Compiler Applies Loop Unswitching To Optimize Loops
Matt Godbolt demonstrates on Compiler Explorer how compilers perform loop unswitching when compiling a sum-of-squares example, posted December 12, 2025. He shows that at -O2 the compiler uses a conditional multiply inside the loop, while at higher optimisation it duplicates the loop (unswitching) to hoist a constant boolean check, trading code size for fewer per-iteration multiplies. Practitioners should verify compiler choices and weigh performance versus code-size trade-offs.
Key Points
- 1Demonstrates loop unswitching duplicates loop at -O3 to hoist invariant branch out
- 2Avoids per-iteration multiplication, reducing wasted work when the boolean stays constant
- 3Advises practitioners to verify compiler decisions with Compiler Explorer and weigh code-size trade-offs
Scoring Rationale
Clear technical demonstration and practical guidance, limited novelty since loop unswitching is established compiler technique.
Sources
Public references used for this report.
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