Auto merge of #1158 - glyn:large-bitfield-units, r=emilio

Support bitfield allocation units larger than 64 bits

Individual bitfields are still limited to at most 64 bits, but this restriction can be weakened when Rust supports `u128`.

This implements issue #816.

Usage notes:

* Since common code is added to each generated binding, a program which uses
  more than one binding may need to work around the duplication by including
  each binding in its own module.
* The values created by bitfield allocation unit constructors can be assigned
  directly to the corresponding struct fields with no need for transmutation.

Implementation notes:

`__BindgenBitfieldUnit` represents a bitfield allocation unit using a `Storage`
type accessible as a slice of `u8`. The alignment of the unit is inherited from
an `Align` type by virtue of the field:
```
align: [Align; 0],
```
The position of this field in the struct is irrelevant.

It is assumed that the alignment of the `Storage` type is no larger than the
alignment of the `Align` type, which will be true if the `Storage` type is, for
example, an array of `u8`. This assumption is checked in a debug assertion.

Although the double underscore (__) prefix is reserved for implementations of
C++, there are precedents for this convention elsewhere in bindgen and so the
convention is adopted here too.

Acknowledgement:

Thanks to @fitzgen for an initial implementation of `__BindgenBitfieldUnit` and
code to integrate it into bindgen.

r? @emilio

1 files changed, 82 insertions, 0 deletions

diff --git a/src/codegen/bitfield_unit.rs b/src/codegen/bitfield_unit.rs
new file mode 100755
index 00000000..3c7c9b7b
--- /dev/null
+++ b/src/codegen/bitfield_unit.rs
@@ -0,0 +1,82 @@
+#[repr(C)]
+#[derive(Copy, Clone, Debug, Default, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub struct __BindgenBitfieldUnit<Storage, Align>
+where
+    Storage: AsRef<[u8]> + AsMut<[u8]>,
+{
+    storage: Storage,
+    align: [Align; 0],
+}
+
+impl<Storage, Align> __BindgenBitfieldUnit<Storage, Align>
+where
+    Storage: AsRef<[u8]> + AsMut<[u8]>,
+{
+    #[inline]
+    pub fn new(storage: Storage) -> Self {
+        Self {
+            storage,
+            align: [],
+        }
+    }
+
+    #[inline]
+    pub fn get_bit(&self, index: usize) -> bool {
+        debug_assert!(index / 8 < self.storage.as_ref().len());
+
+        let byte_index = index / 8;
+        let byte = self.storage.as_ref()[byte_index];
+
+        let bit_index = index % 8;
+        let mask = 1 << bit_index;
+
+        byte & mask == mask
+    }
+
+    #[inline]
+    pub fn set_bit(&mut self, index: usize, val: bool) {
+        debug_assert!(index / 8 < self.storage.as_ref().len());
+
+        let byte_index = index / 8;
+        let byte = &mut self.storage.as_mut()[byte_index];
+
+        let bit_index = index % 8;
+        let mask = 1 << bit_index;
+
+        if val {
+            *byte |= mask;
+        } else {
+            *byte &= !mask;
+        }
+    }
+
+    #[inline]
+    pub fn get(&self, bit_offset: usize, bit_width: u8) -> u64 {
+        debug_assert!(bit_width <= 64);
+        debug_assert!(bit_offset / 8 < self.storage.as_ref().len());
+        debug_assert!((bit_offset + (bit_width as usize)) / 8 <= self.storage.as_ref().len());
+
+        let mut val = 0;
+
+        for i in 0..(bit_width as usize) {
+            if self.get_bit(i + bit_offset) {
+                val |= 1 << i;
+            }
+        }
+
+        val
+    }
+
+    #[inline]
+    pub fn set(&mut self, bit_offset: usize, bit_width: u8, val: u64) {
+        debug_assert!(bit_width <= 64);
+        debug_assert!(bit_offset / 8 < self.storage.as_ref().len());
+        debug_assert!((bit_offset + (bit_width as usize)) / 8 <= self.storage.as_ref().len());
+
+        for i in 0..(bit_width as usize) {
+            let mask = 1 << i;
+            let val_bit_is_set = val & mask == mask;
+            self.set_bit(i + bit_offset, val_bit_is_set);
+        }
+    }
+}