Unreachable code yielding compilation warnings with PSA driver-only configurations

[PatrikNemethNXP]

Suggested enhancement

Modify code guarded by MBEDTLS_PSA_BUILTIN_xxx macros to support PSA driver-only builds. The same would apply to the PSA repository builtins after the split in version 4.0.0.

For example here https://github.com/Mbed-TLS/mbedtls/blob/v3.6.5/library/psa_crypto_aead.c#L256, if no builtin is compiled in, we exit early and unconditionally, resulting in unreachable dead code further down.

There are multiple instances of configuration-based dead code, namely in psa_crypto_cipher.c, psa_crypto_aead.c, and psa_crypto.c.

Justification

Mbed TLS needs this because some configurations of driver-only builds result in dead code that can raise warnings in some toolchains (e.g. Pe111 (unreachable statements) in IAR).

[davidhorstmann-arm]

Hi, thanks for this report! Apologies for the late response.

I'm afraid we don't have the bandwidth to fix compiler warnings like this ourselves but we'd be happy to review and take a patch if you'd be willing to submit one.