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Small String Optimization (#5690)

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Reduce String memory overhead from 24 bytes to 16 bytes by limiting the
maximum string length to <64Kbytes (which is larger than heap so no
effective problem).

Add Small String Optimization, SSO, which instead of allocating pointers
to small strings on the heap will store the string in place of the
pointer in the class.  This should reduce memory fragmentation as

Save up to 12 chars (11 + \0) in String itself by using the terminating
\0 in the inline string as a flag to identify if this is a SSO or a heap
string.

Add a host test that verifies that no memory is allocated until a
full 11 characters are assigned to a string, as well as checking all
intermediate values.

No user code changes should be required to work with this optimization.
This commit is contained in:
Earle F. Philhower, III
2019-02-08 17:37:43 +00:00
committed by GitHub
parent 1959311180
commit 7369133681
4 changed files with 390 additions and 230 deletions
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93
tests/host/core/test_string.cpp
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@ -268,3 +268,96 @@ TEST_CASE("String sizes near 8b", "[core][String]")
REQUIRE(!strcmp(s16.c_str(),"123456789012345_"));
REQUIRE(!strcmp(s17.c_str(),"1234567890123456_"));
}
TEST_CASE("String SSO works", "[core][String]")
{
// This test assumes that SSO_SIZE==8, if that changes the test must as well
String s;
s += "0";
REQUIRE(s == "0");
REQUIRE(s.length() == 1);
const char *savesso = s.c_str();
s += 1;
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "01");
REQUIRE(s.length() == 2);
s += "2";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "012");
REQUIRE(s.length() == 3);
s += 3;
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "0123");
REQUIRE(s.length() == 4);
s += "4";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "01234");
REQUIRE(s.length() == 5);
s += "5";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "012345");
REQUIRE(s.length() == 6);
s += "6";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "0123456");
REQUIRE(s.length() == 7);
s += "7";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "01234567");
REQUIRE(s.length() == 8);
s += "8";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "012345678");
REQUIRE(s.length() == 9);
s += "9";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "0123456789");
REQUIRE(s.length() == 10);
s += "a";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "0123456789a");
REQUIRE(s.length() == 11);
if (sizeof(savesso) == 4) {
s += "b";
REQUIRE(s.c_str() != savesso);
REQUIRE(s == "0123456789ab");
REQUIRE(s.length() == 12);
s += "c";
REQUIRE(s.c_str() != savesso);
REQUIRE(s == "0123456789abc");
REQUIRE(s.length() == 13);
} else {
s += "bcde";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "0123456789abcde");
REQUIRE(s.length() == 15);
s += "fghi";
REQUIRE(s.c_str() == savesso);
REQUIRE(s == "0123456789abcdefghi");
REQUIRE(s.length() == 19);
s += "j";
REQUIRE(s.c_str() != savesso);
REQUIRE(s == "0123456789abcdefghij");
REQUIRE(s.length() == 20);
s += "k";
REQUIRE(s.c_str() != savesso);
REQUIRE(s == "0123456789abcdefghijk");
REQUIRE(s.length() == 21);
s += "l";
REQUIRE(s.c_str() != savesso);
REQUIRE(s == "0123456789abcdefghijkl");
REQUIRE(s.length() == 22);
s += "m";
REQUIRE(s.c_str() != savesso);
REQUIRE(s == "0123456789abcdefghijklm");
REQUIRE(s.length() == 23);
s += "nopq";
REQUIRE(s.c_str() != savesso);
REQUIRE(s == "0123456789abcdefghijklmnopq");
REQUIRE(s.length() == 27);
s += "rstu";
REQUIRE(s.c_str() != savesso);
REQUIRE(s == "0123456789abcdefghijklmnopqrstu");
REQUIRE(s.length() == 31);
}
}