target/hexagon/README

   1 Hexagon is Qualcomm's very long instruction word (VLIW) digital signal
   2 processor(DSP).  We also support Hexagon Vector eXtensions (HVX).  HVX
   3 is a wide vector coprocessor designed for high performance computer vision,
   4 image processing, machine learning, and other workloads.
   5
   6 The following versions of the Hexagon core are supported
   7     Scalar core: v67
   8     https://developer.qualcomm.com/downloads/qualcomm-hexagon-v67-programmer-s-reference-manual
   9     HVX extension: v66
  10     https://developer.qualcomm.com/downloads/qualcomm-hexagon-v66-hvx-programmer-s-reference-manual
  11
  12 We presented an overview of the project at the 2019 KVM Forum.
  13     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
  14
  15 *** Tour of the code ***
  16
  17 The qemu-hexagon implementation is a combination of qemu and the Hexagon
  18 architecture library (aka archlib).  The three primary directories with
  19 Hexagon-specific code are
  20
  21     qemu/target/hexagon
  22         This has all the instruction and packet semantics
  23     qemu/target/hexagon/imported
  24         These files are imported with very little modification from archlib
  25         *.idef                  Instruction semantics definition
  26         macros.def              Mapping of macros to instruction attributes
  27         encode*.def             Encoding patterns for each instruction
  28         iclass.def              Instruction class definitions used to determine
  29                                 legal VLIW slots for each instruction
  30     qemu/target/hexagon/idef-parser
  31         Parser that, given the high-level definitions of an instruction,
  32         produces a C function generating equivalent tiny code instructions.
  33         See README.rst.
  34     qemu/linux-user/hexagon
  35         Helpers for loading the ELF file and making Linux system calls,
  36         signals, etc
  37
  38 We start with scripts that generate a bunch of include files.  This
  39 is a two step process.  The first step is to use the C preprocessor to expand
  40 macros inside the architecture definition files.  This is done in
  41 target/hexagon/gen_semantics.c.  This step produces
  42     <BUILD_DIR>/target/hexagon/semantics_generated.pyinc.
  43 That file is consumed by the following python scripts to produce the indicated
  44 header files in <BUILD_DIR>/target/hexagon
  45         gen_opcodes_def.py              -> opcodes_def_generated.h.inc
  46         gen_op_regs.py                  -> op_regs_generated.h.inc
  47         gen_printinsn.py                -> printinsn_generated.h.inc
  48         gen_op_attribs.py               -> op_attribs_generated.h.inc
  49         gen_helper_protos.py            -> helper_protos_generated.h.inc
  50         gen_shortcode.py                -> shortcode_generated.h.inc
  51         gen_tcg_funcs.py                -> tcg_funcs_generated.c.inc
  52         gen_tcg_func_table.py           -> tcg_func_table_generated.c.inc
  53         gen_helper_funcs.py             -> helper_funcs_generated.c.inc
  54         gen_idef_parser_funcs.py        -> idef_parser_input.h
  55         gen_analyze_funcs.py            -> analyze_funcs_generated.c.inc
  56
  57 Qemu helper functions have 3 parts
  58     DEF_HELPER declaration indicates the signature of the helper
  59     gen_helper_<NAME> will generate a TCG call to the helper function
  60     The helper implementation
  61
  62 Here's an example of the A2_add instruction.
  63     Instruction tag        A2_add
  64     Assembly syntax        "Rd32=add(Rs32,Rt32)"
  65     Instruction semantics  "{ RdV=RsV+RtV;}"
  66
  67 By convention, the operands are identified by letter
  68     RdV is the destination register
  69     RsV, RtV are source registers
  70
  71 The generator uses the operand naming conventions (see large comment in
  72 hex_common.py) to determine the signature of the helper function.  Here are the
  73 results for A2_add
  74
  75 helper_protos_generated.h.inc
  76     DEF_HELPER_3(A2_add, s32, env, s32, s32)
  77
  78 tcg_funcs_generated.c.inc
  79     static void generate_A2_add(
  80                     CPUHexagonState *env,
  81                     DisasContext *ctx,
  82                     Insn *insn,
  83                     Packet *pkt)
  84     {
  85         TCGv RdV = tcg_temp_new();
  86         const int RdN = insn->regno[0];
  87         TCGv RsV = hex_gpr[insn->regno[1]];
  88         TCGv RtV = hex_gpr[insn->regno[2]];
  89         gen_helper_A2_add(RdV, cpu_env, RsV, RtV);
  90         gen_log_reg_write(RdN, RdV);
  91     }
  92
  93 helper_funcs_generated.c.inc
  94     int32_t HELPER(A2_add)(CPUHexagonState *env, int32_t RsV, int32_t RtV)
  95     {
  96         uint32_t slot __attribute__((unused)) = 4;
  97         int32_t RdV = 0;
  98         { RdV=RsV+RtV;}
  99         return RdV;
 100     }
 101
 102 Note that generate_A2_add updates the disassembly context to be processed
 103 when the packet commits (see "Packet Semantics" below).
