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+//! This module contains the LLVM intrinsics bindings that provide the functionality for this
+//! crate.
+//!
+//! The LLVM assembly language is documented here: <https://llvm.org/docs/LangRef.html>
+//!
+//! A quick glossary of jargon that may appear in this module, mostly paraphrasing LLVM's LangRef:
+//! - poison: "undefined behavior as a value". specifically, it is like uninit memory (such as padding bytes). it is "safe" to create poison, BUT
+//!   poison MUST NOT be observed from safe code, as operations on poison return poison, like NaN. unlike NaN, which has defined comparisons,
+//!   poison is neither true nor false, and LLVM may also convert it to undef (at which point it is both). so, it can't be conditioned on, either.
+//! - undef: "a value that is every value". functionally like poison, insofar as Rust is concerned. poison may become this. note:
+//!   this means that division by poison or undef is like division by zero, which means it inflicts...
+//! - "UB": poison and undef cover most of what people call "UB". "UB" means this operation immediately invalidates the program:
+//!   LLVM is allowed to lower it to `ud2` or other opcodes that may cause an illegal instruction exception, and this is the "good end".
+//!   The "bad end" is that LLVM may reverse time to the moment control flow diverged on a path towards undefined behavior,
+//!   and destroy the other branch, potentially deleting safe code and violating Rust's `unsafe` contract.
+//!
+//! Note that according to LLVM, vectors are not arrays, but they are equivalent when stored to and loaded from memory.
+//!
+//! Unless stated otherwise, all intrinsics for binary operations require SIMD vectors of equal types and lengths.
+
+// These intrinsics aren't linked directly from LLVM and are mostly undocumented, however they are
+// mostly lowered to the matching LLVM instructions by the compiler in a fairly straightforward manner.
+// The associated LLVM instruction or intrinsic is documented alongside each Rust intrinsic function.
+extern "platform-intrinsic" {
+    /// add/fadd
+    pub(crate) fn simd_add<T>(x: T, y: T) -> T;
+
+    /// sub/fsub
+    pub(crate) fn simd_sub<T>(lhs: T, rhs: T) -> T;
+
+    /// mul/fmul
+    pub(crate) fn simd_mul<T>(x: T, y: T) -> T;
+
+    /// udiv/sdiv/fdiv
+    /// ints and uints: {s,u}div incur UB if division by zero occurs.
+    /// ints: sdiv is UB for int::MIN / -1.
+    /// floats: fdiv is never UB, but may create NaNs or infinities.
+    pub(crate) fn simd_div<T>(lhs: T, rhs: T) -> T;
+
+    /// urem/srem/frem
+    /// ints and uints: {s,u}rem incur UB if division by zero occurs.
+    /// ints: srem is UB for int::MIN / -1.
+    /// floats: frem is equivalent to libm::fmod in the "default" floating point environment, sans errno.
+    pub(crate) fn simd_rem<T>(lhs: T, rhs: T) -> T;
+
+    /// shl
+    /// for (u)ints. poison if rhs >= lhs::BITS
+    pub(crate) fn simd_shl<T>(lhs: T, rhs: T) -> T;
+
+    /// ints: ashr
+    /// uints: lshr
+    /// poison if rhs >= lhs::BITS
+    pub(crate) fn simd_shr<T>(lhs: T, rhs: T) -> T;
+
+    /// and
+    pub(crate) fn simd_and<T>(x: T, y: T) -> T;
+
+    /// or
+    pub(crate) fn simd_or<T>(x: T, y: T) -> T;
+
+    /// xor
+    pub(crate) fn simd_xor<T>(x: T, y: T) -> T;
+
+    /// getelementptr (without inbounds)
+    pub(crate) fn simd_arith_offset<T, U>(ptrs: T, offsets: U) -> T;
+
+    /// fptoui/fptosi/uitofp/sitofp
+    /// casting floats to integers is truncating, so it is safe to convert values like e.g. 1.5
+    /// but the truncated value must fit in the target type or the result is poison.
+    /// use `simd_as` instead for a cast that performs a saturating conversion.
+    pub(crate) fn simd_cast<T, U>(x: T) -> U;
+    /// follows Rust's `T as U` semantics, including saturating float casts
+    /// which amounts to the same as `simd_cast` for many cases
+    pub(crate) fn simd_as<T, U>(x: T) -> U;
+
+    /// neg/fneg
+    /// ints: ultimately becomes a call to cg_ssa's BuilderMethods::neg. cg_llvm equates this to `simd_sub(Simd::splat(0), x)`.
