Andrew Pinski <andrew.pin...@caviumnetworks.com> writes:
> On Thu, Sep 27, 2012 at 11:13 AM, Uros Bizjak <ubiz...@gmail.com> wrote:
>> 2012-09-27 Uros Bizjak <ubiz...@gmail.com>
>>
>> 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.
Hmm, hadn't thought of that. I think some of the existing subreg
optimisations suffer the same problem. I.e. we can't assume that
subreg truncations of nested operands are OK just because the outer
subreg is OK.
I've got a patch I'm testing.
BTW, I haven't forgotten about your other ext patch. Was hoping
to see whether we could finally take the opportunity to parameterise
the ext* patterns by mode, but got distracted with other patches.
Maybe I'll just have to admit I won't get time to try it for 4.8...
Richard
