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[PATCH v6 02/35] Hexagon (target/hexagon) README

From: Taylor Simpson
Subject: [PATCH v6 02/35] Hexagon (target/hexagon) README
Date: Thu, 7 Jan 2021 22:28:33 -0600

Gives an introduction and overview to the Hexagon target

Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
 target/hexagon/README | 235 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 235 insertions(+)
 create mode 100644 target/hexagon/README

diff --git a/target/hexagon/README b/target/hexagon/README
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+Hexagon is Qualcomm's very long instruction word (VLIW) digital signal
+The following versions of the Hexagon core are supported
+    Scalar core: v67
+We presented an overview of the project at the 2019 KVM Forum.
+*** 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
+        gen_op_regs.py                  -> op_regs_generated.h
+        gen_printinsn.py                -> printinsn_generated.h
+        gen_op_attribs.py               -> op_attribs_generated.h
+        gen_helper_protos.py            -> helper_protos_generated.h
+        gen_shortcode.py                -> shortcode_generated.h
+        gen_tcg_funcs.py                -> tcg_funcs_generated.h
+        gen_tcg_func_table.py           -> tcg_func_table_generated.h
+        gen_helper_funcs.py             -> helper_funcs_generated.h
+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
+    DEF_HELPER_3(A2_add, s32, env, s32, s32)
+    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);
+    }
+    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
+    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) \
+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
+*** Key Files ***
+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.
+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.
+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
+These files create a function for each instruction.  It is mostly composed of
+fGEN_TCG_<tag> definitions followed by including tcg_funcs_generated.h.
+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.h.  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
+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
+    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

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