ds_core/bits: add mask_bits; convert one_at_bit-s

microsoft · Jun 13, 2024 · 5c31127 · 5c31127
1 parent c1e0cce
commit 5c31127
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Showing 4 changed files with 20 additions and 9 deletions.
diff --git a/src/snmalloc/backend_helpers/largebuddyrange.h b/src/snmalloc/backend_helpers/largebuddyrange.h
@@ -354,7 +354,7 @@ namespace snmalloc
         SNMALLOC_ASSERT(size >= MIN_CHUNK_SIZE);
         SNMALLOC_ASSERT(bits::is_pow2(size));
 
-        if (size >= (bits::one_at_bit(MAX_SIZE_BITS) - 1))
+        if (size >= bits::mask_bits(MAX_SIZE_BITS))
         {
           if (ParentRange::Aligned)
             return parent.alloc_range(size);
@@ -378,7 +378,7 @@ namespace snmalloc
 
         if constexpr (MAX_SIZE_BITS != (bits::BITS - 1))
         {
-          if (size >= (bits::one_at_bit(MAX_SIZE_BITS) - 1))
+          if (size >= bits::mask_bits(MAX_SIZE_BITS))
           {
             parent_dealloc_range(base, size);
             return;

diff --git a/src/snmalloc/ds_core/bits.h b/src/snmalloc/ds_core/bits.h
@@ -59,6 +59,19 @@ namespace snmalloc
       return (static_cast<T>(1)) << shift;
     }
 
+    /**
+     * Returns a value of type T that has its n LSBs all set.
+     *
+     * S is a template parameter because callers use either `int` or `size_t`
+     * and either is valid to represent a number in the range 0-63 (or 0-127 if
+     * we want to use `__uint128_t` as `T`).
+     */
+    template<typename T = size_t, typename S>
+    constexpr T mask_bits(S n)
+    {
+      return one_at_bit<T>(n) - 1;
+    }
+
     inline SNMALLOC_FAST_PATH size_t clz(size_t x)
     {
       SNMALLOC_ASSERT(x != 0); // Calling with 0 is UB on some implementations
@@ -326,7 +339,7 @@ namespace snmalloc
     constexpr size_t to_exp_mant_const(size_t value)
     {
       constexpr size_t LEADING_BIT = one_at_bit(MANTISSA_BITS + LOW_BITS) >> 1;
-      constexpr size_t MANTISSA_MASK = one_at_bit(MANTISSA_BITS) - 1;
+      constexpr size_t MANTISSA_MASK = mask_bits(MANTISSA_BITS);
 
       value = value - 1;
 
@@ -344,7 +357,7 @@ namespace snmalloc
       if (MANTISSA_BITS > 0)
       {
         m_e = m_e + 1;
-        constexpr size_t MANTISSA_MASK = one_at_bit(MANTISSA_BITS) - 1;
+        constexpr size_t MANTISSA_MASK = mask_bits(MANTISSA_BITS);
         size_t m = m_e & MANTISSA_MASK;
         size_t e = m_e >> MANTISSA_BITS;
         size_t b = e == 0 ? 0 : 1;

diff --git a/src/snmalloc/mem/entropy.h b/src/snmalloc/mem/entropy.h
@@ -116,8 +116,7 @@ namespace snmalloc
         fresh_bits = get_next();
         count = 64;
       }
-      uint16_t result =
-        static_cast<uint16_t>(fresh_bits & (bits::one_at_bit(n) - 1));
+      uint16_t result = static_cast<uint16_t>(fresh_bits & bits::mask_bits(n));
       fresh_bits >>= n;
       count -= n;
       return result;

diff --git a/src/snmalloc/mem/sizeclasstable.h b/src/snmalloc/mem/sizeclasstable.h
@@ -220,7 +220,7 @@ namespace snmalloc
         size_t slab_bits = bits::max(
           bits::next_pow2_bits_const(MIN_OBJECT_COUNT * rsize), MIN_CHUNK_BITS);
 
-        meta.slab_mask = bits::one_at_bit(slab_bits) - 1;
+        meta.slab_mask = bits::mask_bits(slab_bits);
 
         auto& meta_slow = slow(sizeclass_t::from_small_class(sizeclass));
         meta_slow.capacity =
@@ -245,8 +245,7 @@ namespace snmalloc
       {
         // Calculate reciprocal division constant.
         auto& meta = fast_small(sizeclass);
-        meta.div_mult =
-          ((bits::one_at_bit(DIV_MULT_SHIFT) - 1) / meta.size) + 1;
+        meta.div_mult = (bits::mask_bits(DIV_MULT_SHIFT) / meta.size) + 1;
 
         size_t zero = 0;
         meta.mod_zero_mult = (~zero / meta.size) + 1;