handy.h: Convert XDIGIT_VALUE to branchless

This removes a branch and an array lookup in the XDIGIT_VALUE() macro.
It adds some shifts, masks, and additions instead, though replacing a
mask and addition in the old way.

A somewhat more complicated version could be made for EBCDIC, but I'm
not bothering to do that, using an array lookup to convert the salient
value to ASCII, so on EBCDIC there isn't an array lookup removal.

1 files changed, 16 insertions, 11 deletions

diff --git a/handy.h b/handy.h
index 8da2a15eea..08f832ac59 100644
--- a/handy.h
+++ b/handy.h
@@ -2290,17 +2290,22 @@ typedef U32 line_t;
 	} \
 	return a;
 
-/* Converts a character known to represent a hexadecimal digit (0-9, A-F, or
- * a-f) to its numeric value.  READ_XDIGIT's argument is a string pointer,
- * which is advanced.  The input is validated only by an assert() in DEBUGGING
- * builds.  In both ASCII and EBCDIC the last 4 bits of the digits are 0-9; and
- * the last 4 bits of A-F and a-f are 1-6, so adding 9 yields 10-15 */
-#define XDIGIT_VALUE(c) (__ASSERT_(isXDIGIT(c)) (0xf & (isDIGIT(c)        \
-                                                        ? (c)             \
-                                                        : ((c) + 9))))
-#define READ_XDIGIT(s)  (__ASSERT_(isXDIGIT(*s)) (0xf & (isDIGIT(*(s))     \
-                                                        ? (*(s)++)         \
-                                                        : (*(s)++ + 9))))
+/* Converts a character KNOWN to represent a hexadecimal digit (0-9, A-F, or
+ * a-f) to its numeric value without using any branches.  The input is
+ * validated only by an assert() in DEBUGGING builds.
+ *
+ * It works by right shifting and isolating the bit that is 0 for the digits,
+ * and 1 for at least the alphas A-F, a-f.  The bit is shifted to the ones
+ * position, and then to the eights position.  Both are added together to form
+ * 0 if the input is '0'-'9' and to form 9 if alpha.  This is added to the
+ * final four bits of the input to form the correct value. */
+#define XDIGIT_VALUE(c) (__ASSERT_(isXDIGIT(c))                             \
+           ((NATIVE_TO_LATIN1(c) >> 6) & 1)  /* 1 if alpha; 0 if not */     \
+         + ((NATIVE_TO_LATIN1(c) >> 3) & 8)  /* 8 if alpha; 0 if not */     \
+         + ((c) & 0xF))   /* 0-9 if input valid hex digit */
+
+/* The argument is a string pointer, which is advanced. */
+#define READ_XDIGIT(s)  ((s)++, XDIGIT_VALUE(*((s) - 1)))
 
 /* Converts a character known to represent an octal digit (0-7) to its numeric
  * value.  The input is validated only by an assert() in DEBUGGING builds.  In