[PATCH] D25347: [VirtRegRewriter] Eliminate COPYs before re-writing by renaming.

Matthias Braun via llvm-commits llvm-commits at lists.llvm.org
Tue Nov 15 12:46:01 PST 2016 

Ah right,

that is indeed a different issue. I had that issue on my TODO list as well (but somewhere deep down). Without actually trying it, I hope we can do that by having a smarter spill creation callback which matches a spill(copy(zero)) pattern.

- Matthias

> On Nov 15, 2016, at 12:13 PM, Geoff Berry <gberry at codeaurora.org> wrote:
> 
> Hi Matthias,
> 
> Those changes don't catch the particular cases I'm looking at.  The problem is that the xzr/wzr copies don't become candidates for coalescing until the middle of register allocation when some live ranges have been split/spilled.  Here's an example test case:
> declare i32 @bar()
> declare i32 @baz()
> ; Check that the spill of the zero value gets stored directly instead
> ; of being copied from wzr and then stored.
> define i32 @test_zr_spill_copyprop(i1 %c) {
> ; CHECK-LABEL: test_zr_spill_copyprop:
> entry:
>   br i1 %c, label %if.else, label %if.then
> 
> if.else:
> ; CHECK: bl bar
> ; CHECK-NEXT: str w0, [sp, #[[SLOT:[0-9]+]]]
>   %call1 = tail call i32 @bar()
>   br label %if.end
> 
> if.then:
> ; CHECK: bl baz
> ; CHECK-NEXT: str wzr, [sp, #[[SLOT]]]
>   %call2 = tail call i32 @baz()
>   br label %if.end
> 
> if.end:
>   %x.0 = phi i32 [ 0, %if.then ], [ %call1, %if.else ]
>   call void asm sideeffect "", "~{x0},~{x1},~{x2},~{x3},~{x4},~{x5},~{x6},~{x7},~{x8},~{x9},~{x10},~{x11},~{x12},~{x13},~{x14},~{x15},~{x16},~{x17},~{x18},~{x19},~{x20},~{x21},~{x22},~{x23},~{x24},~{x25},~{x26},~{x27},~{x28},~{fp},~{lr},~{sp}"() nounwind
>   %x.1 = add i32 %x.0, 1
>   ret i32 %x.1
> }
> 
> 
> On 11/15/2016 2:58 PM, Matthias Braun wrote:
>> I would xzr/wzr problems expect to be handled by the changes in https://reviews.llvm.org/D26106 <https://reviews.llvm.org/D26106> as soon as https://reviews.llvm.org/D26111 <https://reviews.llvm.org/D26111> is committed.
>> 
>> - Matthias
>> 
>>> On Nov 15, 2016, at 11:01 AM, Geoff Berry <gberry at codeaurora.org <mailto:gberry at codeaurora.org>> wrote:
>>> 
>>> Hi Quentin,
>>> 
>>> Thanks for taking a look.  I would greatly prefer a one line change :)  My original motivation for looking into this was a case involving XZR/WZR copys on aarch64, so I would like to catch those cases as well.                  I'll take a look into adding constant reg support to the register allocator unless you already know how you'd like to add that functionality.  Once we have that I can compare the two approaches to make sure there aren't more cases being missed.
>>> On 11/14/2016 9:37 PM, Quentin Colombet wrote:
>>>> Hi Geoff,
>>>> 
>>>> I had a closer look at the problem and in particular checked why the “hint fix-up” thing was not catching those cases.
>>>> The reason why it didn’t catch them is because we were doing the fix-up only for live-range that regalloc split not all missed hints.
>>>> Fixing that seems to solve your motivating cases (but the wzr case because this is a reserved reg and we don’t touch it, though I could improve the code to handle constant reg… let me know if you want me to have a look).
>>>> 
>>>> Could you try the attached patch and tell me how it works for you?
>>>> Assuming it solves the cases you wanted to catch, I’ll push it and we can drop that patch.
>>>> 
>>>> Cheers,
>>>> -Quentin
>>>> 
>>>> 
>>>>> On Nov 10, 2016, at 4:37 PM, Quentin Colombet <qcolombet at apple.com> <mailto:qcolombet at apple.com> wrote:
