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Minor optimization of partial load/v 

When loading a part select from the least significant bits, it is
OK to use the %load/v instruction to strip the high bits off. This
allows the zero-based part select to work in one step, without
loading excess bits.
This commit is contained in:
Stephen Williams 2007-12-09 17:28:49 -08:00
parent bee4772ac8
commit 8d9998c44e
2 changed files with 9 additions and 30 deletions
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34
tgt-vvp/eval_expr.c
View File

@ -1861,42 +1861,20 @@ static struct vector_info draw_select_signal(ivl_expr_t sube,
use_word = get_number_immediate(ix);
}
/* Try the special case that the base is 0 and the width
exactly matches the signal. Just load the signal in one
instruction. */
/* Try the special case that the part is at the beginning of
the signal (or the entire width). Just load the early bits
in one go. */
if (number_is_immediate(bit_idx, 32)
&& get_number_immediate(bit_idx) == 0
&& ivl_expr_width(sube) == wid) {
&& (ivl_expr_width(sube) >= wid)) {
res.base = allocate_vector(wid);
res.wid = wid;
fprintf(vvp_out, " %%load/v %u, v%p_%u, %u;\n",
res.base, sig, use_word, ivl_expr_width(sube));
return res;
}
/* Try the special case that the part is at the beginning and
nearly the width of the signal. In this case, just load the
entire signal in one go, then simply drop the excess bits. */
if (number_is_immediate(bit_idx, 32)
&& get_number_immediate(bit_idx) == 0
&& (ivl_expr_width(sube) > wid)
&& (ivl_expr_width(sube) < (wid+wid/10))) {
res.base = allocate_vector(ivl_expr_width(sube));
res.wid = ivl_expr_width(sube);
fprintf(vvp_out, " %%load/v %u, v%p_%u, %u; Only need %u of %u bits\n",
res.base, sig, use_word, ivl_expr_width(sube), wid, res.wid);
res.base, sig, use_word, wid, wid, ivl_expr_width(sube));
save_signal_lookaside(res.base, sig, use_word, res.wid);
{
struct vector_info tmp;
tmp.base = res.base + wid;
tmp.wid = res.wid - wid;
clr_vector(tmp);
res.wid = wid;
}
return res;
}

5
vvp/opcodes.txt
View File

@ -418,8 +418,9 @@ This instruction loads a vector value from the given functor node into
the specified thread register bit. The functor-label can refer to a
.net, a .var or a .functor with a vector output. The entire vector,
from the least significant up to <wid> bits, is loaded starting at
thread bit <bit>. It is an error for the width to not match the vector
width at the functor.
thread bit <bit>. It is an OK for the width to not match the vector
width at the functor. If the <wid> is less then the width at the
functor, the the most significant bits are dropped.
* %load/vp0 <bit>, <functor-label>, <wid>