Embedded systems and IoT devices are designed to perform specific tasks within strict limits on energy consumption and computing power. To ensure secure communication, these devices require cryptography, nowadays including post-quantum algorithms designed to resist emerging threats. However, the complexity of such algorithms often makes their implementation inefficient on constrained platforms. Although hardware optimisations have proven effective in this domain, they typically require deep algorithm-specific knowledge and manual intervention. This work proposes an alternative approach: CIRCO (Custom Instruction RISC-V Code Optimizer). This tool automatically analyses the assembly code of an application to identify patterns of instructions that can be merged into new, custom RISC-V-compliant instructions.