[PATCH v1 1/2] hash: Remove unnecessary `isupper` check before `tolower`

[Noah Goldstein]

tolower only modifies the character if there is a valid lower
conversion:

https://linux.die.net/man/3/tolower:
```
The value returned is that of the converted letter, or c if the
conversion was not possible.
```

Both `isupper` and `tolower` are implemented with essentially the same
lookup table so this doesn't even make a real fast path in the
best case.

Comparing the performance of the old ISTRING_HASH to the new one
on all the files present in GLIBC.

Data Type            Old Time,  New Time
Alternating case -> 24985.754, 13883.422
random case      -> 35211.777, 13569.051
all lower        -> 18818.974, 13706.645
all upper        -> 38859.454, 13506.315

All tests pass:
```
$> git clean -xf; ./bootstrap && ./configure && make check
...

977 Tests in 132 Categories Complete ... No Failures :-)

=========================================================================
 Regression PASSED: GNU Make 4.3.90 (x86_64-pc-linux-gnu) built with gcc
=========================================================================
```
---
 src/hash.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/hash.h b/src/hash.h
index 667d6508..e1aaa30e 100644
--- a/src/hash.h
+++ b/src/hash.h
@@ -154,7 +154,7 @@ extern void *hash_deleted_item;
 #define ISTRING_HASH_1(KEY, RESULT) do { \
   unsigned char const *_key_ = (unsigned char const *) (KEY) - 1; \
   while (*++_key_) \
-    (RESULT) += ((isupper (*_key_) ? tolower (*_key_) : *_key_) << (_key_[1] & 
0xf)); \
+    (RESULT) += (tolower (*_key_) << (_key_[1] & 0xf)); \
 } while (0)
 #define return_ISTRING_HASH_1(KEY) do { \
   unsigned long _result_ = 0; \
@@ -165,7 +165,7 @@ extern void *hash_deleted_item;
 #define ISTRING_HASH_2(KEY, RESULT) do { \
   unsigned char const *_key_ = (unsigned char const *) (KEY) - 1; \
   while (*++_key_) \
-    (RESULT) += ((isupper (*_key_) ? tolower (*_key_) : *_key_) << (_key_[1] & 
0x7)); \
+    (RESULT) += (tolower (*_key_) << (_key_[1] & 0x7); \
 } while (0)
 #define return_ISTRING_HASH_2(KEY) do { \
   unsigned long _result_ = 0; \
-- 
2.25.1