1 files changed, 149 insertions, 0 deletions
diff --git a/vendor/compiler_builtins/src/int/leading_zeros.rs b/vendor/compiler_builtins/src/int/leading_zeros.rs
new file mode 100644
index 000000000..9e60ab0d7
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+++ b/vendor/compiler_builtins/src/int/leading_zeros.rs
@@ -0,0 +1,149 @@
+// Note: these functions happen to produce the correct `usize::leading_zeros(0)` value
+// without a explicit zero check. Zero is probably common enough that it could warrant
+// adding a zero check at the beginning, but `__clzsi2` has a precondition that `x != 0`.
+// Compilers will insert the check for zero in cases where it is needed.
+
+public_test_dep! {
+/// Returns the number of leading binary zeros in `x`.
+#[allow(dead_code)]
+pub(crate) fn usize_leading_zeros_default(x: usize) -> usize {
+    // The basic idea is to test if the higher bits of `x` are zero and bisect the number
+    // of leading zeros. It is possible for all branches of the bisection to use the same
+    // code path by conditionally shifting the higher parts down to let the next bisection
+    // step work on the higher or lower parts of `x`. Instead of starting with `z == 0`
+    // and adding to the number of zeros, it is slightly faster to start with
+    // `z == usize::MAX.count_ones()` and subtract from the potential number of zeros,
+    // because it simplifies the final bisection step.
+    let mut x = x;
+    // the number of potential leading zeros
+    let mut z = usize::MAX.count_ones() as usize;
+    // a temporary
+    let mut t: usize;
+    #[cfg(target_pointer_width = "64")]
+    {
+        t = x >> 32;
+        if t != 0 {
+            z -= 32;
+            x = t;
+        }
+    }
+    #[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
+    {
+        t = x >> 16;
+        if t != 0 {
+            z -= 16;
+            x = t;
+        }
+    }
+    t = x >> 8;
+    if t != 0 {
+        z -= 8;
+        x = t;
+    }
+    t = x >> 4;
+    if t != 0 {
+        z -= 4;
+        x = t;
+    }
+    t = x >> 2;
+    if t != 0 {
+        z -= 2;
+        x = t;
+    }
+    // the last two bisections are combined into one conditional
+    t = x >> 1;
+    if t != 0 {
+        z - 2
+    } else {
+        z - x
+    }
+
+    // We could potentially save a few cycles by using the LUT trick from
+    // "https://embeddedgurus.com/state-space/2014/09/
+    // fast-deterministic-and-portable-counting-leading-zeros/".
+    // However, 256 bytes for a LUT is too large for embedded use cases. We could remove
+    // the last 3 bisections  and use this 16 byte LUT for the rest of the work:
+    //const LUT: [u8; 16] = [0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4];
+    //z -= LUT[x] as usize;
+    //z
+    // However, it ends up generating about the same number of instructions. When benchmarked
+    // on x86_64, it is slightly faster to use the LUT, but this is probably because of OOO
+    // execution effects. Changing to using a LUT and branching is risky for smaller cores.
+}
+}
+
+// The above method does not compile well on RISC-V (because of the lack of predicated
+// instructions), producing code with many branches or using an excessively long
+// branchless solution. This method takes advantage of the set-if-less-than instruction on
+// RISC-V that allows `(x >= power-of-two) as usize` to be branchless.
+
+public_test_dep! {
+/// Returns the number of leading binary zeros in `x`.
+#[allow(dead_code)]
+pub(crate) fn usize_leading_zeros_riscv(x: usize) -> usize {
+    let mut x = x;
+    // the number of potential leading zeros
+    let mut z = usize::MAX.count_ones() as usize;
+    // a temporary
+    let mut t: usize;
+
+    // RISC-V does not have a set-if-greater-than-or-equal instruction and
+    // `(x >= power-of-two) as usize` will get compiled into two instructions, but this is
+    // still the most optimal method. A conditional set can only be turned into a single
+    // immediate instruction if `x` is compared with an immediate `imm` (that can fit into
+    // 12 bits) like `x < imm` but not `imm < x` (because the immediate is always on the
+    // right). If we try to save an instruction by using `x < imm` for each bisection, we
+    // have to shift `x` left and compare with powers of two approaching `usize::MAX + 1`,
+    // but the immediate will never fit into 12 bits and never save an instruction.
+    #[cfg(target_pointer_width = "64")]
+    {
+        // If the upper 32 bits of `x` are not all 0, `t` is set to `1 << 5`, otherwise
+        // `t` is set to 0.
+        t = ((x >= (1 << 32)) as usize) << 5;
+        // If `t` was set to `1 << 5`, then the upper 32 bits are shifted down for the
+        // next step to process.
+        x >>= t;
+        // If `t` was set to `1 << 5`, then we subtract 32 from the number of potential
+        // leading zeros
+        z -= t;
+    }
+    #[cfg(any(target_pointer_width = "32", target_pointer_width = "64"))]
+    {
+        t = ((x >= (1 << 16)) as usize) << 4;
+        x >>= t;
+        z -= t;
+    }
+    t = ((x >= (1 << 8)) as usize) << 3;
+    x >>= t;
+    z -= t;
+    t = ((x >= (1 << 4)) as usize) << 2;
+    x >>= t;
+    z -= t;
+    t = ((x >= (1 << 2)) as usize) << 1;
+    x >>= t;
+    z -= t;
+    t = (x >= (1 << 1)) as usize;
+    x >>= t;
+    z -= t;
+    // All bits except the LSB are guaranteed to be zero for this final bisection step.
+    // If `x != 0` then `x == 1` and subtracts one potential zero from `z`.
+    z - x
+}
+}
+
+intrinsics! {
+    #[maybe_use_optimized_c_shim]
+    #[cfg(any(
+        target_pointer_width = "16",
+        target_pointer_width = "32",
+        target_pointer_width = "64"
+    ))]
+    /// Returns the number of leading binary zeros in `x`.
+    pub extern "C" fn __clzsi2(x: usize) -> usize {
+        if cfg!(any(target_arch = "riscv32", target_arch = "riscv64")) {
+            usize_leading_zeros_riscv(x)
+        } else {
+            usize_leading_zeros_default(x)
+        }
+    }
+}