Commit Graph

345 Commits

Author SHA1 Message Date
Daniel Sanders 0c47611131 Apply llvm-prefer-register-over-unsigned from clang-tidy to LLVM
Summary:
This clang-tidy check is looking for unsigned integer variables whose initializer
starts with an implicit cast from llvm::Register and changes the type of the
variable to llvm::Register (dropping the llvm:: where possible).

Partial reverts in:
X86FrameLowering.cpp - Some functions return unsigned and arguably should be MCRegister
X86FixupLEAs.cpp - Some functions return unsigned and arguably should be MCRegister
X86FrameLowering.cpp - Some functions return unsigned and arguably should be MCRegister
HexagonBitSimplify.cpp - Function takes BitTracker::RegisterRef which appears to be unsigned&
MachineVerifier.cpp - Ambiguous operator==() given MCRegister and const Register
PPCFastISel.cpp - No Register::operator-=()
PeepholeOptimizer.cpp - TargetInstrInfo::optimizeLoadInstr() takes an unsigned&
MachineTraceMetrics.cpp - MachineTraceMetrics lacks a suitable constructor

Manual fixups in:
ARMFastISel.cpp - ARMEmitLoad() now takes a Register& instead of unsigned&
HexagonSplitDouble.cpp - Ternary operator was ambiguous between unsigned/Register
HexagonConstExtenders.cpp - Has a local class named Register, used llvm::Register instead of Register.
PPCFastISel.cpp - PPCEmitLoad() now takes a Register& instead of unsigned&

Depends on D65919

Reviewers: arsenm, bogner, craig.topper, RKSimon

Reviewed By: arsenm

Subscribers: RKSimon, craig.topper, lenary, aemerson, wuzish, jholewinski, MatzeB, qcolombet, dschuff, jyknight, dylanmckay, sdardis, nemanjai, jvesely, wdng, nhaehnle, sbc100, jgravelle-google, kristof.beyls, hiraditya, aheejin, kbarton, fedor.sergeev, javed.absar, asb, rbar, johnrusso, simoncook, apazos, sabuasal, niosHD, jrtc27, MaskRay, zzheng, edward-jones, atanasyan, rogfer01, MartinMosbeck, brucehoult, the_o, tpr, PkmX, jocewei, jsji, Petar.Avramovic, asbirlea, Jim, s.egerton, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65962

llvm-svn: 369041
2019-08-15 19:22:08 +00:00
Jinsong Ji 9fd81dc139 [PowerPC] Use xxleqv to set all one vector IMM(-1).
Summary:
xxspltib/vspltisb are 3 cycle PM instructions,
xxleqv is 2 cycle ALU instruction.

We should use xxleqv to set all one vectors.

Reviewers: hfinkel, nemanjai, steven.zhang

Subscribers: hiraditya, kbarton, MaskRay, shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65529

llvm-svn: 369006
2019-08-15 14:32:51 +00:00
Daniel Sanders 2bea69bf65 Finish moving TargetRegisterInfo::isVirtualRegister() and friends to llvm::Register as started by r367614. NFC
llvm-svn: 367633
2019-08-01 23:27:28 +00:00
Kai Luo 5c8af53806 [PowerPC][NFC] Added `getDefMIPostRA` method
Summary:
In PostRA phase, we often have to find out the most recent definition
of a register.  This patch adds getDefMIPostRA so that other methods
can use it rather than implementing it repeatedly.

Differential Revision: https://reviews.llvm.org/D65131

llvm-svn: 366990
2019-07-25 07:47:52 +00:00
Chen Zheng 8b7e82be12 [PowerPC][NFC] use opcode instead of MachineInstr for instrHasImmForm().
llvm-svn: 366867
2019-07-24 04:50:23 +00:00
Jason Liu 8dd563ef4b [NFC][PowerPC]Change ADDIStocHA to ADDIStocHA8 to follow 64-bit naming convention
Summary:

Since we are planning to add ADDIStocHA for 32bit in later patch, we decided
 to change 64bit one first to follow naming convention with 8 behind opcode.

