[LLVMdev] ScheduleDAGInstrs/R600 test potential issue with implicit defs

[Stefan Hepp]

Hi Tom,

Thanks a lot for your explanations, now it makes a lot more sense ;)

I had a slightly closer look at the R600 packetizer, and the issue is 
that the third LSHL instruction has both an implicit use and 
*afterwards* an implicit def of T1_XYZW. The latter def causes the 
current ScheduleDAGInstrs implementation to ignore the implicit use, 
thus the ScheduleDAG only contains an anti-dependency from the second to 
the third LSHL and the packetizer can bundle the instructions.

If the order of the implicit-defs and implicit-use would be different 
(e.g., like TableGen adds them), or if I apply my patch so that the 
implicit-use is not ignored, there is a true dependency edge between 
those two instructions, preventing bundling.

Removing only the implicit-defs alone has no effect in this example, as 
then the implicit-use of T1_XYZW is then no longer removed and a true 
dependency to the def of T1_W is added. Removing all implicit operands 
however solves that (see attached patch/hack).

I would argue that the behaviour of ScheduleDAGInstrs::buildSchedGraph 
for implicit operands is not correct, simply because changing the order 
of the operands would already prevent the R600 packetizer from bundling 
in this example.

Cheers,
  Stefan

On 2014-02-25 17:35, Tom Stellard wrote:
> On Tue, Feb 25, 2014 at 04:38:46PM +0100, Stefan Hepp wrote:
>> Hello,
>>
>> The ScheduleDAGInstrs::buildSchedGraph() function creates def/uses lists by iterating over all instruction operands and calls addPhysRegDeps() if used post-RA (line ~770 ff.). If an operand is a def, the uses of that registers are cleared (ScheduleDAGInstrs.cpp:333:  Uses.eraseAll(Reg); ).
>>
>> As a consequence, if an instruction has an explicit use of a register and an implicit def of the same register, the implicit def causes the use to be removed. In our case this leads to missing scheduling dependencies for indirect calls where a return register is used as operand of the indirect call. Therefore I patched the buildSchedGraph() function to iterate over all defs first, then over all uses in a second iteration (maybe not the most efficient solution, but it solves the issue).
>> I am attaching the corresponding patch.
>>
>> However, while upgrading our code to LLVM 3.4, I found that the test/CodeGen/R600/store.ll test fails for -mcpu=redwood due to this change. The difference is in the following line in @store_i8: With my patch applied I get:
>>
>>      LSHL                  * T0.W, PV.W, literal.x,
>>    3(4.203895e-45), 0(0.000000e+00)
>>      LSHL                  * T1.X, T1.W, PV.W,
>>      LSHL                  * T1.W, literal.x, T0.W,
>>    255(3.573311e-43), 0(0.000000e+00)
>>
>> versus the orginal output of LLVM 3.4:
>>
>>      LSHL                  * T0.W, PV.W, literal.x,
>>    3(4.203895e-45), 0(0.000000e+00)
>>      LSHL                    T1.X, T1.W, PV.W,
>>      LSHL                  * T1.W, literal.x, PV.W,
>>    255(3.573311e-43), 0(0.000000e+00)
>>
>> The difference is the '*' in the second LSHL and the T0.W versus PV.W in the third LSHL.
>>
>> Looking at the output of -print-after-all, llc generates the following MC in both plain LLVM 3.4 and LLVM 3.4 with my patch applied right after the "Finalize machine instruction bundles" pass:
>>
>>      %T0_W<def> = LSHL_eg 0, 0, 1, 0, 0, 0, %T0_W<kill>, 0, 0, 0, -1, %ALU_LITERAL_X, 0, 0, 0, -1, 1, pred:%PRED_SEL_OFF, 3, 0
>>      %T1_X<def> = LSHL_eg 0, 0, 1, 0, 0, 0, %T1_W<kill>, 0, 0, 0, -1, %T0_W, 0, 0, 0, -1, 1, pred:%PRED_SEL_OFF, 0, 0, %T1_XYZW<imp-def>
>>      %T1_W<def> = LSHL_eg 0, 0, 1, 0, 0, 0, %ALU_LITERAL_X, 0, 0, 0, -1, %T0_W<kill>, 0, 0, 0, -1, 1, pred:%PRED_SEL_OFF, 255, 0, %T1_XYZW<imp-use,kill>, %T1_XYZW<imp-def>
>>
>> Hence, my patch affects either the R600 Packetizer or the R600 Control Flow Finalizer pass. A closer look reveals that the 17th operand (the one before the 'pred:$PRED_SEL_OFF' operand) controls the '*' output and is actually set for all three LSHL operands before the Packetizer/Control Flow Finalizer pass, and from a quick glance at the AMD ISA docs I gather that 'PV' versus 'T0' seems to be related to this, but the packetizer (?) clears this flag for the second LSHR for redwood/VLIW5 GPUs.
>>
>>
>> Now, here is my question: Is the R600/store.ll test actually correct (I am not yet familiar enough with the R600 ISA to tell), or does the R600 backend rely on the fact that implicit defs kill the uses of registers, or is there a bug/.. in the R600 backend that counteracts the implicit-defs scheduling issue?
>> And is removing the explicit use by an implicit def at an instruction a bug or a feature?
>>
>> If this is an actual bug in the ScheduleDAGInstrs implementation, I would like to adapt my patch so that it can be applied (i.e., include a fix for the store.ll test), although there might be a better way of doing this (e.g., only erase uses that are not from the same SU, or iterate over defs, implicit-defs, uses, implicit-uses instead of iterating over all operands twice), and I would need someone more familiar with the R600 backend to check if the new output of LLC for store.ll is actually correct or not.
>>
>
> Hi Stefan,
>
> Thanks for the detailed analysis.  The '*' marks the end of a packet, so
> with your patch, the sequence uses 3 packets, and it uses only 2 with
> LLVM 3.4.  So, this is a performance regression.
>
> The root of this problem is that LLVM assumes it is impossible to
> concurrently access two sub-registers of the same super-register, so
> it creates a scheduling dependency between the two sub-register defs.
>
> This is a problem with R600's VLIW architecture, because we want to be
> able to access all four sub-registers of the same packet, but scheduling
> dependencies prevent this.
>
> This is why there are implicit uses and defs of the T*_XYZW registers
> added to these instructions.  Some of these are added by LLVM passes
> and I think some are added by the R600MachineScheduler to work around
> the issue described (I'm not that familiar with the scheduler, so I may
> be wrong about this).
>
> I think there was an effort recently to try allow concurrent
> sub-register accesses, but I don't remember who was working on this.
>
> In this case, it looks like problem is that the last two instructions
> both define the same super-register, so the packetizer isn't putting
> them in the same packet.
>
> So, I think the conclusion is that your patch increases the exposure of
> the R600 to an already present problem in the register allocator /
> scheduler.  I wonder if we could work around this by stripping all the
> implicit defs and uses off the instructions once scheduling and register
> allocation has completed.  They are technically incorrect, and I'm not
> sure if any other passes use them.  Do you have any suggestions?
>
> Thanks,
> Tom
>
>

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