Commit Graph

53158 Commits

Author SHA1 Message Date
Sam Parker
414dd1c946 [NFC][ARM[ParallelDSP] Cleanup of BinOpChain
- Remove some unused typedefs.
- Rename BinOpChain struct to MulCandidate.
- Remove the size method of MulCandidate.
- Store only the first input of the ValueList provided to
  MulCandidate, as it's the only value we care about. This means we
  don't have to perform any ugly (and unnecessary) iterations of the 
  list later on.

llvm-svn: 367208
2019-07-29 08:41:51 +00:00
David Stuttard
20235ef3e7 [AMDGPU] Enable v4f16 and above for v_pk_fma instructions
Summary:
If isel is presented with <2 x half> vectors then it will correctly select
v_pk_fma style instructions.
If isel is presented with e.g. <4 x half> vectors it will scalarize, unlike for
other instruction types (such as fadd, fmul etc.)

Added extra support to enable this. Updated one of the tests to include a test
for this (as well as extending the test to GFX9)

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, tpr, t-tye, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65325

Change-Id: I50a4577a3f8223fb53992af3b7d26121f65b71ee
llvm-svn: 367206
2019-07-29 08:15:10 +00:00
Sam Parker
8538060103 [NFC][ARM][ParallelDSP] Remove AreSymmetrical
We explicitly search for a parallel mac and we only care about its
inputs, checking for symmetry doesn't add anything here.

llvm-svn: 367205
2019-07-29 08:12:24 +00:00
Sam Parker
11ad33ede6 [NFC][ARM][ParallelDSP] Remove PopulateLoads
We no longer have to check what loads are used, all this
is performed at the start of the transform, so it's not
doing anything now.

llvm-svn: 367204
2019-07-29 08:07:23 +00:00
Craig Topper
eb1beabad9 [X86] Don't use PMADDWD for vector add reductions of multiplies if the mul inputs have an additional user.
The pmaddwd inserts a truncate, if that truncate would end up
creating additional instructions instead of making a zext
narrower, then we shouldn't do it.

I've restricted this to only sse4.1 targets since on prior
targets the zext will be done in stages. So the truncate will
probably not create additional instructions. Might need some
more investigation of mul shrinking and the other pmaddwd
transform to be sure this is the right decision.

There might be a slight regression on AVX1 targets due to add
splitting. Hard to say for sure. Maybe we need to look into
using the vector reduction flag to use 2 narrow loads and a
blend instead of extracting and inserting.

llvm-svn: 367198
2019-07-29 01:36:58 +00:00
Craig Topper
894916cac9 [X86] In combineLoopMAddPattern and combineLoopSADPattern, preserve the vector reduction flag on the final add. Handle unrolled loops by letting DAG combine revisit.
This reverts r340478 and r340631 and replaces them with a simpler
method of just letting DAG combine revisit the nodes to handle
the other operand.

llvm-svn: 367195
2019-07-28 18:45:42 +00:00
David Green
b8b8b46a51 [ARM] MVE VPNOT
This adds the patterns required to transform xor P0, -1 to a VPNOT. The
instruction operands have to change a little for this, adding an in and an out
VCCR reg and using a custom DecodeMVEVPNOT for the decode.

Differential Revision: https://reviews.llvm.org/D65133

llvm-svn: 367192
2019-07-28 14:07:48 +00:00
David Green
9cf344e739 [ARM] Better patterns for fp <> predicate vectors
These are some better patterns for converting between predicates and floating
points. Much like the extends, we select "1"/"-1" or "0" depending on the
predicate value. Or we perform a compare against 0 to convert to a predicate.

Differential Revision: https://reviews.llvm.org/D65103

llvm-svn: 367191
2019-07-28 13:53:39 +00:00
Simon Pilgrim
353a848473 [X86][SSE] Replace PMULDQ GetDemandedBits combine with SimplifyMultipleUseDemandedBits handler (Reapplied)
Recommit rL367100 which was reverted at rL367141. Until PR42777 is fixed, we no longer get the benefits of peeking through bitcasts but it does still remove a GetDemandedBits user and gives us the equivalent combines.

llvm-svn: 367172
2019-07-27 13:30:29 +00:00
Amara Emerson
7bc4fad0fb [AArch64][GlobalISel] Implement narrowing of G_SEXT.
We need this to narrow a sext to s128.