Patch by: Xiangling_L

Differential Revision: https://reviews.llvm.org/D64814

llvm-svn: 366731
2019-07-22 19:55:33 +00:00
Justin Hibbits 0257c6b659 PowerPC: Fix register spilling for SPE registers
Summary:
Missed in the original commit, use the correct callee-saved register
list for spilling, instead of the standard SVR432 list.  This avoids
needlessly spilling the SPE non-volatile registers when they're not used.

As part of this, also add where missing, and sort, the spill opcode
checks for SPE and SPE4 register classes.

Reviewers: nemanjai, hfinkel, joerg

Subscribers: kbarton, jsji, llvm-commits

Differential Revision: https://reviews.llvm.org/D56703

llvm-svn: 366319
2019-07-17 12:30:48 +00:00
Fangrui Song b251cc0d91 Delete dead stores
llvm-svn: 365903
2019-07-12 14:58:15 +00:00
Chen Zheng 25ab27e6ef [PowerPC][NFC] remove redundant function isVFReg().
llvm-svn: 365429
2019-07-09 02:48:30 +00:00
QingShan Zhang 7fdb3a293b [PowerPC] Implement the areMemAccessesTriviallyDisjoint hook
After implemented this hook, we will model the memory dependency in the scheduling dependency graph more precise,
and will have more opportunity to reorder the load/stores, as they didn't have the dependency at some condition

Differential Revision: https://reviews.llvm.org/D63804

llvm-svn: 364886
2019-07-02 03:28:52 +00:00
Matt Arsenault e3a676e9ad CodeGen: Introduce a class for registers
Avoids using a plain unsigned for registers throughoug codegen.
Doesn't attempt to change every register use, just something a little
more than the set needed to build after changing the return type of
MachineOperand::getReg().

llvm-svn: 364191
2019-06-24 15:50:29 +00:00
Jinsong Ji ef2d6d99c0 [PowerPC] Enable MachinePipeliner for P9 with -ppc-enable-pipeliner
Implement necessary target hooks to enable MachinePipeliner for P9 only.
The pass is off by default, can be enabled with -ppc-enable-pipeliner for P9.

Differential Revision: https://reviews.llvm.org/D62164

llvm-svn: 363085
2019-06-11 17:40:39 +00:00
Nemanja Ivanovic bad43d8f49 [PowerPC] Look through copies for compare elimination
We currently miss the opportunities for optmizing comparisons in the peephole
optimizer if the input is the result of a COPY since we look for record-form
versions of the producing instruction.

This patch simply lets the optimization peek through copies.

Differential revision: https://reviews.llvm.org/D59633

llvm-svn: 362438
2019-06-03 19:09:15 +00:00
Nick Desaulniers 7fcad2f171 [PowerPC] check for INLINEASM_BR along w/ INLINEASM
Summary:
It looks like since INLINEASM_BR was created off of INLINEASM (r353563),
a few checks for INLINEASM needed to be updated to check for either
case.

pr/41999

Reviewers: hfinkel

Reviewed By: hfinkel

Subscribers: nemanjai, hiraditya, kbarton, jsji, llvm-commits, craig.topper, srhines

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62403

llvm-svn: 362278
2019-05-31 23:02:13 +00:00
Stefan Pintilie 522307fa40 [PowerPC] Remove CRBits Copy Of Unset/set CBit
For the situation, where we generate the following code:

       crxor 8, 8, 8
       < Some instructions>
.LBB0_1:
       < Some instructions>
       cror 1, 8, 8

cror (COPY of CRbit) depends on the result of the crxor instruction.
CR8 is known to be zero as crxor is equivalent to CRUNSET. We can simply use
crxor 1, 1, 1 instead to zero out CR1, which does not have any dependency on
any previous instruction.

This patch will optimize it to:

        < Some instructions>
.LBB0_1:
        < Some instructions>
        cror 1, 1, 1

Patch By: Victor Huang (NeHuang)

Differential Revision: https://reviews.llvm.org/D62044

llvm-svn: 361632
2019-05-24 12:05:37 +00:00
Jinsong Ji 9dc2c1d564 Set useful flags for vector imm setting instructions
Vector imm setting instructions like XXLXORz/XXLXORspz/XXLXORdpz
Should behave like LI8.

We should set corresponding flags to allow rematerialization and other
opts in LICM, RA, Scheduling etc.

