Re: [PATCH, ARM] correctly encode the CC reg data flow

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

Hi Kyrill,

Thanks for your review!


On 09/04/17 15:55, Kyrill Tkachov wrote:
> Hi Bernd,
> 
> On 18/01/17 15:36, Bernd Edlinger wrote:
>> On 01/13/17 19:28, Bernd Edlinger wrote:
>>> On 01/13/17 17:10, Bernd Edlinger wrote:
>>>> On 01/13/17 14:50, Richard Earnshaw (lists) wrote:
>>>>> On 18/12/16 12:58, Bernd Edlinger wrote:
>>>>>> Hi,
>>>>>>
>>>>>> this is related to PR77308, the follow-up patch will depend on this
>>>>>> one.
>>>>>>
>>>>>> When trying the split the *arm_cmpdi_insn and *arm_cmpdi_unsigned
>>>>>> before reload, a mis-compilation in libgcc function 
>>>>>> __gnu_satfractdasq
>>>>>> was discovered, see [1] for more details.
>>>>>>
>>>>>> The reason seems to be that when the *arm_cmpdi_insn is directly
>>>>>> followed by a *arm_cmpdi_unsigned instruction, both are split
>>>>>> up into this:
>>>>>>
>>>>>>     [(set (reg:CC CC_REGNUM)
>>>>>>           (compare:CC (match_dup 0) (match_dup 1)))
>>>>>>      (parallel [(set (reg:CC CC_REGNUM)
>>>>>>                      (compare:CC (match_dup 3) (match_dup 4)))
>>>>>>                 (set (match_dup 2)
>>>>>>                      (minus:SI (match_dup 5)
>>>>>>                               (ltu:SI (reg:CC_C CC_REGNUM) (const_int
>>>>>> 0))))])]
>>>>>>
>>>>>>     [(set (reg:CC CC_REGNUM)
>>>>>>           (compare:CC (match_dup 2) (match_dup 3)))
>>>>>>      (cond_exec (eq:SI (reg:CC CC_REGNUM) (const_int 0))
>>>>>>                 (set (reg:CC CC_REGNUM)
>>>>>>                      (compare:CC (match_dup 0) (match_dup 1))))]
>>>>>>
>>>>>> The problem is that the reg:CC from the *subsi3_carryin_compare
>>>>>> is not mentioning that the reg:CC is also dependent on the reg:CC
>>>>>> from before.  Therefore the *arm_cmpsi_insn appears to be
>>>>>> redundant and thus got removed, because the data values are 
>>>>>> identical.
>>>>>>
>>>>>> I think that applies to a number of similar pattern where data
>>>>>> flow is happening through the CC reg.
>>>>>>
>>>>>> So this is a kind of correctness issue, and should be fixed
>>>>>> independently from the optimization issue PR77308.
>>>>>>
>>>>>> Therefore I think the patterns need to specify the true
>>>>>> value that will be in the CC reg, in order for cse to
>>>>>> know what the instructions are really doing.
>>>>>>
>>>>>>
>>>>>> Bootstrapped and reg-tested on arm-linux-gnueabihf.
>>>>>> Is it OK for trunk?
>>>>>>
>>>>> I agree you've found a valid problem here, but I have some issues with
>>>>> the patch itself.
>>>>>
>>>>>
>>>>> (define_insn_and_split "subdi3_compare1"
>>>>>    [(set (reg:CC_NCV CC_REGNUM)
>>>>>      (compare:CC_NCV
>>>>>        (match_operand:DI 1 "register_operand" "r")
>>>>>        (match_operand:DI 2 "register_operand" "r")))
>>>>>     (set (match_operand:DI 0 "register_operand" "=&r")
>>>>>      (minus:DI (match_dup 1) (match_dup 2)))]
>>>>>    "TARGET_32BIT"
>>>>>    "#"
>>>>>    "&& reload_completed"
>>>>>    [(parallel [(set (reg:CC CC_REGNUM)
>>>>>             (compare:CC (match_dup 1) (match_dup 2)))
>>>>>            (set (match_dup 0) (minus:SI (match_dup 1) (match_dup 
>>>>> 2)))])
>>>>>     (parallel [(set (reg:CC_C CC_REGNUM)
>>>>>             (compare:CC_C
>>>>>               (zero_extend:DI (match_dup 4))
>>>>>               (plus:DI (zero_extend:DI (match_dup 5))
>>>>>                    (ltu:DI (reg:CC_C CC_REGNUM) (const_int 0)))))
>>>>>            (set (match_dup 3)
>>>>>             (minus:SI (minus:SI (match_dup 4) (match_dup 5))
>>>>>                   (ltu:SI (reg:CC_C CC_REGNUM) (const_int 0))))])]
>>>>>
>>>>>
>>>>> This pattern is now no-longer self consistent in that before the split
>>>>> the overall result for the condition register is in mode CC_NCV, but
>>>>> afterwards it is just CC_C.
>>>>>
>>>>> I think CC_NCV is correct mode (the N, C and V bits all correctly
>>>>> reflect the result of the 64-bit comparison), but that then implies 
>>>>> that
>>>>> the cc mode of subsi3_carryin_compare is incorrect as well and 
>>>>> should in
>>>>> fact also be CC_NCV.  Thinking about this pattern, I'm inclined to 
>>>>> agree
>>>>> that CC_NCV is the correct mode for this operation
>>>>>
>>>>> I'm not sure if there are other consequences that will fall out from
>>>>> fixing this (it's possible that we might need a change to 
>>>>> select_cc_mode
>>>>> as well).
>>>>>
>>>> Yes, this is still a bit awkward...
>>>>
>>>> The N and V bit will be the correct result for the subdi3_compare1
>>>> a 64-bit comparison, but zero_extend:DI (match_dup 4) (plus:DI ...)
>>>> only gets the C bit correct, the expression for N and V is a different
>>>> one.
>>>>
>>>> It probably works, because the subsi3_carryin_compare instruction sets
>>>> more CC bits than the pattern does explicitly specify the value.
>>>> We know the subsi3_carryin_compare also computes the NV bits, but it is
>>>> hard to write down the correct rtl expression for it.
>>>>
>>>> In theory the pattern should describe everything correctly,
>>>> maybe, like:
>>>>
>>>> set (reg:CC_C CC_REGNUM)
>>>>      (compare:CC_C
>>>>        (zero_extend:DI (match_dup 4))
>>>>        (plus:DI (zero_extend:DI (match_dup 5))
>>>>                 (ltu:DI (reg:CC_C CC_REGNUM) (const_int 0)))))
>>>> set (reg:CC_NV CC_REGNUM)
>>>>      (compare:CC_NV
>>>>       (match_dup 4))
