Re: [Qemu-devel] [PATCH 2/6] Add copy and constant propagation.

[Aurelien Jarno <aurelien@aurel32.net>]

On Fri, May 20, 2011 at 04:39:29PM +0400, Kirill Batuzov wrote:
> Make tcg_constant_folding do copy and constant propagation. It is a
> preparational work before actual constant folding.
> 
> Signed-off-by: Kirill Batuzov <batuzovk@ispras.ru>
> ---
>  tcg/optimize.c |  123 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 123 insertions(+), 0 deletions(-)
> 
> diff --git a/tcg/optimize.c b/tcg/optimize.c
> index cf31d18..a761c51 100644
> --- a/tcg/optimize.c
> +++ b/tcg/optimize.c
> @@ -31,22 +31,139 @@
>  #include "qemu-common.h"
>  #include "tcg-op.h"
>  
> +typedef enum {
> +    TCG_TEMP_UNDEF = 0,
> +    TCG_TEMP_CONST,
> +    TCG_TEMP_COPY,
> +    TCG_TEMP_ANY
> +} tcg_temp_state;
> +
> +static int mov_to_movi(int op)
> +{
> +    switch (op) {
> +    case INDEX_op_mov_i32: return INDEX_op_movi_i32;
> +#if TCG_TARGET_REG_BITS == 64
> +    case INDEX_op_mov_i64: return INDEX_op_movi_i64;
> +#endif
> +    default:
> +        fprintf(stderr, "Unrecognized operation %d in mov_to_movi.\n", op);
> +        tcg_abort();
> +    }
> +}
> +
> +/* Reset TEMP's state to TCG_TEMP_ANY.  If TEMP was a representative of some
> +   class of equivalent temp's, a new representative should be chosen in this
> +   class. */
> +static void reset_temp(tcg_temp_state *state, tcg_target_ulong *vals,
> +                       TCGArg temp, int nb_temps, int nb_globals)
> +{
> +    int i;
> +    TCGArg new_base;
> +    new_base = (TCGArg)-1;
> +    for (i = nb_globals; i < nb_temps; i++) {
> +        if (state[i] == TCG_TEMP_COPY && vals[i] == temp) {
> +            if (new_base == ((TCGArg)-1)) {
> +                new_base = (TCGArg)i;
> +                state[i] = TCG_TEMP_ANY;
> +            } else {
> +                vals[i] = new_base;
> +            }
> +        }
> +    }
> +    for (i = 0; i < nb_globals; i++) {
> +        if (state[i] == TCG_TEMP_COPY && vals[i] == temp) {
> +            if (new_base == ((TCGArg)-1)) {
> +                state[i] = TCG_TEMP_ANY;
> +            } else {
> +                vals[i] = new_base;
> +            }
> +        }
> +    }
> +    state[temp] = TCG_TEMP_ANY;
> +}
> +
> +/* Propagate constants and copies, fold constant expressions. */
>  static TCGArg *tcg_constant_folding(TCGContext *s, uint16_t *tcg_opc_ptr,
>                                      TCGArg *args, TCGOpDef *tcg_op_defs)
>  {
>      int i, nb_ops, op_index, op, nb_temps, nb_globals;
>      const TCGOpDef *def;
>      TCGArg *gen_args;
> +    /* Array VALS has an element for each temp.
> +       If this temp holds a constant then its value is kept in VALS' element.
> +       If this temp is a copy of other ones then this equivalence class'
> +       representative is kept in VALS' element.
> +       If this temp is neither copy nor constant then corresponding VALS'
> +       element is unused. */
> +    static tcg_target_ulong vals[TCG_MAX_TEMPS];
> +    static tcg_temp_state state[TCG_MAX_TEMPS];
>  
>      nb_temps = s->nb_temps;
>      nb_globals = s->nb_globals;
> +    memset(state, 0, nb_temps * sizeof(tcg_temp_state));
>  
>      nb_ops = tcg_opc_ptr - gen_opc_buf;
>      gen_args = args;
>      for (op_index = 0; op_index < nb_ops; op_index++) {
>          op = gen_opc_buf[op_index];
>          def = &tcg_op_defs[op];
> +        /* Do copy propagation */
> +        if (op != INDEX_op_call) {
> +            for (i = def->nb_oargs; i < def->nb_oargs + def->nb_iargs; i++) {
