source: vbox/trunk/src/libs/liblzma-5.4.1/simple/arm64.c

///////////////////////////////////////////////////////////////////////////////
//
/// \file       arm64.c
/// \brief      Filter for ARM64 binaries
///
/// This converts ARM64 relative addresses in the BL and ADRP immediates
/// to absolute values to increase redundancy of ARM64 code.
///
/// Converting B or ADR instructions was also tested but it's not useful.
/// A majority of the jumps for the B instruction are very small (+/- 0xFF).
/// These are typical for loops and if-statements. Encoding them to their
/// absolute address reduces redundancy since many of the small relative
/// jump values are repeated, but very few of the absolute addresses are.
//
//  Authors:    Lasse Collin
//              Jia Tan
//              Igor Pavlov
//
//  This file has been put into the public domain.
//  You can do whatever you want with this file.
//
///////////////////////////////////////////////////////////////////////////////

#include "simple_private.h"


static size_t
arm64_code(void *simple lzma_attribute((__unused__)),
                uint32_t now_pos, bool is_encoder,
                uint8_t *buffer, size_t size)
{
        size_t i;

        // Clang 14.0.6 on x86-64 makes this four times bigger and 40 % slower
        // with auto-vectorization that is enabled by default with -O2.
        // Such vectorization bloat happens with -O2 when targeting ARM64 too
        // but performance hasn't been tested.
#ifdef __clang__
#       pragma clang loop vectorize(disable)
#endif
        for (i = 0; i + 4 <= size; i += 4) {
                uint32_t pc = (uint32_t)(now_pos + i);
                uint32_t instr = read32le(buffer + i);

                if ((instr >> 26) == 0x25) {
                        // BL instruction:
                        // The full 26-bit immediate is converted.
                        // The range is +/-128 MiB.
                        //
                        // Using the full range is helps quite a lot with
                        // big executables. Smaller range would reduce false
                        // positives in non-code sections of the input though
                        // so this is a compromise that slightly favors big
                        // files. With the full range only six bits of the 32
                        // need to match to trigger a conversion.
                        const uint32_t src = instr;
                        instr = 0x94000000;

                        pc >>= 2;
                        if (!is_encoder)
                                pc = 0U - pc;

                        instr |= (src + pc) & 0x03FFFFFF;
                        write32le(buffer + i, instr);

                } else if ((instr & 0x9F000000) == 0x90000000) {
                        // ADRP instruction:
                        // Only values in the range +/-512 MiB are converted.
                        //
                        // Using less than the full +/-4 GiB range reduces
                        // false positives on non-code sections of the input
                        // while being excellent for executables up to 512 MiB.
                        // The positive effect of ADRP conversion is smaller
                        // than that of BL but it also doesn't hurt so much in
                        // non-code sections of input because, with +/-512 MiB
                        // range, nine bits of 32 need to match to trigger a
                        // conversion (two 10-bit match choices = 9 bits).
                        const uint32_t src = ((instr >> 29) & 3)
                                        | ((instr >> 3) & 0x001FFFFC);

                        // With the addition only one branch is needed to
                        // check the +/- range. This is usually false when
                        // processing ARM64 code so branch prediction will
                        // handle it well in terms of performance.
                        //
                        //if ((src & 0x001E0000) != 0
                        // && (src & 0x001E0000) != 0x001E0000)
                        if ((src + 0x00020000) & 0x001C0000)
                                continue;

                        instr &= 0x9000001F;

                        pc >>= 12;
                        if (!is_encoder)
                                pc = 0U - pc;

                        const uint32_t dest = src + pc;
                        instr |= (dest & 3) << 29;
                        instr |= (dest & 0x0003FFFC) << 3;
                        instr |= (0U - (dest & 0x00020000)) & 0x00E00000;
                        write32le(buffer + i, instr);
                }
        }

        return i;
}


static lzma_ret
arm64_coder_init(lzma_next_coder *next, const lzma_allocator *allocator,
                const lzma_filter_info *filters, bool is_encoder)
{
        return lzma_simple_coder_init(next, allocator, filters,
                        &arm64_code, 0, 4, 4, is_encoder);
}


#ifdef HAVE_ENCODER_ARM64
extern lzma_ret
lzma_simple_arm64_encoder_init(lzma_next_coder *next,
                const lzma_allocator *allocator,
                const lzma_filter_info *filters)
{
        return arm64_coder_init(next, allocator, filters, true);
}
#endif


#ifdef HAVE_DECODER_ARM64
extern lzma_ret
lzma_simple_arm64_decoder_init(lzma_next_coder *next,
                const lzma_allocator *allocator,
                const lzma_filter_info *filters)
{
        return arm64_coder_init(next, allocator, filters, false);
}
#endif