CODEWITHSUNDEEP
cstringperformancecompare
chriskirk
chriskirk

Reputation: 761

Fastest way to compare two same-length strings

Strings are fixed length 8, contain alphanumeric characters and right-padded with spaces.

I.e,

"STRING1 "
"STR2    "
"S       "

etc..

I was thinking memcmp might be the fastest here?

Upvotes: 1

Views: 705

Answers (2)

Blagovest Buyukliev
Blagovest Buyukliev

Reputation: 43508

If you ensure that the strings are aligned on an 8-byte boundary via compiler-specific attributes, you can do:

uint64_t a = *((uint64_t *) "STRING1 ");
uint64_t b = *((uint64_t *) "STR2    ");

Then a == b should yield to a single 64-bit instruction.

Or, if they are just constant immutable strings (stored in a read-only area of the process), you can go on with comparing the const char * pointers themselves. It's still a reliable test since a string literal that appears twice in the current translation unit should refer to the same memory:

/* fails because the two strings are stored at different locations */
"STRING1 " == "STR2    "
/* should succeed, even the silliest compiler should merge both literals */
"STRING1 " == "STRING1 "

Upvotes: 10

K-ballo
K-ballo

Reputation: 81349

If the strings have fixed equal length, then memcmp is a good approach.

Upvotes: 4

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