Hexagon is Qualcomm's very long instruction word (VLIW) digital signal
processor(DSP).

The following versions of the Hexagon core are supported
    Scalar core: v67
    https://developer.qualcomm.com/downloads/qualcomm-hexagon-v67-programmer-s-reference-manual

We presented an overview of the project at the 2019 KVM Forum.
    https://kvmforum2019.sched.com/event/Tmwc/qemu-hexagon-automatic-translation-of-the-isa-manual-pseudcode-to-tiny-code-instructions-of-a-vliw-architecture-niccolo-izzo-revng-taylor-simpson-qualcomm-innovation-center

*** Tour of the code ***

The qemu-hexagon implementation is a combination of qemu and the Hexagon
architecture library (aka archlib).  The three primary directories with
Hexagon-specific code are

    qemu/target/hexagon
        This has all the instruction and packet semantics
    qemu/target/hexagon/imported
        These files are imported with very little modification from archlib
        *.idef                  Instruction semantics definition
        macros.def              Mapping of macros to instruction attributes
        encode*.def             Encoding patterns for each instruction
        iclass.def              Instruction class definitions used to determine
                                legal VLIW slots for each instruction
    qemu/linux-user/hexagon
        Helpers for loading the ELF file and making Linux system calls,
        signals, etc

We start with scripts that generate a bunch of include files.  This
is a two step process.  The first step is to use the C preprocessor to expand
macros inside the architecture definition files.  This is done in
target/hexagon/gen_semantics.c.  This step produces
    <BUILD_DIR>/target/hexagon/semantics_generated.pyinc.
That file is consumed by the following python scripts to produce the indicated
header files in <BUILD_DIR>/target/hexagon
        gen_opcodes_def.py              -> opcodes_def_generated.h.inc
        gen_op_regs.py                  -> op_regs_generated.h.inc
        gen_printinsn.py                -> printinsn_generated.h.inc
        gen_op_attribs.py               -> op_attribs_generated.h.inc
        gen_helper_protos.py            -> helper_protos_generated.h.inc
        gen_shortcode.py                -> shortcode_generated.h.inc
        gen_tcg_funcs.py                -> tcg_funcs_generated.c.inc
        gen_tcg_func_table.py           -> tcg_func_table_generated.c.inc
        gen_helper_funcs.py             -> helper_funcs_generated.c.inc

Qemu helper functions have 3 parts
    DEF_HELPER declaration indicates the signature of the helper
    gen_helper_<NAME> will generate a TCG call to the helper function
    The helper implementation

Here's an example of the A2_add instruction.
    Instruction tag        A2_add
    Assembly syntax        "Rd32=add(Rs32,Rt32)"
    Instruction semantics  "{ RdV=RsV+RtV;}"

By convention, the operands are identified by letter
    RdV is the destination register
    RsV, RtV are source registers

The generator uses the operand naming conventions (see large comment in
hex_common.py) to determine the signature of the helper function.  Here are the
results for A2_add

helper_protos_generated.h.inc
    DEF_HELPER_3(A2_add, s32, env, s32, s32)

tcg_funcs_generated.c.inc
    static void generate_A2_add(
                    CPUHexagonState *env,
                    DisasContext *ctx,
                    Insn *insn,
                    Packet *pkt)
    {
        TCGv RdV = tcg_temp_local_new();
        const int RdN = insn->regno[0];
        TCGv RsV = hex_gpr[insn->regno[1]];
        TCGv RtV = hex_gpr[insn->regno[2]];
        gen_helper_A2_add(RdV, cpu_env, RsV, RtV);
        gen_log_reg_write(RdN, RdV);
        ctx_log_reg_write(ctx, RdN);
        tcg_temp_free(RdV);
    }

helper_funcs_generated.c.inc
    int32_t HELPER(A2_add)(CPUHexagonState *env, int32_t RsV, int32_t RtV)
    {
        uint32_t slot __attribute__((unused)) = 4;
        int32_t RdV = 0;
        { RdV=RsV+RtV;}
        return RdV;
    }

Note that generate_A2_add updates the disassembly context to be processed
when the packet commits (see "Packet Semantics" below).

The generator checks for fGEN_TCG_<tag> macro.  This allows us to generate
TCG code instead of a call to the helper.  If defined, the macro takes 1
argument.
    C semantics (aka short code)

This allows the code generator to override the auto-generated code.  In some
cases this is necessary for correct execution.  We can also override for
faster emulation.  For example, calling a helper for add is more expensive
than generating a TCG add operation.