Differential Revision: https://reviews.llvm.org/D58645

llvm-svn: 355948
2019-03-12 18:27:09 +00:00
Jinsong Ji 06bee01d2b [NFC][PowerPC]Assert when trying to generate directmove below P8.
This was found when we generated COPY from G8RC to F8RC in
EmitInstrWithCustomInserter without checking proper architecture,
we silently generated mtvsrd, which require P8 and up.

This is a NFC patch to add assert when we call copyPhysReg, in case
someone accidentally generate COPY between G8RC to F8RC for P7 and
below.

llvm-svn: 355920
2019-03-12 14:01:29 +00:00
Chen Zheng 9cfe7e81f1 [PowerPC] fix killed/dead flag after convert x-form to d-form tranformation.
Differential Revision: https://reviews.llvm.org/D58428

llvm-svn: 355378
2019-03-05 04:56:54 +00:00
Stefan Pintilie bd5429ef38 [PowerPC] Move the stack pointer update instruction later in the prologue and earlier in the epilogue.
Move the stdu instruction in the prologue and epilogue.
This should provide a small performance boost in functions that are able to do
this. I've kept this change rather conservative at the moment and functions
with frame pointers or base pointers will not try to move the stack pointer
update.

Differential Revision: https://reviews.llvm.org/D42590

llvm-svn: 355085
2019-02-28 12:23:28 +00:00
Chen Zheng be589423d8 [PowerPC] delete no more needed workaround for readsRegister() in PowerPC
Differential Revision: https://reviews.llvm.org/D57439

llvm-svn: 352689
2019-01-30 23:18:38 +00:00
Chen Zheng ca26039cc7 [PowerPC] more opportunity for converting reg+reg to reg+imm
Differential Revision: https://reviews.llvm.org/D57314

llvm-svn: 352583
2019-01-30 01:57:01 +00:00
Chandler Carruth 2946cd7010 Update the file headers across all of the LLVM projects in the monorepo
to reflect the new license.

We understand that people may be surprised that we're moving the header
entirely to discuss the new license. We checked this carefully with the
Foundation's lawyer and we believe this is the correct approach.

Essentially, all code in the project is now made available by the LLVM
project under our new license, so you will see that the license headers
include that license only. Some of our contributors have contributed
code under our old license, and accordingly, we have retained a copy of
our old license notice in the top-level files in each project and
repository.

llvm-svn: 351636
2019-01-19 08:50:56 +00:00
QingShan Zhang f2d9df61c7 [PowerPC] Remove the implicit use of the register if it is replaced by Imm
If we are changing the MI operand from Reg to Imm, we need also handle its implicit use if have.

Differential Revision: https://reviews.llvm.org/D56078

llvm-svn: 350115
2018-12-28 03:38:09 +00:00
Chen Zheng 5ede950df9 [PowerPC] fix register class after converting X-FORM instruction to D-FORM instruction
Differential Revision: https://reviews.llvm.org/D55806

llvm-svn: 350111
2018-12-28 01:02:35 +00:00
Kewen Lin 44ace92596 [PowerPC] Exploit power9 new instruction setb
Check the expected pattens feeding to SELECT_CC like:
   (select_cc lhs, rhs,  1, (sext (setcc [lr]hs, [lr]hs, cc2)), cc1)
   (select_cc lhs, rhs, -1, (zext (setcc [lr]hs, [lr]hs, cc2)), cc1)
   (select_cc lhs, rhs,  0, (select_cc [lr]hs, [lr]hs,  1, -1, cc2), seteq)
   (select_cc lhs, rhs,  0, (select_cc [lr]hs, [lr]hs, -1,  1, cc2), seteq)
Further transform the sequence to comparison + setb if hits.

Differential Revision: https://reviews.llvm.org/D53275

llvm-svn: 349445
2018-12-18 07:53:26 +00:00
Zi Xuan Wu cf4d477b0b [PowerPC] Fix assert from machine verify pass that missing undef register flag
Fix assert about using an undefined physical register in machine instruction verify pass. 
The reason is that register flag undef is missing when doing transformation from If Conversion Pass.

```
Bad machine code: Using an undefined physical register 
- function:    func_65
- basic block: %bb.0 entry (0x10024740738)
- instruction: BCLR killed $cr5lt, implicit $lr8, implicit $rm, implicit undef $x3
- operand 0:   killed $cr5lt
LLVM ERROR: Found 1 machine code errors.
```

There are also other existing testcases with same issue. So I add -verify-machineinstrs option to open verifying.

