My bad Ian,
Thanks for the input and the target was private and gcc 4.8.1 version
used and your are on same page on reg pairing .
Let me have a look on the port .
Thanks Again
~Umesh
On Tue, Jan 21, 2014 at 8:12 PM, Ian Lance Taylor <i...@google.com> wrote:
> On Tue, Jan 21, 2014 at 12:52 AM, Umesh Kalappa
> <umesh.kalap...@gmail.com> wrote:
>>
>> The following C code snippet
>>
>> unsigned char c ;
>> int d ;
>>
>> int test ()
>> {
>> d = c;
>> return d;
>> }
>>
>> below is the RTL without optimisation enabled
>>
>> (insn 6 5 0 (set (reg:QI 18 [ c.0 ])
>>
>> (mem/c:QI (symbol_ref:HI ("c") <var_decl 0xb73a50b8 c>) [0
>> c+0 S1 A8])) cnv.c:5 -1
>>
>> (nil))
>>
>>
>> (insn 7 6 8 (set (reg:QI 19)
>>
>> (const_int 0 [0])) cnv.c:5 -1
>>
>> (nil))
>>
>>
>>
>> (insn 8 7 0 (set (reg:HI 19 [ d.1 ])
>>
>> (subreg:HI (reg:QI 18 [ c.0 ]) 0)) cnv.c:5 -1
>>
>> (expr_list:REG_EQUAL (zero_extend:HI (reg:QI 18 [ c.0 ]))
>>
>> (nil)))
>>
>>
>>
>> (insn 9 8 0 (set (mem/c:HI (symbol_ref:HI ("d") <var_decl 0xb73a5114
>> d>) [0 d+0 S2 A16])
>>
>> (reg:HI 19 [ d.1 ])) cnv.c:5 -1
>>
>> (nil))
>>
>>
>>
>> (insn 10 9 0 (set (reg:HI 20 [ D.1323 ])
>>
>> (mem/c:HI (symbol_ref:HI ("d") <var_decl 0xb73a5114 d>) [0
>> d+0 S2 A16])) cnv.c:6 -1
>>
>> (nil))
>>
>> and respective ASM
>>
>> ld C, (c)
>> ld B, 0
>> ld (d), BC
>> ld WA, (d)
>> ret
>>
>>
>> But problem arises when i enabled the optimisation -O3 ( with -da) .
>>
>> RTL is expanded as show above, but after subreg pass(*.subreg) we
>> see the below RTL
>>
>>
>> (note 4 0 3 2 [bb 2] NOTE_INSN_BASIC_BLOCK)
>>
>> (note 3 4 8 2 NOTE_INSN_FUNCTION_BEG)
>>
>> (insn 8 3 9 2 (set (mem/c:HI (symbol_ref:HI ("d") <var_decl
>> 0xb7356114 d>) [2 d+0 S2 A16])
>>
>> (reg:HI 19 [ d.1 ])) cnv.c:5 9 {*movhi}
>>
>> (nil))
>>
>> (insn 9 8 13 2 (set (reg:HI 20 [ <retval> ])
>>
>> (reg:HI 19 [ d.1 ])) cnv.c:6 9 {*movhi}
>>
>> (nil))
>>
>> (insn 13 9 16 2 (set (reg/i:HI 0 W)
>>
>> (reg:HI 20 [ <retval> ])) cnv.c:7 9 {*movhi}
>>
>> (nil))
>>
>> (insn 16 13 0 2 (use (reg/i:HI 0 W)) cnv.c:7 -1
>>
>> (nil))
>>
>> ;; su
>>
>> and respective asm emited as
>>
>>
>> ld C, (c)
>> ld (d), BC ;This is invalid since B can have a clobbered value
>> ld WA, (d)
>> ret
>>
>> Would like know there exist any target hook to surpass the above
>> optimization so that i ended up emitting valid instructions.
>>
>> Any lights on the above problem is appreciated .
>
>
> You didn't say what target you are using, you didn't say what version
> of GCC you are using, and you didn't give enough information to
> understand what is happening. I don't know what the "ld (d),BC"
> syntax means, but perhaps it means that two different registers, B and
> C, are combined to form a single value. If that is true, then it
> appears that the subreg pass has dropped the insn setting
> pseudo-register 19. I have no idea how that could happen; it suggests
> a bug in your backend port when handling a movqi of a subreg.
>
> Ian
