improved decompression speed

while reviewing #1364,
I found a decompression speed improvement.

On my laptop, the new code decompresses +5-6% faster on clang
and +2-3% faster on gcc.

not bad for an accidental optimization...

diff --git a/lib/common/bitstream.h b/lib/common/bitstream.h
index 2f91460c5eb0a0e319c8f0b91e4f8e041ff59da5..00b22049a48e136c1e39fc6e7e74c56eec1606e7 100644 (file)
--- a/lib/common/bitstream.h
+++ b/lib/common/bitstream.h
@@ -339,17 +339,8 @@ MEM_STATIC size_t BIT_getUpperBits(size_t bitContainer, U32 const start)
 
 MEM_STATIC size_t BIT_getMiddleBits(size_t bitContainer, U32 const start, U32 const nbBits)
 {
-#if defined(__BMI__) && defined(__GNUC__) && __GNUC__*1000+__GNUC_MINOR__ >= 4008  /* experimental */
-#  if defined(__x86_64__)
-    if (sizeof(bitContainer)==8)
-        return _bextr_u64(bitContainer, start, nbBits);
-    else
-#  endif
-        return _bextr_u32(bitContainer, start, nbBits);
-#else
     assert(nbBits < BIT_MASK_SIZE);
     return (bitContainer >> start) & BIT_mask[nbBits];
-#endif
 }
 
 MEM_STATIC size_t BIT_getLowerBits(size_t bitContainer, U32 const nbBits)
@@ -366,9 +357,11 @@ MEM_STATIC size_t BIT_getLowerBits(size_t bitContainer, U32 const nbBits)
  * @return : value extracted */
 MEM_STATIC size_t BIT_lookBits(const BIT_DStream_t* bitD, U32 nbBits)
 {
-#if defined(__BMI__) && defined(__GNUC__)   /* experimental; fails if bitD->bitsConsumed + nbBits > sizeof(bitD->bitContainer)*8 */
+#if 1
+    assert(bitD->bitsConsumed + nbBits <= sizeof(bitD->bitContainer)*8);
     return BIT_getMiddleBits(bitD->bitContainer, (sizeof(bitD->bitContainer)*8) - bitD->bitsConsumed - nbBits, nbBits);
 #else
+    /* previous code path, seems slower */
     U32 const regMask = sizeof(bitD->bitContainer)*8 - 1;
     return ((bitD->bitContainer << (bitD->bitsConsumed & regMask)) >> 1) >> ((regMask-nbBits) & regMask);
 #endif