Differential Revision: https://reviews.llvm.org/D55408

llvm-svn: 348566
2018-12-07 05:25:16 +00:00
Kang Zhang 51986417f9 [PowerPC] Fix inconsistent ImmMustBeMultipleOf for same instruction
Summary:
There are 4 instructions which have Inconsistent ImmMustBeMultipleOf in the
function PPCInstrInfo::instrHasImmForm, they are LFS, LFD, STFS, STFD.
These four instructions should set the ImmMustBeMultipleOf to 1 instead of 4.

Reviewed By: steven.zhang

Differential Revision: https://reviews.llvm.org/D54738

llvm-svn: 348109
2018-12-03 03:32:57 +00:00
Kang Zhang 840e98f9f1 Revert "[PowerPC] Fix inconsistent ImmMustBeMultipleOf for same instruction"
This reverts commits r347532. Forget add the option 
-mtriple powerpc64-unknown-linux-gnu. So other platform is error except
for PowerPC.

llvm-svn: 347534
2018-11-26 07:15:31 +00:00
Kang Zhang e98d4f511c [PowerPC] Fix inconsistent ImmMustBeMultipleOf for same instruction
Summary:
There are 4 instructions which have Inconsistent ImmMustBeMultipleOf in the
function PPCInstrInfo::instrHasImmForm, they are LFS, LFD, STFS, STFD.
These four instructions should set the ImmMustBeMultipleOf to 1 instead of 4.

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D54738

llvm-svn: 347532
2018-11-26 06:03:25 +00:00
Nemanja Ivanovic 674581afbb [PowerPC][NFC] Fix bugs in r+r to r+i conversion
The D-Form VSX loads introduced in ISA 3.0 are not direct D-Form equivalent of
the corresponding X-Forms since they only target the Altivec registers.
Namely LXSSPX can load into any of the 64 VSX registers whereas LXSSP can only
load into the upper 32 VSX registers. Similarly with the remaining affected
instructions.

There is currently no way that I can see to trigger the bug, but as we add other
ways of exploiting these instructions, there may very well be instances that do.

This is an NFC patch in practical terms since the changes it introduces can not
be triggered without an MIR test.

Differential revision: https://reviews.llvm.org/D53323

llvm-svn: 344894
2018-10-22 11:22:59 +00:00
Stefan Pintilie 5d32a86f44 [PowerPC] Folding XForm to DForm loads requires alignment for some DForm loads.
Going from XForm Load to DSForm Load requires that the immediate be 4 byte
aligned.
If we are not aligned we must leave the load as LDX (XForm).
This bug is causing a compile-time failure in the benchmark h264ref.

Differential Revision: https://reviews.llvm.org/D51988

llvm-svn: 343525
2018-10-01 20:16:27 +00:00
Nemanja Ivanovic 87c31a6113 [PowerPC] Do not emit record-form rotates when record-form andi/andis suffices
This is a follow-up to the previous patch that eliminated some of the rotates.
With this addition, we will also emit the record-form andis.

This patch increases the number of record-form rotates we eliminate by
more than 70%.

Differential revision: https://reviews.llvm.org/D44897

llvm-svn: 342478
2018-09-18 13:43:16 +00:00
Nemanja Ivanovic 6a39d32e66 [PowerPC] Optimize compares fed by ANDISo
Both ANDIo and ANDISo (and the 64-bit versions) are record-form instructions.
When optimizing compares, we handle the former in order to eliminate the compare
instruction but not the latter. This patch just adds the latter to the set of
instructions we optimize.
The reason these instructions need to be handled separately is that they are not
part of the RecFormRel map (since they don't have a non-record-form). The
missing "and-immediate-shifted" is just an oversight in the initial
implementation.

Differential revision: https://reviews.llvm.org/D51353

llvm-svn: 342472
2018-09-18 13:21:58 +00:00
Stefan Pintilie f384606799 [PowerPC] Emit xscpsgndp instead of xxlor when copying floating point scalar registers for P9
This patch will address using the xscpsgndp instruction to copy floating point
scalar registers instead of the xxlor (specifically XXLORf) instruction that is
currently used. Additionally, this patch of utilizing xscpsgndp will apply to
P9, while pre-P9 will still use xxlor.

