On Thu, Sep 27, 2012 at 11:13 AM, Uros Bizjak <[email protected]> wrote:
> On Thu, Sep 27, 2012 at 8:08 PM, <[email protected]> wrote:
>
>>>>>>> I agree (subreg:M (op:N A C) 0) to (op:M (subreg:N (A 0)) C) is
>>>>>>> a good transformation, but why do we need to handle as special
>>>>>>> the case where the subreg is itself the operand of a plus or minus?
>>>>>>> I think it should happen regardless of where the subreg occurs.
>>>>>>
>>>>>> Don't we need to restrict this to the low part though?
>>>>>
>>>>> ...
>>>
>>> After some off-line discussion with Richard, attached is v2 of the patch.
>>>
>>> 2012-09-27 Uros Bizjak <[email protected]>
>>>
>>> PR rtl-optimization/54457
>>> * simplify-rtx.c (simplify_subreg):
>>> Simplify (subreg:SI (op:DI ((x:DI) (y:DI)), 0)
>>> to (op:SI (subreg:SI (x:DI) 0) (subreg:SI (x:DI) 0)).
>>> ...
>>
>> Is it just specific to DI -> SI, or is it for any large mode -> smaller
>> mode, like SI -> HI?
>
> Oh, I just copied v1 ChangeLog. The patch converts all modes where
> size of mode M < size of mode N. Updated ChangeLog reads:
>
> 2012-09-27 Uros Bizjak <[email protected]>
>
> PR rtl-optimization/54457
> * simplify-rtx.c (simplify_subreg):
> Simplify (subreg:M (op:N ((x:N) (y:N)), 0)
> to (op:M (subreg:M (x:N) 0) (subreg:M (x:N) 0)), where
> the outer subreg is effectively a truncation to the original mode M.
When I was doing something similar on our internal toolchain at
Cavium. I found doing this caused a regression on MIPS64 n32 in
gcc.c-torture/execute/20040709-1.c Where:
(insn 15 14 16 2 (set (reg/v:DI 200 [ y ])
(reg:DI 2 $2)) t.c:16 301 {*movdi_64bit}
(expr_list:REG_DEAD (reg:DI 2 $2)
(nil)))
(insn 16 15 17 2 (set (reg:DI 210)
(zero_extract:DI (reg/v:DI 200 [ y ])
(const_int 29 [0x1d])
(const_int 0 [0]))) t.c:16 249 {extzvdi}
(expr_list:REG_DEAD (reg/v:DI 200 [ y ])
(nil)))
(insn 17 16 23 2 (set (reg:SI 211)
(truncate:SI (reg:DI 210))) t.c:16 175 {truncdisi2}
(expr_list:REG_DEAD (reg:DI 210)
(nil)))
Gets converted to:
(insn 23 17 26 2 (set (reg/i:SI 2 $2)
(and:SI (reg:SI 2 $2 [+4 ])
(const_int 536870911 [0x1fffffff]))) t.c:18 156 {*andsi3}
(nil))
Which is considered an ext instruction
And with the Octeon simulator which causes undefined arguments to
32bit word operations to come out as 0xDEADBEEF which showed the
regression. I fixed it by changing it to produce TRUNCATE instead of
the subreg.
I did the simplification on ior/and rather than plus/minus/mult so the
issue is only when expanding to this to and/ior.
Thanks,
Andrew Pinski
>
> testsuite/ChangeLog:
>
> 2012-09-27 Uros Bizjak <[email protected]>
>
> PR rtl-optimization/54457
> * gcc.target/i386/pr54457.c: New test.
>
> Uros.
