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libexpr: Overalign Value to 16 bytes

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This is necessary to make use of 128 bit atomics on x86_64 [1],
since MOVAPD, MOVAPS, and MOVDQA need memory operands to be 16-byte
aligned. We are not losing anything here, because Value is already 16-byte
wide and Boehm allocates memory in granules that are 16 bytes by default
on 64 bit systems [2].

[1]: https://patchwork.sourceware.org/project/gcc/patch/YhxkfzGEEQ9KHbBC@tucnak/
[2]: 54ac18ccbc/include/gc/gc_tiny_fl.h (L31-L33)
This commit is contained in:
Sergei Zimmerman 2025-09-09 22:18:52 +03:00
parent 371623bf0c
commit 4524235af4
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2 changed files with 15 additions and 2 deletions
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12
src/libexpr/eval-gc.cc
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@ -16,6 +16,7 @@
# endif
# include <gc/gc_allocator.h>
# include <gc/gc_tiny_fl.h> // For GC_GRANULE_BYTES
# include <boost/coroutine2/coroutine.hpp>
# include <boost/coroutine2/protected_fixedsize_stack.hpp>
@ -23,6 +24,17 @@
#endif
/*
* Ensure that Boehm satisfies our alignment requirements. This is the default configuration [^]
* and this assertion should never break for any platform. Let's assert it just in case.
*
* This alignment is particularly useful to be able to use aligned
* load/store instructions for loading/writing Values.
*
* [^]: https://github.com/bdwgc/bdwgc/blob/54ac18ccbc5a833dd7edaff94a10ab9b65044d61/include/gc/gc_tiny_fl.h#L31-L33
*/
static_assert(sizeof(void *) * 2 == GC_GRANULE_BYTES, "Boehm GC must use GC_GRANULE_WORDS = 2");
namespace nix {
#if NIX_USE_BOEHMGC

5
src/libexpr/include/nix/expr/value.hh
View file

@ -369,7 +369,7 @@ namespace detail {
/* Whether to use a specialization of ValueStorage that does bitpacking into
alignment niches. */
template<std::size_t ptrSize>
inline constexpr bool useBitPackedValueStorage = (ptrSize == 8) && (__STDCPP_DEFAULT_NEW_ALIGNMENT__ >= 8);
inline constexpr bool useBitPackedValueStorage = (ptrSize == 8) && (__STDCPP_DEFAULT_NEW_ALIGNMENT__ >= 16);
} // namespace detail
@ -378,7 +378,8 @@ inline constexpr bool useBitPackedValueStorage = (ptrSize == 8) && (__STDCPP_DEF
* Packs discriminator bits into the pointer alignment niches.
*/
template<std::size_t ptrSize>
class ValueStorage<ptrSize, std::enable_if_t<detail::useBitPackedValueStorage<ptrSize>>> : public detail::ValueBase
class alignas(16) ValueStorage<ptrSize, std::enable_if_t<detail::useBitPackedValueStorage<ptrSize>>>
: public detail::ValueBase
{
/* Needs a dependent type name in order for member functions (and
* potentially ill-formed bit casts) to be SFINAE'd out.