Add an x86 optimized version of extend().

[fjl] / dehuff.h

diff --git a/dehuff.h b/dehuff.h
index 9243e101530018b346e80e2db8b64b96dee1db61..7621f6095e99791feb6138e81c2574ae9dda6bce 100644 (file)
--- a/dehuff.h
+++ b/dehuff.h
@@ -81,13 +81,25 @@ static const int bit_thresholds[16] = {
        0, 1 << 0, 1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7, 1 << 8, 1 << 9, 1 << 10, 1 << 11, 1 << 12, 1 << 13, 1 << 14
 };
 
        0, 1 << 0, 1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7, 1 << 8, 1 << 9, 1 << 10, 1 << 11, 1 << 12, 1 << 13, 1 << 14
 };
 

-static inline unsigned extend(int val, int bits)
+static inline unsigned extend(int val, unsigned bits)

 {
 {

+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+       // GCC should ideally be able to figure out that the conditional move is better, but
+       // it doesn't for various reasons, and this is pretty important for speed, so we hardcode.
+       asm("cmp %2, %0 ; cmovl %3, %0"
+               : "=r" (val)
+               : "0" (val),
+                 "g" (bit_thresholds[bits]),
+                 "r" (val + (-1 << bits) + 1)
+               : "cc");
+       return val;
+#else

        if (val < bit_thresholds[bits]) {
                return val + (-1 << bits) + 1;
        } else {
                return val;
        }
        if (val < bit_thresholds[bits]) {
                return val + (-1 << bits) + 1;
        } else {
                return val;
        }

+#endif

 }
 
 #endif /* !defined(_DEHUFF_H) */
 }
 
 #endif /* !defined(_DEHUFF_H) */