>>>>       (plus:SI (match_dup 5) (ltu:SI (reg:CC_C CC_REGNUM) (const_int 
>>>> 0)))
>>>> set (match_dup 3)
>>>>      (minus:SI (minus:SI (match_dup 4) (match_dup 5))
>>>>                (ltu:SI (reg:CC_C CC_REGNUM) (const_int 0)))))
>>>>
>>>>
>>>> But I doubt that will work to set CC_REGNUM with two different modes
>>>> in parallel?
>>>>
>>>> Another idea would be to invent a CC_CNV_NOOV mode, that implicitly
>>>> defines C from the DImode result, and NV from the SImode result,
>>>> similar to the CC_NOOVmode, that also leaves something open what
>>>> bits it really defines?
>>>>
>>>>
>>>> What do you think?
>>>>
>>>>
>>>> Thanks
>>>> Bernd.
>>> I think maybe the right solution is to invent a new CCmode
>>> that defines C as if the comparison is done in DImode
>>> but N and V as if the comparison is done in SImode.
>>>
>>> I thought maybe I would call it CC_NCV_CIC (CIC = Carry-In-Compare),
>>> furthermore I think the CC_NOOV should be renamed to CC_NZ (because
>>> only N and Z are set correctly), but in a different patch of course.
>>>
>>> Attached is a new version that implements the new CCmode.
>>>
>>> How do you like this new version?
>>>
>>> It seems to be able to build a cross-compiler at least.
>>>
>>> I will start a new bootstrap with this new patch, but that can take some
>>> time until I have definitive results.
>>>
>>> Is there still a chance that it can go into gcc-7 or should it wait
>>> for the next stage1?
>>>
>>> Thanks
>>> Bernd.
>>
>> I thought I should also look at where the subdi_compare1 amd the
>> negdi2_compare patterns are used, and look if the caller is fine with
>> not having all CC bits available.
>>
>> And indeed usubv<mode>4 turns out to be questionabe, because it
>> emits gen_sub<mode>3_compare1 and uses arm_gen_unlikely_cbranch (LTU,
>> CCmode) which is inconsistent when subdi3_compare1 no longer uses
>> CCmode.
>>
>> To correct this, the branch should use CC_Cmode which is always defined.
>>
>> So I tried to test this pattern, with the following test programs,
>> and found that the code actually improves when the branch uses CC_Cmode
>> instead of CCmode, both for SImode and DImode data, which was a bit
>> surprising.
>>
>> I used this test program to see how the usubv<mode>4 pattern works:
>>
>> cat test.c (DImode)
>> unsigned long long x, y, z;
>> int b;
>> void test()
>> {
>>     b = __builtin_sub_overflow (y,z, &x);
>> }
>>
>>
>> unpatched code used 8 byte more stack than patched,
>> because the DImode subtraction is effectively done twice.
>>
>> cat test1.c (SImode)
>> unsigned long x, y, z;
>> int b;
>> void test()
>> {
>>     b = __builtin_sub_overflow (y,z, &x);
>> }
>>
>> which generates (unpatched):
>>           cmp     r3, r0
>>           sub     ip, r3, r0
>>
>> instead of expected (patched):
>>     subs    r3, r3, r2
>>
>>
>> The condition is extracted by ifconversion and/or combine
>> and complicates the resulting code instead of simplifying.
>>
>> I think this happens only when the branch and the subsi/di3_compare1
>> is using the same CC mode.
>>
>> That does not happen when the CC modes disagree, as with the
>> proposed patch.  All other uses of the pattern are already using
>> CC_Cmode or CC_Vmode in the branch, and these do not change.
>>
>> Attached is an updated version of the patch, that happens to
>> improve the code generation of the usubsi4 and usubdi4 pattern,
>> as a side effect.
>>
>>
>> Bootstrapped and reg-tested on arm-linux-gnueabihf.
>> Is it OK for trunk?
> 
> I'm very sorry it has taken so long to review.
> I've been ramping up on the context recently now so I'll try to move 
> this along...
> 
> This patch looks mostly ok to me from reading the patterns and the 
> discussion around it.
> I have one concern:
> 
> 
>   (define_insn_and_split "negdi2_compare"
> -  [(set (reg:CC CC_REGNUM)
> -    (compare:CC
> +  [(set (reg:CC_NCV CC_REGNUM)
> +    (compare:CC_NCV
>         (const_int 0)
>         (match_operand:DI 1 "register_operand" "0,r")))
>      (set (match_operand:DI 0 "register_operand" "=r,&r")
> @@ -4647,8 +4650,12 @@
>              (compare:CC (const_int 0) (match_dup 1)))
>             (set (match_dup 0) (minus:SI (const_int 0)
>                          (match_dup 1)))])
> -   (parallel [(set (reg:CC CC_REGNUM)
> -           (compare:CC (const_int 0) (match_dup 3)))
> +   (parallel [(set (reg:CC_NCV_CIC CC_REGNUM)
> +           (compare:CC_NCV_CIC
> +             (const_int 0)
> +             (plus:DI
> +               (zero_extend:DI (match_dup 3))
> +               (ltu:DI (reg:CC_C CC_REGNUM) (const_int 0)))))
>            (set (match_dup 2)
>             (minus:SI
>              (minus:SI (const_int 0) (match_dup 3))
> 
> 
> I was somewhat concerned with having the first operand of the COMPARE 
> being a const_int 0 and the second being
> a complex expression as the RTL canonicalization rules usually require 
> the complex operand going first if possible.
> Reading the RTL rules in rtl.texi I see it says this:
> "If one of the operands is a constant, it should be placed in the
> second operand and the comparison code adjusted as appropriate."
> So it seems that the pre-existing pattern that puts const_int 0 as the 
> first operand already breaks that rule.
> I think we should fix that and update the use of condition code to a GEU 
> rather than LTU as well.
> 