 104
 105 The generator checks for fGEN_TCG_<tag> macro.  This allows us to generate
 106 TCG code instead of a call to the helper.  If defined, the macro takes 1
 107 argument.
 108     C semantics (aka short code)
 109
 110 This allows the code generator to override the auto-generated code.  In some
 111 cases this is necessary for correct execution.  We can also override for
 112 faster emulation.  For example, calling a helper for add is more expensive
 113 than generating a TCG add operation.
 114
 115 The gen_tcg.h file has any overrides. For example, we could write
 116     #define fGEN_TCG_A2_add(GENHLPR, SHORTCODE) \
 117         tcg_gen_add_tl(RdV, RsV, RtV)
 118
 119 The instruction semantics C code relies heavily on macros.  In cases where the
 120 C semantics are specified only with macros, we can override the default with
 121 the short semantics option and #define the macros to generate TCG code.  One
 122 example is L2_loadw_locked:
 123     Instruction tag        L2_loadw_locked
 124     Assembly syntax        "Rd32=memw_locked(Rs32)"
 125     Instruction semantics  "{ fEA_REG(RsV); fLOAD_LOCKED(1,4,u,EA,RdV) }"
 126
 127 In gen_tcg.h, we use the shortcode
 128 #define fGEN_TCG_L2_loadw_locked(SHORTCODE) \
 129     SHORTCODE
 130
 131 There are also cases where we brute force the TCG code generation.
 132 Instructions with multiple definitions are examples.  These require special
 133 handling because qemu helpers can only return a single value.
 134
 135 For HVX vectors, the generator behaves slightly differently.  The wide vectors
 136 won't fit in a TCGv or TCGv_i64, so we pass TCGv_ptr variables to pass the
 137 address to helper functions.  Here's an example for an HVX vector-add-word
 138 istruction.
 139     static void generate_V6_vaddw(DisasContext *ctx)
 140     {
 141         Insn *insn __attribute__((unused)) = ctx->insn;
 142         const int VdN = insn->regno[0];
 143         const intptr_t VdV_off =
 144             ctx_future_vreg_off(ctx, VdN, 1, true);
 145         TCGv_ptr VdV = tcg_temp_new_ptr();
 146         tcg_gen_addi_ptr(VdV, cpu_env, VdV_off);
 147         const int VuN = insn->regno[1];
 148         const intptr_t VuV_off =
 149             vreg_src_off(ctx, VuN);
 150         TCGv_ptr VuV = tcg_temp_new_ptr();
 151         const int VvN = insn->regno[2];
 152         const intptr_t VvV_off =
 153             vreg_src_off(ctx, VvN);
 154         TCGv_ptr VvV = tcg_temp_new_ptr();
 155         tcg_gen_addi_ptr(VuV, cpu_env, VuV_off);
 156         tcg_gen_addi_ptr(VvV, cpu_env, VvV_off);
 157         gen_helper_V6_vaddw(cpu_env, VdV, VuV, VvV);
 158     }
 159
 160 Notice that we also generate a variable named <operand>_off for each operand of
 161 the instruction.  This makes it easy to override the instruction semantics with
 162 functions from tcg-op-gvec.h.  Here's the override for this instruction.
 163     #define fGEN_TCG_V6_vaddw(SHORTCODE) \
 164         tcg_gen_gvec_add(MO_32, VdV_off, VuV_off, VvV_off, \
 165                          sizeof(MMVector), sizeof(MMVector))
 166
 167 Finally, we notice that the override doesn't use the TCGv_ptr variables, so
 168 we don't generate them when an override is present.  Here is what we generate
 169 when the override is present.
 170     static void generate_V6_vaddw(DisasContext *ctx)
 171     {
 172         Insn *insn __attribute__((unused)) = ctx->insn;
 173         const int VdN = insn->regno[0];
 174         const intptr_t VdV_off =
 175             ctx_future_vreg_off(ctx, VdN, 1, true);
 176         const int VuN = insn->regno[1];
 177         const intptr_t VuV_off =
 178             vreg_src_off(ctx, VuN);
 179         const int VvN = insn->regno[2];
 180         const intptr_t VvV_off =
 181             vreg_src_off(ctx, VvN);
 182         fGEN_TCG_V6_vaddw({ fHIDE(int i;) fVFOREACH(32, i) { VdV.w[i] = VuV.w[i] + VvV.w[i] ; } });
 183     }
 184
 185 We also generate an analyze_<tag> function for each instruction.  Currently,
 186 these functions record the writes to registers by calling ctx_log_*.  During
 187 gen_start_packet, we invoke the analyze_<tag> function for each instruction in
 188 the packet, and we mark the implicit writes.  After the analysis is performed,
 189 we initialize hex_new_value for each of the predicated assignments.
