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// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
// ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
// ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
// OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
// ----------------------------------------------------------------------------
// Count leading zero bits in a single word
// Input a; output function return
//
// extern uint64_t word_clz (uint64_t a);
//
// Standard x86-64 ABI: RDI = a, returns RAX
// Microsoft x64 ABI: RCX = a, returns RAX
// ----------------------------------------------------------------------------
#include "s2n_bignum_internal.h"
.intel_syntax noprefix
S2N_BN_SYM_VISIBILITY_DIRECTIVE(word_clz)
S2N_BN_SYM_PRIVACY_DIRECTIVE(word_clz)
.text
S2N_BN_SYMBOL(word_clz):
endbr64
#if WINDOWS_ABI
push rdi
push rsi
mov rdi, rcx
#endif
// First do rax = 63 - bsr(a), which is right except (maybe) for zero inputs
bsr rax, rdi
xor rax, 63
// Force return of 64 in the zero-input case
mov edx, 64
test rdi, rdi
cmove rax, rdx
#if WINDOWS_ABI
pop rsi
pop rdi
#endif
ret
#if defined(__linux__) && defined(__ELF__)
.section .note.GNU-stack,"",%progbits
#endif
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