Structure size reduction #623

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opened 2025-12-27 06:09:51 -06:00 by irwir · 0 comments

irwir commented

2025-12-27 06:09:51 -06:00

(Migrated from github.com)

github.com/Mbed-TLS/TF-PSA-Crypto@e704e187b2/drivers/builtin/src/pk_wrap.h (L28-L33)
Visual C in 64-bit mode aligns the pointer to 8-byte address, and this means 4 bytes were lost twice - on padding of the first and the third fields.
Ideally, fields should be ordered by size in decreasing order (moved op_type down as it could be redeclared with char size):

typedef struct { 
     void *op; 
     mbedtls_svc_key_id_t pub_id; 
     mbedtls_pk_rs_op_t op_type; 
 } mbedtls_pk_psa_restartable_ctx_t;

However, in this particular case, swapping of the second and the third fields should eliminate padding and reduce the structure size from 24 bytes to 16:

typedef struct { 
     mbedtls_pk_rs_op_t op_type; 
     mbedtls_svc_key_id_t pub_id; 
     void *op; 
 } mbedtls_pk_psa_restartable_ctx_t;

Changed field order should not be an API break.

https://github.com/Mbed-TLS/TF-PSA-Crypto/blob/e704e187b2e90d5e7d97ffd4ad961b5a04ec8a13/drivers/builtin/src/pk_wrap.h#L28-L33 Visual C in 64-bit mode aligns the pointer to 8-byte address, and this means 4 bytes were lost twice - on padding of the first and the third fields. Ideally, fields should be ordered by size in decreasing order (moved `op_type` down as it could be redeclared with `char` size): ``` typedef struct { void *op; mbedtls_svc_key_id_t pub_id; mbedtls_pk_rs_op_t op_type; } mbedtls_pk_psa_restartable_ctx_t; ``` However, in this particular case, swapping of the second and the third fields should eliminate padding and reduce the structure size from 24 bytes to 16: ``` typedef struct { mbedtls_pk_rs_op_t op_type; mbedtls_svc_key_id_t pub_id; void *op; } mbedtls_pk_psa_restartable_ctx_t; ``` Changed field order should not be an API break.

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gh-Mbed-TLS/TF-PSA-Crypto#623

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