xdiff: refactor xdl_hash_record()

Inline the check for whitespace flags so that the compiler can hoist
it out of the loop in xdl_prepare_ctx(). This improves the performance
by 8%.

$ hyperfine --warmup=1 -L rev HEAD,HEAD^  --setup='git checkout {rev} -- :/ && make git' ': {rev}; GIT_CONFIG_GLOBAL=/dev/null ./git log --oneline --shortstat v2.0.0..v2.5.0'
Benchmark 1: : HEAD; GIT_CONFIG_GLOBAL=/dev/null ./git log --oneline --shortstat v2.0.0..v2.5.0
  Time (mean ± σ):      1.670 s ±  0.044 s    [User: 1.473 s, System: 0.196 s]
  Range (min … max):    1.619 s …  1.754 s    10 runs

Benchmark 2: : HEAD^; GIT_CONFIG_GLOBAL=/dev/null ./git log --oneline --shortstat v2.0.0..v2.5.0
  Time (mean ± σ):      1.801 s ±  0.021 s    [User: 1.605 s, System: 0.192 s]
  Range (min … max):    1.766 s …  1.831 s    10 runs

Summary
  ': HEAD^; GIT_CONFIG_GLOBAL=/dev/null ./git log --oneline --shortstat v2.0.0..v2.5.0' ran
    1.08 ± 0.03 times faster than ': HEAD^^; GIT_CONFIG_GLOBAL=/dev/null ./git log --oneline --shortstat v2.0.0..v2.5.0'

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: phillipwood

xdiff/xutils.c

@@ -249,7 +249,7 @@ int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags)
 	return 1;
 }
 
-static unsigned long xdl_hash_record_with_whitespace(char const **data,
+unsigned long xdl_hash_record_with_whitespace(char const **data,
 		char const *top, long flags) {
 	unsigned long ha = 5381;
 	char const *ptr = *data;
@@ -294,13 +294,10 @@ static unsigned long xdl_hash_record_with_whitespace(char const **data,
 	return ha;
 }
 
-unsigned long xdl_hash_record(char const **data, char const *top, long flags) {
+unsigned long xdl_hash_record_verbatim(char const **data, char const *top) {
 	unsigned long ha = 5381;
 	char const *ptr = *data;
 
-	if (flags & XDF_WHITESPACE_FLAGS)
-		return xdl_hash_record_with_whitespace(data, top, flags);
-
 	for (; ptr < top && *ptr != '\n'; ptr++) {
 		ha += (ha << 5);
 		ha ^= (unsigned long) *ptr;

xdiff/xutils.h

@@ -34,7 +34,15 @@ void *xdl_cha_alloc(chastore_t *cha);
 long xdl_guess_lines(mmfile_t *mf, long sample);
 int xdl_blankline(const char *line, long size, long flags);
 int xdl_recmatch(const char *l1, long s1, const char *l2, long s2, long flags);
-unsigned long xdl_hash_record(char const **data, char const *top, long flags);
+unsigned long xdl_hash_record_verbatim(char const **data, char const *top);
+unsigned long xdl_hash_record_with_whitespace(char const **data, char const *top, long flags);
+static inline unsigned long xdl_hash_record(char const **data, char const *top, long flags)
+{
+	if (flags & XDF_WHITESPACE_FLAGS)
+		return xdl_hash_record_with_whitespace(data, top, flags);
+	else
+		return xdl_hash_record_verbatim(data, top);
+}
 unsigned int xdl_hashbits(unsigned int size);
 int xdl_num_out(char *out, long val);
 int xdl_emit_hunk_hdr(long s1, long c1, long s2, long c2,