On 02/14/2013 03:19 PM, Richard Henderson wrote:
>> > + tcg_gen_brcond_i32(TCG_COND_EQ, REG(a), REG(b), label_equal);
>> > +
>> > +#define CMPTEST(t,CODE) \
>> > + { \
>> > + int lyes = gen_new_label(); \
>> > + int lno = gen_new_label(); \
>> > + TCGv t1 = new_tmp(); \
>> > + tcg_gen_brcond_i32(t, REG(a), REG(b), lyes); \
>> > + tcg_gen_br(lno); \
>> > + gen_set_label(lyes); \
>> > + tcg_gen_ori_i32(t1, cc, CODE); \
>> > + tcg_gen_mov_i32(cc, t1); \
>> > + gen_set_label(lno); \
>> > + dead_tmp(t1); \
> Consider making use of tcg_setcond_i32 here. It's probably new since
> you wrote all this in the first place.
>
That said, I wonder if the amount of code generated here even with
setcond would be too big.
The quick and easy option is to use a helper function. If we mark it
as-if gcc's pure, aka TCG_CALL_NO_RWG_SE, it should be relatively
efficient, as far as TCG goes:
target_long helper_cmp(target_long x, target_long y)
{
target_ulong ux = x, uy = y;
if (x == y) {
return CC_EQ;
}
return (x < y ? CC_LT : CC_GT) | (ux < uy ? CC_LTU : CC_GTU);
}
Optimizes pretty well inside gcc:
cmpl %esi, %edi
movl $4, %eax
je .L11
setl %dl
movzbl %dl, %edx
addl $1, %edx
cmpl %esi, %edi
sbbl %eax, %eax
andl $8, %eax
addl $8, %eax
orl %edx, %eax
.L11:
rep
ret
Now, given that CC is only settable via CMP, and apparently only
readable via branches. means that we could optimize things a bit.
What if the CMP insn merely copies its inputs to TCG globals CMP1 and
CMP2? Then the actual implementation of BEQ (et al) is
tcg_gen_brcond_i32(TCG_COND_EQ, cpu_cmp1, cpu_cmp2, lab_true);
Since these are globals, a subsequent BGT in a different TB will still
read the same values and perform the appropriate comparison in
performing its branch.
What I don't know is how this affects the moxie exception model, of
which I can find no information. There doesn't seem to be any
implementation of such a model in the patches sent here.
When saving/restoring the CC value, one could presumably translate the
two variables into a canonical CC code as above, and for the reverse
transformation chose two values that create any valid comparison.
cmp1 cmp2
EQ 0 0
LT & LTU 0 1
LT & GTU -1 1
GT & LTU 1 -1
GT & GTU 1 0
as everything else is logically impossible.
r~