> +                if (state[args[i]] == TCG_TEMP_COPY
> +                    && !(def->args_ct[i].ct & TCG_CT_IALIAS)
> +                    && (def->args_ct[i].ct & TCG_CT_REG)) {
> +                    args[i] = vals[args[i]];
> +                }
> +            }
> +        }
> +
> +        /* Propagate constants through copy operations and do constant
> +           folding.  Constants will be substituted to arguments by register
> +           allocator where needed and possible.  Also detect copies. */
>          switch (op) {
> +        case INDEX_op_mov_i32:
> +#if TCG_TARGET_REG_BITS == 64
> +        case INDEX_op_mov_i64:
> +#endif
> +            if ((state[args[1]] == TCG_TEMP_COPY
> +                && vals[args[1]] == args[0])
> +                || args[0] == args[1]) {
> +                args += 2;
> +                gen_opc_buf[op_index] = INDEX_op_nop;
> +                break;
> +            }
> +            if (state[args[1]] != TCG_TEMP_CONST) {
> +                reset_temp(state, vals, args[0], nb_temps, nb_globals);
> +                if (args[1] >= s->nb_globals) {
> +                    state[args[0]] = TCG_TEMP_COPY;
> +                    vals[args[0]] = args[1];
> +                }
> +                gen_args[0] = args[0];
> +                gen_args[1] = args[1];
> +                gen_args += 2;
> +                args += 2;
> +                break;
> +            } else {
> +                /* Source argument is constant.  Rewrite the operation and
> +                   let movi case handle it. */
> +                op = mov_to_movi(op);
> +                gen_opc_buf[op_index] = op;
> +                args[1] = vals[args[1]];
> +                /* fallthrough */
> +            }
> +        case INDEX_op_movi_i32:
> +#if TCG_TARGET_REG_BITS == 64
> +        case INDEX_op_movi_i64:
> +#endif
> +            reset_temp(state, vals, args[0], nb_temps, nb_globals);
> +            state[args[0]] = TCG_TEMP_CONST;
> +            vals[args[0]] = args[1];
> +            gen_args[0] = args[0];
> +            gen_args[1] = args[1];
> +            gen_args += 2;
> +            args += 2;
> +            break;
>          case INDEX_op_call:
>          case INDEX_op_jmp:
>          case INDEX_op_br:
> @@ -55,6 +172,7 @@ static TCGArg *tcg_constant_folding(TCGContext *s, uint16_t *tcg_opc_ptr,
>  #if TCG_TARGET_REG_BITS == 64
>          case INDEX_op_brcond_i64:
>  #endif
> +            memset(state, 0, nb_temps * sizeof(tcg_temp_state));
>              i = (op == INDEX_op_call) ?
>                  (args[0] >> 16) + (args[0] & 0xffff) + 3 :
>                  def->nb_args;
> @@ -66,6 +184,11 @@ static TCGArg *tcg_constant_folding(TCGContext *s, uint16_t *tcg_opc_ptr,
>              }
>              break;
>          default:
> +            /* Default case: we do know nothing about operation so no
> +               propagation is done.  We only trash output args.  */
> +            for (i = 0; i < def->nb_oargs; i++) {
> +                reset_temp(state, vals, args[i], nb_temps, nb_globals);
> +            }
>              for (i = 0; i < def->nb_args; i++) {
>                  gen_args[i] = args[i];
>              }

It's not possible to do any optimization across certain ops that have
side effect or helper which access globals directly. For helpers this
can be easily detected looking at the TCG_CALL_PURE and TCG_CALL_CONST
flags. For ops, you should look at TCG_OPF_BB_END, TCG_OPF_CALL_CLOBBER
and TCG_OPF_SIDE_EFFECTS flags.

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net