>>>>> 
>>>>> qcolombet added inline comments.
>>>>> 
>>>>> 
>>>>> ================
>>>>> Comment at: lib/CodeGen/VirtRegMap.cpp:158
>>>>> +// Map from physical regunit to list of def SlotIndexes.
>>>>> +using PhysDefsMap = DenseMap<unsigned, SmallVector<SlotIndex, 4>>;
>>>>> +
>>>>> ----------------
>>>>> For consistency with the rest of LLVM source code I would use typedef.
>>>>> 
>>>>> 
>>>>> ================
>>>>> Comment at: lib/CodeGen/VirtRegMap.cpp:158
>>>>> +// Map from physical regunit to list of def SlotIndexes.
>>>>> +using PhysDefsMap = DenseMap<unsigned, SmallVector<SlotIndex, 4>>;
>>>>> +
>>>>> ----------------
>>>>> qcolombet wrote:
>>>>>> For consistency with the rest of LLVM source code I would use typedef.
>>>>> Having read the code now, I believe it would be more efficient to map PhysReg to LI.
>>>>> It would greatly simplify add/removeMappedPhysReg.
>>>>> 
>>>>> 
>>>>> ================
>>>>> Comment at: lib/CodeGen/VirtRegMap.cpp:221
>>>>>   AU.addRequired<VirtRegMap>();
>>>>> +  AU.addRequired<MachineBlockFrequencyInfo>();
>>>>> +  AU.addPreserved<MachineBlockFrequencyInfo>();
>>>>> ----------------
>>>>> We could gate that dependency on DisableRenameCopys.
>>>>> That being said, if we move all that code in its own pass, that's not going to be a problem :).
>>>>> 
>>>>> 
>>>>> ================
>>>>> Comment at: lib/CodeGen/VirtRegMap.cpp:296
>>>>> +      bool FoundDef = false;
>>>>> +      for (auto DI = DefIndexes.begin(); DI != DefIndexes.end(); ++DI)
>>>>> +        if (*DI == DefIndex) {
>>>>> ----------------
>>>>> Seems inefficient to use a vector here.
>>>>> At the very least it should be sorted.
>>>>> 
>>>>> 
>>>>> ================
>>>>> Comment at: lib/CodeGen/VirtRegMap.cpp:336
>>>>> +                                    const TargetRegisterInfo &TRI,
>>>>> +                                    const VirtRegMap &VRM) {
>>>>> +
>>>>> ----------------
>>>>> Looks to me like this is reimplementing functionalities already available in the LiveRegMatrix class.
>>>>> Reuse that instead.
>>>>> 
>>>>> 
>>>>> ================
>>>>> Comment at: lib/CodeGen/VirtRegMap.cpp:428
>>>>> +    // Check the cost/benefit of renaming.
>>>>> +    BlockFrequency DefFreq = MBFI->getBlockFreq(MI->getParent());
>>>>> +    BlockFrequency Benefit(DefFreq);
>>>>> ----------------
>>>>> Wasn't the goal to propagate useless copies, i.e., that should always be beneficial, right?
>>>>> 
>>>>> If you want to do more, then please add test cases covering that. Indeed, as far as I can tell, the changes in the test cases only cover the simple case of propagating useless copies.
>>>>> 
>>>>> 
>>>>> ================
>>>>> Comment at: lib/CodeGen/VirtRegMap.cpp:491
>>>>> +    // reg so kill flags computation will be correct.
>>>>> +    assert(LI.getNumValNums() == 1);
>>>>> +    SlotIndex SrcDefPrevSlot = LI.getValNumInfo(0)->def.getPrevSlot();
>>>>> ----------------
>>>>> Add && "Msg" in the assert.
>>>>> 
>>>>> Also explain in the comment before the assert what would it take to extend that to multiple VNs.
>>>>> 
>>>>> 
>>>>> https://reviews.llvm.org/D25347 <https://reviews.llvm.org/D25347>
>>>>> 
>>>>> 
>>>>> 
>>> 
>>> -- 
>>> Geoff Berry
>>> Employee of Qualcomm Datacenter Technologies, Inc.
>>>  Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
>> 
> 
> -- 
> Geoff Berry
> Employee of Qualcomm Datacenter Technologies, Inc.
>  Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.

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