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change(mbedtls/port): optimize gcm_mult()

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1) pre-shift GCM last4 to use 32-bit shift

On 32-bit architectures like Aarch32, RV32, Xtensa,
shifting a 64-bit variable by 32-bits is free,
since it changes the register representing half of the 64-bit var.
Pre-shift the last4 array to take advantage of this.

2) unroll first GCM iteration

The first loop of gcm_mult() is different from
the others. By unrolling it separately from the
others, the other iterations may take advantage
of the zero-overhead loop construct, in addition
to saving a conditional branch in the loop.
This commit is contained in:
Richard Allen 2024-08-06 08:44:59 -05:00 committed by Mahavir Jain
parent 357e28826d
commit 0b51c24238
1 changed files with 22 additions and 17 deletions
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39
components/mbedtls/port/aes/esp_aes_gcm.c
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@ -192,11 +192,11 @@ static int gcm_gen_table( esp_gcm_context *ctx )
* last4[x] = x times P^128
* where x and last4[x] are seen as elements of GF(2^128) as in [MGV]
*/
static const uint64_t last4[16] = {
0x0000, 0x1c20, 0x3840, 0x2460,
0x7080, 0x6ca0, 0x48c0, 0x54e0,
0xe100, 0xfd20, 0xd940, 0xc560,
0x9180, 0x8da0, 0xa9c0, 0xb5e0
static const uint32_t last4[16] = {
0x00000000, 0x1c200000, 0x38400000, 0x24600000,
0x70800000, 0x6ca00000, 0x48c00000, 0x54e00000,
0xe1000000, 0xfd200000, 0xd9400000, 0xc5600000,
0x91800000, 0x8da00000, 0xa9c00000, 0xb5e00000
};
/* Based on MbedTLS's implementation
*
@ -211,28 +211,33 @@ static void gcm_mult( esp_gcm_context *ctx, const unsigned char x[16],
uint64_t zh, zl;
lo = x[15] & 0xf;
hi = x[15] >> 4;
zh = ctx->HH[lo];
zl = ctx->HL[lo];
for ( i = 15; i >= 0; i-- ) {
rem = (unsigned char) zl & 0xf;
zl = ( zh << 60 ) | ( zl >> 4 );
zh = ( zh >> 4 );
zh ^= (uint64_t) last4[rem] << 32;
zh ^= ctx->HH[hi];
zl ^= ctx->HL[hi];
for ( i = 14; i >= 0; i-- ) {
lo = x[i] & 0xf;
hi = x[i] >> 4;
if ( i != 15 ) {
rem = (unsigned char) zl & 0xf;
zl = ( zh << 60 ) | ( zl >> 4 );
zh = ( zh >> 4 );
zh ^= (uint64_t) last4[rem] << 48;
zh ^= ctx->HH[lo];
zl ^= ctx->HL[lo];
}
rem = (unsigned char) zl & 0xf;
zl = ( zh << 60 ) | ( zl >> 4 );
zh = ( zh >> 4 );
zh ^= (uint64_t) last4[rem] << 48;
zh ^= (uint64_t) last4[rem] << 32;
zh ^= ctx->HH[lo];
zl ^= ctx->HL[lo];
rem = (unsigned char) zl & 0xf;
zl = ( zh << 60 ) | ( zl >> 4 );
zh = ( zh >> 4 );
zh ^= (uint64_t) last4[rem] << 32;
zh ^= ctx->HH[hi];
zl ^= ctx->HL[hi];
}