Patch by amyk

Differential Revision: https://reviews.llvm.org/D50004

llvm-svn: 340643
2018-08-24 20:00:24 +00:00
QingShan Zhang f8f9af7ba5 [PowerPC] Add a peephole post RA to transform the inst that fed by add
If the arch is P8, we will select XFLOAD to load the floating point, and then, expand it to vsx and non-vsx X-form instruction post RA. This patch is trying to convert the X-form to D-form if it meets the requirement that one operand of the x-form inst is the special Zero register, and another operand fed by add inst. i.e.
y = add imm, reg
LFDX. 0, y
-->
LFD imm(reg)

Reviewers: Nemanjai
Differential Revision: https://reviews.llvm.org/D49007

llvm-svn: 340149
2018-08-20 02:52:55 +00:00
Fangrui Song f78650a8de Remove trailing space
sed -Ei 's/[[:space:]]+$//' include/**/*.{def,h,td} lib/**/*.{cpp,h}

llvm-svn: 338293
2018-07-30 19:41:25 +00:00
Justin Hibbits d52990c71b Introduce codegen for the Signal Processing Engine
Summary:
The Signal Processing Engine (SPE) is found on NXP/Freescale e500v1,
e500v2, and several e200 cores.  This adds support targeting the e500v2,
as this is more common than the e500v1, and is in SoCs still on the
market.

This patch is very intrusive because the SPE is binary incompatible with
the traditional FPU.  After discussing with others, the cleanest
solution was to make both SPE and FPU features on top of a base PowerPC
subset, so all FPU instructions are now wrapped with HasFPU predicates.

Supported by this are:
* Code generation following the SPE ABI at the LLVM IR level (calling
conventions)
* Single- and Double-precision math at the level supported by the APU.

Still to do:
* Vector operations
* SPE intrinsics

As this changes the Callee-saved register list order, one test, which
tests the precise generated code, was updated to account for the new
register order.

Reviewed by: nemanjai
Differential Revision: https://reviews.llvm.org/D44830

llvm-svn: 337347
2018-07-18 04:25:10 +00:00
Nemanja Ivanovic 080c35050e [PowerPC] Materialize more constants with CR-field set in late peephole
Revision r322373 fixed a bug in how we materialize constants when the CR-field
needs to be set.

However the fix is overly conservative. It will only do the transform if
AND-ing the input with the new constant produces the same new constant.
This is of course correct, but not necessarily required.

If there are no futher uses of the constant, the constant can be changed.
If there are no uses of the GPR result, the final result of the materialization
isn't important other than it needs to compare to zero correctly (lt, gt, eq).

Differential revision: https://reviews.llvm.org/D42109

llvm-svn: 337008
2018-07-13 15:21:03 +00:00
QingShan Zhang 9f0fe9a3f8 If the arch is P9, we will select the DFLOADf32/DFLOADf64 pseudo instruction when we are loading a floating,
and expand it post RA basing on the register pressure. However, we miss to do the add-imm peephole for these pseudo instruction.

Differential Revision: https://reviews.llvm.org/D47568
Reviewed By: Nemanjai

llvm-svn: 335024
2018-06-19 06:54:51 +00:00
Hiroshi Inoue 0f7f59f073 [PowerPC] fix trivial typos in comment, NFC
llvm-svn: 334583
2018-06-13 08:54:13 +00:00
Nicola Zaghen d34e60ca85 Rename DEBUG macro to LLVM_DEBUG.
The DEBUG() macro is very generic so it might clash with other projects.
The renaming was done as follows:
- git grep -l 'DEBUG' | xargs sed -i 's/\bDEBUG\s\?(/LLVM_DEBUG(/g'
- git diff -U0 master | ../clang/tools/clang-format/clang-format-diff.py -i -p1 -style LLVM
- Manual change to APInt
- Manually chage DOCS as regex doesn't match it.

In the transition period the DEBUG() macro is still present and aliased
to the LLVM_DEBUG() one.

Differential Revision: https://reviews.llvm.org/D43624

llvm-svn: 332240
2018-05-14 12:53:11 +00:00
Nemanja Ivanovic c564dc060a [PowerPC] Fix condition for 64-bit rotate when replacing r+r instr with r+i
This patch fixes https://bugs.llvm.org/show_bug.cgi?id=37039
The condition only covers one of the two 64-bit rotate instructions. This just
adds the second (RLDICLo).

