Hi,
Got it, Thanks :)
Jojo
在 2020年9月27日 +0800 AM3:22，Segher Boessenkool <seg...@kernel.crashing.org>，写道：
> Hi!
>
> On Wed, Sep 23, 2020 at 10:50:52AM +0800, Jojo R wrote:
> > insn seqs:
> >
> > s1:
> >
> > __builtin_set_float_convert_mode(0);
> > r1 = __builtin_load(a1, a2);
> > r2 = __builtin_float_convert(r1);
> > __builtin_store(a3, r2);
> > __builtin_set_float_convert_mode(0);
> >
> > s2:
> > __builtin_set_float_convert_mode(1);
> > r1 = __builtin_load(a1, a2);
> > r2 = __builtin_float_convert(r1);
> > __builtin_store(a3, r2);
> > __builtin_set_float_convert_mode(0);
> >
> >
> > the diference of s1 and s2 only is "__builtin_set_float_convert_mode"
> >
> > from beginning, the s1 mode is set 0, but s2 mode is set 1.
> >
> > From optimization 'fwprop', the s2 insn seqs is deleted
> >
> > as dead code with compiler option ‘-O3', is it gcc bug ?
> >
> >
> > builtin patten of __builtin_set_float_convert_mode:
> >
> > (define_insn "target_fcvtmode"
> > [(set (reg:SI FCVTMODE_REGNUM)
> > (unspec_volatile:SI [(match_operand:SI 0 "operand" "rK")]
> > UNSPECV_FCVTMODE))]
> > ""
> > "fcvtmode\t%1"
> > )
> >
> >
> > builtin patten of __builtin_set_float_convert_mode:
> >
> > (define_insn "target_fcvt"
> > [(set (match_operand:SI 0 "register_operand" "=r")
> > (unspec:SI [(match_operand:SF 1 "register_operand" "f")]
> > UNSPEC_FCVT))
> > (use (reg:SI FCVTMODE_REGNUM))]
> > ""
> > "fcvt\t%0,%1"
> > )
>
> That "use" is mostly useless; what you should do afaics is make that
> FCVTMODE_REGNUM an input of the unspec (and it can just be an unspec,
> unspec_volatile isn't needed).
>
> > As far as i know, __builtin_set_float_convert_mode depend on
> > 'FCVTMODE_REGNUM' which is from
> > __builtin_set_float_convert_mode, it's not dead code.
>
> The output (operands[0]) does not depend on the convert mode setting, so
> the second one can be replaced by the first (in CSE or fwprop perhaps),
> and as end result you get what you observed. Look at what all RTL
> passes did to your code (-dap is easiest) to see what happened.
>
> HtH,
>
>
> Segher