+    /// floats: LLVM's fneg, which changes the floating point sign bit. Some arches have instructions for it.
+    /// Rust panics for Neg::neg(int::MIN) due to overflow, but it is not UB in LLVM without `nsw`.
+    pub(crate) fn simd_neg<T>(x: T) -> T;
+
+    /// fabs
+    pub(crate) fn simd_fabs<T>(x: T) -> T;
+
+    // minnum/maxnum
+    pub(crate) fn simd_fmin<T>(x: T, y: T) -> T;
+    pub(crate) fn simd_fmax<T>(x: T, y: T) -> T;
+
+    // these return Simd<int, N> with the same BITS size as the inputs
+    pub(crate) fn simd_eq<T, U>(x: T, y: T) -> U;
+    pub(crate) fn simd_ne<T, U>(x: T, y: T) -> U;
+    pub(crate) fn simd_lt<T, U>(x: T, y: T) -> U;
+    pub(crate) fn simd_le<T, U>(x: T, y: T) -> U;
+    pub(crate) fn simd_gt<T, U>(x: T, y: T) -> U;
+    pub(crate) fn simd_ge<T, U>(x: T, y: T) -> U;
+
+    // shufflevector
+    // idx: LLVM calls it a "shuffle mask vector constant", a vector of i32s
+    pub(crate) fn simd_shuffle<T, U, V>(x: T, y: T, idx: U) -> V;
+
+    /// llvm.masked.gather
+    /// like a loop of pointer reads
+    /// val: vector of values to select if a lane is masked
+    /// ptr: vector of pointers to read from
+    /// mask: a "wide" mask of integers, selects as if simd_select(mask, read(ptr), val)
+    /// note, the LLVM intrinsic accepts a mask vector of <N x i1>
+    /// FIXME: review this if/when we fix up our mask story in general?
+    pub(crate) fn simd_gather<T, U, V>(val: T, ptr: U, mask: V) -> T;
+    /// llvm.masked.scatter
+    /// like gather, but more spicy, as it writes instead of reads
+    pub(crate) fn simd_scatter<T, U, V>(val: T, ptr: U, mask: V);
+
+    // {s,u}add.sat
+    pub(crate) fn simd_saturating_add<T>(x: T, y: T) -> T;
+
+    // {s,u}sub.sat
+    pub(crate) fn simd_saturating_sub<T>(lhs: T, rhs: T) -> T;
+
+    // reductions
+    // llvm.vector.reduce.{add,fadd}
+    pub(crate) fn simd_reduce_add_ordered<T, U>(x: T, y: U) -> U;
+    // llvm.vector.reduce.{mul,fmul}
+    pub(crate) fn simd_reduce_mul_ordered<T, U>(x: T, y: U) -> U;
+    #[allow(unused)]
+    pub(crate) fn simd_reduce_all<T>(x: T) -> bool;
+    #[allow(unused)]
+    pub(crate) fn simd_reduce_any<T>(x: T) -> bool;
+    pub(crate) fn simd_reduce_max<T, U>(x: T) -> U;
+    pub(crate) fn simd_reduce_min<T, U>(x: T) -> U;
+    pub(crate) fn simd_reduce_and<T, U>(x: T) -> U;
+    pub(crate) fn simd_reduce_or<T, U>(x: T) -> U;
+    pub(crate) fn simd_reduce_xor<T, U>(x: T) -> U;
+
+    // truncate integer vector to bitmask
+    // `fn simd_bitmask(vector) -> unsigned integer` takes a vector of integers and
+    // returns either an unsigned integer or array of `u8`.
+    // Every element in the vector becomes a single bit in the returned bitmask.
+    // If the vector has less than 8 lanes, a u8 is returned with zeroed trailing bits.
+    // The bit order of the result depends on the byte endianness. LSB-first for little
+    // endian and MSB-first for big endian.
+    //
+    // UB if called on a vector with values other than 0 and -1.
+    #[allow(unused)]
+    pub(crate) fn simd_bitmask<T, U>(x: T) -> U;
+
+    // select
+    // first argument is a vector of integers, -1 (all bits 1) is "true"
+    // logically equivalent to (yes & m) | (no & (m^-1),
+    // but you can use it on floats.
+    pub(crate) fn simd_select<M, T>(m: M, yes: T, no: T) -> T;
+    #[allow(unused)]
+    pub(crate) fn simd_select_bitmask<M, T>(m: M, yes: T, no: T) -> T;
+}