Hmmm...

I think the compare is not a commutative operation, and swapping
the arguments will imply a different value in the flags.

So if I write
(set (reg:CC_NCV CC_REGNUM)
      (compare:CC_NCV
        (const_int 0)
        (reg:DI 123)))

I have C,N,V set to the result of (0 - r123), C = usable for LTU or GEU,
N,V = usable for LT, GE

But if I write
(set (reg:CC_NCV CC_REGNUM)
      (compare:CC_NCV
        (reg:DI 123)
        (const_int 0)))

I have C,N,V set to the result of (r123 - 0), but the expansion stays
the same and the actual value in the flags is defined by the expansion.
Of course there exists probably no matching expansion for that.

Note that both LTU in the above hunk are in a parallel-stmt and operate
on the flags from the previous pattern, so changing these to GEU
will probably be wrong.

Both (ltu:SI (reg:CC_C CC_REGNUM) (const_int 0)) in the negdi2_compare
use the flags from the previous (set (reg:CC CC_REGNUM) (compare:CC
(const_int 0) (match_dup 1)).

One use of the resulting flags (I know of) is in negvdi3 where we
have:

   emit_insn (gen_negdi2_compare (operands[0], operands[1]));
   arm_gen_unlikely_cbranch (NE, CC_Vmode, operands[2]);

I think only 0-x can overflow while x-0 can never overflow.

Of course the CC_NCV_CIC mode bends the definition of the RTL compare
a lot and I guess if this pattern is created by a splitter, this can
only be expanded by an exactly matching pattern, there is (hopefully)
no way how combine could mess with this pattern due to the exotic
CCmode.  So while I think it would work to swap only the notation of
all CC_NCV_CIC patterns, _without_ changing the assembler-parts and the
consuming statements, that would make it quite hard to follow for the
human reader at least.

What do you think?


Bernd.