Signed arithmetic overflow: document the compiler contract before changing optimization

What it is

GCC documents -fwrapv as requesting wrapping behavior for signed addition, subtraction and multiplication, while -ftrapv requests traps for those operations. The options interact, and their order and active state matter. These are explicit compiler contracts with stated scope, not interchangeable ways to make every arithmetic operation safe. GCC optimization options

Why it matters

An agent investigating a release-only bug may disable optimization and stop. That establishes an environmental difference, but leaves the intended arithmetic unspecified. For counts, lengths and offsets, decide whether an out-of-range result should be rejected, saturated, widened or wrapped before choosing an implementation.

How to apply

• Capture the actual compile flags for the failing translation unit, including inherited build settings. Record the compiler and target rather than relying on the build-profile name.
• Write the valid operand ranges and intended result policy beside the calculation. For an allocation size, include intermediate calculations rather than considering only the final destination type.
• Construct proposed boundary cases just inside and outside the permitted range. Keep expected results independent of whatever the current executable happens to print.
• Prefer a checked arithmetic operation or a mathematically justified range check when rejection is required. If wrapping is deliberately part of the contract, state which operations and compilation units rely on it.
• Compare debug and release configurations using the same fixtures, then inspect relevant diagnostics. Retain the smallest example showing why the chosen contract is necessary.

Pitfalls

Do not assume a wider destination rescues an intermediate calculation performed in a narrower type. A flag documented for addition, subtraction and multiplication is not a blanket promise about division, shifts or pointers. Avoid silently changing flags across the whole project to fix one local operation. The proposed checks are an investigation plan; no compiler experiment or passing boundary test is claimed here.