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// generated source. do not edit.
#![allow(non_upper_case_globals, unused_macros, unused_imports)]
use crate::low::macros::*;
// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0 OR ISC OR MIT-0
// ----------------------------------------------------------------------------
// Return size of bignum in bits
// Input x[k]; output function return
//
// extern uint64_t bignum_bitsize(uint64_t k, const uint64_t *x);
//
// In the case of a zero bignum as input the result is 0
//
// In principle this has a precondition k < 2^58, but obviously that
// is always true in practice because of address space limitations.
//
// Standard ARM ABI: X0 = k, X1 = x, returns X0
// ----------------------------------------------------------------------------
macro_rules! k {
() => {
"x0"
};
}
macro_rules! x {
() => {
"x1"
};
}
macro_rules! i {
() => {
"x2"
};
}
macro_rules! w {
() => {
"x3"
};
}
macro_rules! a {
() => {
"x4"
};
}
macro_rules! j {
() => {
"x5"
};
}
/// Return size of bignum in bits
///
/// Input x[k]; output function return
///
/// In the case of a zero bignum as input the result is 0
///
/// In principle this has a precondition k < 2^58, but obviously that
/// is always true in practice because of address space limitations.
pub(crate) fn bignum_bitsize(x: &[u64]) -> usize {
let ret: u64;
// SAFETY: inline assembly. see [crate::low::inline_assembly_safety] for safety info.
unsafe {
core::arch::asm!(
// If the bignum is zero-length, x0 is already the right answer of 0
Q!(" cbz " k!() ", " Label!("bignum_bitsize_end", 2, After)),
// Use w = a[i-1] to store nonzero words in a bottom-up sweep
// Set the initial default to be as if we had a 11...11 word directly below
Q!(" mov " i!() ", xzr"),
Q!(" mov " w!() ", #-1"),
Q!(" mov " j!() ", xzr"),
Q!(Label!("bignum_bitsize_loop", 3) ":"),
Q!(" ldr " a!() ", [" x!() ", " j!() ", lsl #3]"),
Q!(" add " j!() ", " j!() ", #1"),
Q!(" cmp " a!() ", #0"),
Q!(" csel " i!() ", " j!() ", " i!() ", ne"),
Q!(" csel " w!() ", " a!() ", " w!() ", ne"),
Q!(" cmp " j!() ", " k!()),
Q!(" bne " Label!("bignum_bitsize_loop", 3, Before)),
// Now w = a[i-1] is the highest nonzero word, or in the zero case the
// default of the "extra" 11...11 = a[0-1]. We now want 64* i - clz(w).
// Note that this code does not rely on the behavior of the clz instruction
// for zero inputs, though the ARM manual does in fact guarantee clz(0) = 64.
Q!(" lsl " i!() ", " i!() ", #6"),
Q!(" clz " a!() ", " w!()),
Q!(" sub " "x0, " i!() ", " a!()),
Q!(Label!("bignum_bitsize_end", 2) ":"),
inout("x0") x.len() => ret,
inout("x1") x.as_ptr() => _,
// clobbers
out("x2") _,
out("x3") _,
out("x4") _,
out("x5") _,
)
};
ret as usize
}