 190
 191 In addition to instruction semantics, we use a generator to create the decode
 192 tree.  This generation is also a two step process.  The first step is to run
 193 target/hexagon/gen_dectree_import.c to produce
 194     <BUILD_DIR>/target/hexagon/iset.py
 195 This file is imported by target/hexagon/dectree.py to produce
 196     <BUILD_DIR>/target/hexagon/dectree_generated.h.inc
 197
 198 *** Key Files ***
 199
 200 cpu.h
 201
 202 This file contains the definition of the CPUHexagonState struct.  It is the
 203 runtime information for each thread and contains stuff like the GPR and
 204 predicate registers.
 205
 206 macros.h
 207 mmvec/macros.h
 208
 209 The Hexagon arch lib relies heavily on macros for the instruction semantics.
 210 This is a great advantage for qemu because we can override them for different
 211 purposes.  You will also notice there are sometimes two definitions of a macro.
 212 The QEMU_GENERATE variable determines whether we want the macro to generate TCG
 213 code.  If QEMU_GENERATE is not defined, we want the macro to generate vanilla
 214 C code that will work in the helper implementation.
 215
 216 translate.c
 217
 218 The functions in this file generate TCG code for a translation block.  Some
 219 important functions in this file are
 220
 221     gen_start_packet - initialize the data structures for packet semantics
 222     gen_commit_packet - commit the register writes, stores, etc for a packet
 223     decode_and_translate_packet - disassemble a packet and generate code
 224
 225 genptr.c
 226 gen_tcg.h
 227
 228 These files create a function for each instruction.  It is mostly composed of
 229 fGEN_TCG_<tag> definitions followed by including tcg_funcs_generated.c.inc.
 230
 231 op_helper.c
 232
 233 This file contains the implementations of all the helpers.  There are a few
 234 general purpose helpers, but most of them are generated by including
 235 helper_funcs_generated.c.inc.  There are also several helpers used for debugging.
 236
 237
 238 *** Packet Semantics ***
 239
 240 VLIW packet semantics differ from serial semantics in that all input operands
 241 are read, then the operations are performed, then all the results are written.
 242 For exmaple, this packet performs a swap of registers r0 and r1
 243     { r0 = r1; r1 = r0 }
 244 Note that the result is different if the instructions are executed serially.
 245
 246 Packet semantics dictate that we defer any changes of state until the entire
 247 packet is committed.  We record the results of each instruction in a side data
 248 structure, and update the visible processor state when we commit the packet.
 249
 250 The data structures are divided between the runtime state and the translation
 251 context.
 252
 253 During the TCG generation (see translate.[ch]), we use the DisasContext to
 254 track what needs to be done during packet commit.  Here are the relevant
 255 fields
 256
 257     reg_log            list of registers written
 258     reg_log_idx        index into ctx_reg_log
 259     pred_log           list of predicates written
 260     pred_log_idx       index into ctx_pred_log
 261     store_width        width of stores (indexed by slot)
 262
 263 During runtime, the following fields in CPUHexagonState (see cpu.h) are used
 264
 265     new_value             new value of a given register
 266     reg_written           boolean indicating if register was written
 267     new_pred_value        new value of a predicate register
 268     pred_written          boolean indicating if predicate was written
 269     mem_log_stores        record of the stores (indexed by slot)
 270
 271 For Hexagon Vector eXtensions (HVX), the following fields are used
 272     VRegs                       Vector registers
 273     future_VRegs                Registers to be stored during packet commit
 274     tmp_VRegs                   Temporary registers *not* stored during commit
 275     QRegs                       Q (vector predicate) registers
 276     future_QRegs                Registers to be stored during packet commit
 277
 278 *** Debugging ***
 279
 280 You can turn on a lot of debugging by changing the HEX_DEBUG macro to 1 in
 281 internal.h.  This will stream a lot of information as it generates TCG and
 282 executes the code.
 283
 284 To track down nasty issues with Hexagon->TCG generation, we compare the
 285 execution results with actual hardware running on a Hexagon Linux target.
 286 Run qemu with the "-d cpu" option.  Then, we can diff the results and figure
 287 out where qemu and hardware behave differently.
 288
 289 The stacks are located at different locations.  We handle this by changing
 290 env->stack_adjust in translate.c.  First, set this to zero and run qemu.
 291 Then, change env->stack_adjust to the difference between the two stack
 292 locations.  Then rebuild qemu and run again. That will produce a very
 293 clean diff.
 294
 295 Here are some handy places to set breakpoints
 296
 297     At the call to gen_start_packet for a given PC (note that the line number
 298         might change in the future)
 299         br translate.c:602 if ctx->base.pc_next == 0xdeadbeef
 300     The helper function for each instruction is named helper_<TAG>, so here's
 301         an example that will set a breakpoint at the start
 302         br helper_A2_add
 303     If you have the HEX_DEBUG macro set, the following will be useful
 304         At the start of execution of a packet for a given PC
 305             br helper_debug_start_packet if env->gpr[41] == 0xdeadbeef
 306         At the end of execution of a packet for a given PC
 307             br helper_debug_commit_end if env->this_PC == 0xdeadbeef