reftable/record: store "val1" hashes as static arrays

When reading ref records of type "val1", we store its object ID in an
allocated array. This results in an additional allocation for every
single ref record we read, which is rather inefficient especially when
iterating over refs.

Refactor the code to instead use an embedded array of `GIT_MAX_RAWSZ`
bytes. While this means that `struct ref_record` is bigger now, we
typically do not store all refs in an array anyway and instead only
handle a limited number of records at the same point in time.

Using `git show-ref --quiet` in a repository with ~350k refs this leads
to a significant drop in allocations. Before:

    HEAP SUMMARY:
        in use at exit: 21,098 bytes in 192 blocks
      total heap usage: 2,116,683 allocs, 2,116,491 frees, 76,098,060 bytes allocated

After:

    HEAP SUMMARY:
        in use at exit: 21,098 bytes in 192 blocks
      total heap usage: 1,419,031 allocs, 1,418,839 frees, 62,145,036 bytes allocated

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 6 insertions, 10 deletions

diff --git a/reftable/merged_test.c b/reftable/merged_test.c
index d08c16abef..b3927a5d73 100644
--- a/reftable/merged_test.c
+++ b/reftable/merged_test.c
@@ -123,13 +123,11 @@ static void readers_destroy(struct reftable_reader **readers, size_t n)
 
 static void test_merged_between(void)
 {
-	uint8_t hash1[GIT_SHA1_RAWSZ] = { 1, 2, 3, 0 };
-
 	struct reftable_ref_record r1[] = { {
 		.refname = "b",
 		.update_index = 1,
 		.value_type = REFTABLE_REF_VAL1,
-		.value.val1 = hash1,
+		.value.val1 = { 1, 2, 3, 0 },
 	} };
 	struct reftable_ref_record r2[] = { {
 		.refname = "a",
@@ -165,26 +163,24 @@ static void test_merged_between(void)
 
 static void test_merged(void)
 {
-	uint8_t hash1[GIT_SHA1_RAWSZ] = { 1 };
-	uint8_t hash2[GIT_SHA1_RAWSZ] = { 2 };
 	struct reftable_ref_record r1[] = {
 		{
 			.refname = "a",
 			.update_index = 1,
 			.value_type = REFTABLE_REF_VAL1,
-			.value.val1 = hash1,
+			.value.val1 = { 1 },
 		},
 		{
 			.refname = "b",
 			.update_index = 1,
 			.value_type = REFTABLE_REF_VAL1,
-			.value.val1 = hash1,
+			.value.val1 = { 1 },
 		},
 		{
 			.refname = "c",
 			.update_index = 1,
 			.value_type = REFTABLE_REF_VAL1,
-			.value.val1 = hash1,
+			.value.val1 = { 1 },
 		}
 	};
 	struct reftable_ref_record r2[] = { {
@@ -197,13 +193,13 @@ static void test_merged(void)
 			.refname = "c",
 			.update_index = 3,
 			.value_type = REFTABLE_REF_VAL1,
-			.value.val1 = hash2,
+			.value.val1 = { 2 },
 		},
 		{
 			.refname = "d",
 			.update_index = 3,
 			.value_type = REFTABLE_REF_VAL1,
-			.value.val1 = hash1,
+			.value.val1 = { 1 },
 		},
 	};