Rappel.py is a pretty janky assembly REPL. It works by using keystone for R, and GDB for EPL.

rappel.py

""" Assembly REPL in gdb / possible sketchy binary patcher.

    Usage: 
        gdb -q ./target
            -x rappel.py    adds 'rappel' command
            [-write]        patches binary on disk, sometimes!
"""
import gdb, tempfile, keystone as ks

class Rappel(gdb.Command):
    """Assemble a patch at $pc and step over it (interactive assembly repl)

    Extra semicolons will do extra steps. Endianness may require edits.

    Examples on x86:
    
        rappel rdrand eax
        print/x $eax

        rappel inc rax; push rax; pop rbx; cmp rax, rbx
        info reg

        rappel pcmpistri xmm0, xmm1, 5
        info all-registers
    """
    def invoke(self, arg:str, _from_tty:bool):
        if not arg:
            return print('No assembly instructions passed. Try "help rappel".')

        if 'ks' not in self.__dict__:
            # endian/architecture are usually set to 'auto', hard to query :(
            arch = gdb.selected_frame().architecture().name()
            if arch not in TESTED_MAP:
                print('Could not guess options for {0}, set manually!' % arch)
                print('Tested architectures: ' + ', '.join(TESTED_MAP.keys()))
                return
            self.ks = ks.Ks(*TESTED_MAP[arch])

        text, isn_count = self.ks.asm(arg)

        with tempfile.NamedTemporaryFile() as hnd:
            hnd.write(bytes(text))
            hnd.flush()
            gdb.execute('restore %s binary $pc' % hnd.name)
        gdb.execute('disassemble/r $pc, +%d' % len(text))

        [gdb.execute('stepi') for _ in range(isn_count)]

    def __init__(self):
        super(Rappel, self).__init__('rappel', gdb.COMMAND_RUNNING)

Rappel()

KSTOOL = {  
    # Scraped from kstool.cpp, v0.9.1 + armv8/thumbv8* addition
    # Covers most possible settings, MIPS ISA variants being the exception
    # Run `kstool` to check for new additions/see descriptions
    "x16":  [ks.KS_ARCH_X86, ks.KS_MODE_16],
    "x32":  [ks.KS_ARCH_X86, ks.KS_MODE_32],
    "x64":  [ks.KS_ARCH_X86, ks.KS_MODE_64],
    "x16att":   [ks.KS_ARCH_X86, ks.KS_MODE_16],
    "x32att":   [ks.KS_ARCH_X86, ks.KS_MODE_32],
    "x64att":   [ks.KS_ARCH_X86, ks.KS_MODE_64],
    "x16nasm":  [ks.KS_ARCH_X86, ks.KS_MODE_16],
    "x32nasm":  [ks.KS_ARCH_X86, ks.KS_MODE_32],
    "x64nasm":  [ks.KS_ARCH_X86, ks.KS_MODE_64],
    "arm":  [ks.KS_ARCH_ARM, ks.KS_MODE_ARM+ks.KS_MODE_LITTLE_ENDIAN],
    "armbe":    [ks.KS_ARCH_ARM, ks.KS_MODE_ARM+ks.KS_MODE_BIG_ENDIAN],
    "thumb":    [ks.KS_ARCH_ARM, ks.KS_MODE_THUMB+ks.KS_MODE_LITTLE_ENDIAN],
    "thumbbe":  [ks.KS_ARCH_ARM, ks.KS_MODE_THUMB+ks.KS_MODE_BIG_ENDIAN],
    "armv8":    [ks.KS_ARCH_ARM, ks.KS_MODE_ARM+ks.KS_MODE_LITTLE_ENDIAN+ks.KS_MODE_V8],
    "armv8be":  [ks.KS_ARCH_ARM, ks.KS_MODE_ARM+ks.KS_MODE_BIG_ENDIAN+ks.KS_MODE_V8],
    "thumbv8":  [ks.KS_ARCH_ARM, ks.KS_MODE_THUMB+ks.KS_MODE_LITTLE_ENDIAN+ks.KS_MODE_V8],
    "thumbv8be":    [ks.KS_ARCH_ARM, ks.KS_MODE_THUMB+ks.KS_MODE_BIG_ENDIAN+ks.KS_MODE_V8],
    "arm64":    [ks.KS_ARCH_ARM64, ks.KS_MODE_LITTLE_ENDIAN],
    "hex": [ks.KS_ARCH_HEXAGON, ks.KS_MODE_BIG_ENDIAN],
    "mips": [ks.KS_ARCH_MIPS, ks.KS_MODE_MIPS32+ks.KS_MODE_LITTLE_ENDIAN],
    "mipsbe":   [ks.KS_ARCH_MIPS, ks.KS_MODE_MIPS32+ks.KS_MODE_BIG_ENDIAN],
    "mips64":   [ks.KS_ARCH_MIPS, ks.KS_MODE_MIPS64+ks.KS_MODE_LITTLE_ENDIAN],
    "mips64be": [ks.KS_ARCH_MIPS, ks.KS_MODE_MIPS64+ks.KS_MODE_BIG_ENDIAN],
    "ppc32be":  [ks.KS_ARCH_PPC, ks.KS_MODE_PPC32+ks.KS_MODE_BIG_ENDIAN],
    "ppc64":    [ks.KS_ARCH_PPC, ks.KS_MODE_PPC64+ks.KS_MODE_LITTLE_ENDIAN],
    "ppc64be":  [ks.KS_ARCH_PPC, ks.KS_MODE_PPC64+ks.KS_MODE_BIG_ENDIAN],
    "sparc":    [ks.KS_ARCH_SPARC, ks.KS_MODE_SPARC32+ks.KS_MODE_LITTLE_ENDIAN],
    "sparcbe":  [ks.KS_ARCH_SPARC, ks.KS_MODE_SPARC32+ks.KS_MODE_BIG_ENDIAN],
    "sparc64":  [ks.KS_ARCH_SPARC, ks.KS_MODE_SPARC64+ks.KS_MODE_LITTLE_ENDIAN],
    "sparc64be":    [ks.KS_ARCH_SPARC, ks.KS_MODE_SPARC64+ks.KS_MODE_BIG_ENDIAN],
    "systemz": [ks.KS_ARCH_SYSTEMZ, ks.KS_MODE_BIG_ENDIAN],
}

# Needs testing: armv5, armv5t, aarch64, powerpc:common, powerpc:common64,
# mips:isa32, mips:isa64, ...
TESTED_MAP = {  
    # gdb          keystone-engine
    'i386:x86-32': KSTOOL['x32'],
    'i386:x86-64': KSTOOL['x64'],
    # ^            ^
    # |            +- Start by checking if kstool has something close
    # |               Full options in include/keystone/keystone.h
    # +- Type "set architecture <TAB>" in gdb to see built options
    #    Check gdb-multiarch on Ubuntu/Debian for a long list
}