocks away. -freorder-blocks-and-partition is a GCC knob for similar effect.

c
Loading…

Pair with unlikely() on branches into cold helpers
I-cache pressure shows up as frontend stalls in VTune
Do not mark everything hot; the attribute loses meaning
Measure icache misses if you suspect bloat

Working Through the Exercise

Keep the relevant documentation open while you implement. When your output disagrees with the reference, trace one failing case by hand before changing random lines.

Trace first: walk one failing input step by step
Read the manual: skim the official doc for the mechanism named in this task
Compare baselines: save measurements from the prior step so speedups are honest

Why for this exercise

You will mark functions hot/cold and inspect section placement or size changes. This exercise requires showing icache-friendly layout for the common case.

Demonstrate hot/cold function attributes with performance comparison.

Requirements:

Implement hot function (called in tight loop)
Implement cold function (error handling, rarely called)
Use attribute((hot)) for performance-critical code
Use attribute((cold)) for error handling
Compare assembly code with and without attributes
Show code layout difference
Measure performance improvement (5-15% speedup)
Demonstrate I-cache benefit
Print function placement in binary
Explain when to use each attribute

Mark Functions with attribute((hot)) and ((cold))

Topic Overview

Split Hot and Cold Code

Attributes and Layout

Working Through the Exercise

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source

Mark Functions with attribute((hot)) and ((cold))

Topic Overview

Split Hot and Cold Code

Attributes and Layout

Working Through the Exercise

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source