Patch by Josh Stone.

llvm-svn: 329852
2018-04-11 21:25:44 +00:00
Stefan Pintilie 26d4f923c4 [PowerPC] Infrastructure work. Implement getting the opcode for a spill in one place.
A new function getOpcodeForSpill should now be the only place to get
the opcode for a given spilled register.

Differential Revision: https://reviews.llvm.org/D43086

llvm-svn: 328556
2018-03-26 17:39:18 +00:00
Zaara Syeda 6535993625 Re-commit: [MachineLICM] Add functions to MachineLICM to hoist invariant stores
This patch adds functions to allow MachineLICM to hoist invariant stores.
Currently, MachineLICM does not hoist any store instructions, however
when storing the same value to a constant spot on the stack, the store
instruction should be considered invariant and be hoisted. The function
isInvariantStore iterates each operand of the store instruction and checks
that each register operand satisfies isCallerPreservedPhysReg. The store
may be fed by a copy, which is hoisted by isCopyFeedingInvariantStore.
This patch also adds the PowerPC changes needed to consider the stack
register as caller preserved.

Differential Revision: https://reviews.llvm.org/D40196

llvm-svn: 328326
2018-03-23 15:28:15 +00:00
Zaara Syeda 01f414baaa Revert [MachineLICM] This reverts commit rL327856
Failing build bots. Revert the commit now.

llvm-svn: 327864
2018-03-19 16:19:44 +00:00
Zaara Syeda ff05e2b0e6 [MachineLICM] Add functions to MachineLICM to hoist invariant stores
This patch adds functions to allow MachineLICM to hoist invariant stores.
Currently, MachineLICM does not hoist any store instructions, however
when storing the same value to a constant spot on the stack, the store
instruction should be considered invariant and be hoisted. The function
isInvariantStore iterates each operand of the store instruction and checks
that each register operand satisfies isCallerPreservedPhysReg. The store
may be fed by a copy, which is hoisted by isCopyFeedingInvariantStore.
This patch also adds the PowerPC changes needed to consider the stack
register as caller preserved.

Differential Revision: https://reviews.llvm.org/D40196

llvm-svn: 327856
2018-03-19 14:52:25 +00:00
Nemanja Ivanovic 6cc31ca814 [PowerPC] Do not emit record-form rotates when record-form andi suffices
Up until Power9, the performance profile for rlwinm., rldicl. and andi. looked
more or less equivalent. However with Power9, the rotates are still 2-way
cracked whereas the and-immediate is not.

This patch just ensures that we don't emit record-form rotates when an andi.
is adequate.

As first pointed out by Carrot in https://bugs.llvm.org/show_bug.cgi?id=30833
(this patch is a fix for that PR).

Differential Revision: https://reviews.llvm.org/D43977

llvm-svn: 326736
2018-03-05 19:27:16 +00:00
Benjamin Kramer 309124e0b1 [PowerPC] Don't miscompile rotate+mask into an ANDIo if it can't recreate the immediate
I'm not even sure if this transform is ever worth it, but this at least
stops the bleeding.

llvm-svn: 322373
2018-01-12 15:03:24 +00:00
Nemanja Ivanovic 4e1f5e0734 [PowerPC] Fix for PR35688 - handle out-of-range values for r+r to r+i conversion
Revision 320791 introduced a pass that transforms reg+reg instructions to
reg+imm if they're fed by "load immediate". However, it didn't
handle out-of-range shifts correctly as reported in PR35688.
This patch fixes that and therefore the PR.

Furthermore, there was undefined behaviour in the patch where the RHS of an
initialization expression was 32 bits and constant `1` was shifted left 32
bits. This was fixed by ensuring the RHS is 64 bits just like the LHS.

Differential Revision: https://reviews.llvm.org/D41369

llvm-svn: 321551
2017-12-29 12:22:27 +00:00
Matthias Braun f1caa2833f MachineFunction: Return reference from getFunction(); NFC
The Function can never be nullptr so we can return a reference.

llvm-svn: 320884
2017-12-15 22:22:58 +00:00