A retpoline is a software mitigation for risky speculative indirect branches on CPUs, especially in the context of Spectre-v2. Instead of letting the processor speculatively jump to an attacker-influenced target, the compiler rewrites indirect branches into a controlled sequence that traps speculation in a harmless loop. The CPU may still predict aggressively, but the speculative path is kept away from sensitive code and data.
This matters because indirect-branch prediction is a key target in microarchitectural attacks. If an attacker can poison branch history or prediction state, speculative execution may briefly touch secrets even when the final architectural result is safe. Retpoline reduces that exposure in software, which makes it valuable where hardware fixes are absent, incomplete, or too costly. In real defenses, it is often used alongside other mitigations such as IBPB or IBRS, and its effectiveness depends on the exact CPU, compiler, and execution path.