The gen_tcg.h file has any overrides. For example, we could write
    #define fGEN_TCG_A2_add(GENHLPR, SHORTCODE) \
        tcg_gen_add_tl(RdV, RsV, RtV)

The instruction semantics C code relies heavily on macros.  In cases where the
C semantics are specified only with macros, we can override the default with
the short semantics option and #define the macros to generate TCG code.  One
example is L2_loadw_locked:
    Instruction tag        L2_loadw_locked
    Assembly syntax        "Rd32=memw_locked(Rs32)"
    Instruction semantics  "{ fEA_REG(RsV); fLOAD_LOCKED(1,4,u,EA,RdV) }"

In gen_tcg.h, we use the shortcode
#define fGEN_TCG_L2_loadw_locked(SHORTCODE) \
    SHORTCODE

There are also cases where we brute force the TCG code generation.
Instructions with multiple definitions are examples.  These require special
handling because qemu helpers can only return a single value.

In addition to instruction semantics, we use a generator to create the decode
tree.  This generation is also a two step process.  The first step is to run
target/hexagon/gen_dectree_import.c to produce
    <BUILD_DIR>/target/hexagon/iset.py
This file is imported by target/hexagon/dectree.py to produce
    <BUILD_DIR>/target/hexagon/dectree_generated.h.inc

*** Key Files ***

cpu.h

This file contains the definition of the CPUHexagonState struct.  It is the
runtime information for each thread and contains stuff like the GPR and
predicate registers.

macros.h

The Hexagon arch lib relies heavily on macros for the instruction semantics.
This is a great advantage for qemu because we can override them for different
purposes.  You will also notice there are sometimes two definitions of a macro.
The QEMU_GENERATE variable determines whether we want the macro to generate TCG
code.  If QEMU_GENERATE is not defined, we want the macro to generate vanilla
C code that will work in the helper implementation.

translate.c

The functions in this file generate TCG code for a translation block.  Some
important functions in this file are

    gen_start_packet - initialize the data structures for packet semantics
    gen_commit_packet - commit the register writes, stores, etc for a packet
    decode_and_translate_packet - disassemble a packet and generate code

genptr.c
gen_tcg.h

These files create a function for each instruction.  It is mostly composed of
fGEN_TCG_<tag> definitions followed by including tcg_funcs_generated.c.inc.

op_helper.c

This file contains the implementations of all the helpers.  There are a few
general purpose helpers, but most of them are generated by including
helper_funcs_generated.c.inc.  There are also several helpers used for debugging.


*** Packet Semantics ***

VLIW packet semantics differ from serial semantics in that all input operands
are read, then the operations are performed, then all the results are written.
For exmaple, this packet performs a swap of registers r0 and r1
    { r0 = r1; r1 = r0 }
Note that the result is different if the instructions are executed serially.

Packet semantics dictate that we defer any changes of state until the entire
packet is committed.  We record the results of each instruction in a side data
structure, and update the visible processor state when we commit the packet.

The data structures are divided between the runtime state and the translation
context.

During the TCG generation (see translate.[ch]), we use the DisasContext to
track what needs to be done during packet commit.  Here are the relevant
fields

    reg_log            list of registers written
    reg_log_idx        index into ctx_reg_log
    pred_log           list of predicates written
    pred_log_idx       index into ctx_pred_log
    store_width        width of stores (indexed by slot)

During runtime, the following fields in CPUHexagonState (see cpu.h) are used

    new_value             new value of a given register
    reg_written           boolean indicating if register was written
    new_pred_value        new value of a predicate register
    pred_written          boolean indicating if predicate was written
    mem_log_stores        record of the stores (indexed by slot)

*** Debugging ***

You can turn on a lot of debugging by changing the HEX_DEBUG macro to 1 in
internal.h.  This will stream a lot of information as it generates TCG and
executes the code.

To track down nasty issues with Hexagon->TCG generation, we compare the
execution results with actual hardware running on a Hexagon Linux target.
Run qemu with the "-d cpu" option.  Then, we can diff the results and figure
out where qemu and hardware behave differently.

The stacks are located at different locations.  We handle this by changing
env->stack_adjust in translate.c.  First, set this to zero and run qemu.
Then, change env->stack_adjust to the difference between the two stack
locations.  Then rebuild qemu and run again. That will produce a very
clean diff.

Here are some handy places to set breakpoints

    At the call to gen_start_packet for a given PC (note that the line number
        might change in the future)
        br translate.c:602 if ctx->base.pc_next == 0xdeadbeef
    The helper function for each instruction is named helper_<TAG>, so here's
        an example that will set a breakpoint at the start
        br helper_A2_add
    If you have the HEX_DEBUG macro set, the following will be useful
        At the start of execution of a packet for a given PC
            br helper_debug_start_packet if env->gpr[41] == 0xdeadbeef
        At the end of execution of a packet for a given PC
            br helper_debug_commit_end if env->this_PC == 0xdeadbeef