Differential Revision: https://reviews.llvm.org/D65357

llvm-svn: 367164
2019-07-26 23:46:38 +00:00
Jessica Paquette
aa8b9993c2 [AArch64][GlobalISel] Select @llvm.aarch64.stlxr for 32-bit pointers
Add partial instruction selection for intrinsics like this:

```
declare i32 @llvm.aarch64.stlxr(i64, i32*)
```

(This only handles the case where a G_ZEXT is feeding the intrinsic.)

Also make sure that the added store instruction actually has the memory op from
the original G_STORE.

Update select-stlxr-intrin.mir and arm64-ldxr-stxr.ll.

Differential Revision: https://reviews.llvm.org/D65355

llvm-svn: 367163
2019-07-26 23:28:53 +00:00
Vlad Tsyrklevich
485b8789de Revert "[X86][SSE] Replace PMULDQ GetDemandedBits combine with SimplifyMultipleUseDemandedBits handler."
This reverts r367100, it appears to be causing test failures after
Nico's revert of r367091.

llvm-svn: 367141
2019-07-26 18:14:21 +00:00
Sean Fertile
9df6177d38 [PowerPC][AIX]Add lowering of MCSymbol MachineOperand.
Adds machine operand lowering for MCSymbolSDNodes to the PowerPC
backend. This is needed to produce call instructions in assembly for AIX
because the callee operand is a MCSymbolSDNode. The test is XFAIL'ed for
asserts due to a (valid) assertion in PEI that the AIX ABI isn't supported yet.

Differential Revision: https://reviews.llvm.org/D63738

llvm-svn: 367133
2019-07-26 17:25:27 +00:00
Michael Liao
711556e6a8 [AMDGPU] Fix typo.
llvm-svn: 367131
2019-07-26 17:13:59 +00:00
Cullen Rhodes
2cde8b5db6 [AArch64][SVE2] Rename bitperm feature to sve2-bitperm
Summary:
The bitperm feature flag is now prefixed with SVE2, as it is for all other SVE2
extensions

Patch by Maciej Gabka.

Reviewers: sdesmalen, rovka, chill, SjoerdMeijer, rengolin

Reviewed By: SjoerdMeijer, rengolin

Differential Revision: https://reviews.llvm.org/D65327

llvm-svn: 367124
2019-07-26 15:57:50 +00:00
Sam Parker
3da59e5513 [ARM][ParallelDSP] Combine structs
Combine OpChain and BinOpChain structs as OpChain is a base class to
BinOpChain that is never used.

llvm-svn: 367114
2019-07-26 14:11:40 +00:00
Sean Fertile
9bd22fec0d [PowerPC] Add getCRSaveOffset to improve readability. [NFC]
In preperation for AIX support in FrameLowering: replace a number of literal
'8' that represent the stack offset of the condition register save area with
a member in PPCFrameLowering.

Patch by Chris Bowler.

llvm-svn: 367111
2019-07-26 14:02:17 +00:00
Petar Avramovic
cf21794566 [MIPS GlobalISel] Fix check for void return during lowerCall
Void return used to have unsigned with value 0 for virtual register
but with addition of Register class and changes to arguments to lowerCall
this is no longer valid.
Check for void return by inspecting the Ty field in OrigRet.

Differential Revision: https://reviews.llvm.org/D65321

llvm-svn: 367107
2019-07-26 13:19:37 +00:00
Carl Ritson
0b28357053 [AMDGPU] Move WQM/WWM intrinsic instruction selection to AMDGPUISelDAGToDAG
Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65328

llvm-svn: 367105
2019-07-26 13:11:44 +00:00
Petar Avramovic
b1fc6f6130 [MIPS GlobalISel] Select inttoptr and ptrtoint
Select G_INTTOPTR and G_PTRTOINT for MIPS32.

Differential Revision: https://reviews.llvm.org/D65217

llvm-svn: 367104
2019-07-26 13:08:06 +00:00
Simon Pilgrim
d93e8ece7b [X86][SSE] Replace PMULDQ GetDemandedBits combine with SimplifyMultipleUseDemandedBits handler.
This removes a GetDemandedBits user and allows us to benefit from the DemandedElts propagated through SimplifyDemandedBits.

llvm-svn: 367100
2019-07-26 11:10:20 +00:00
Sam Parker
7440065bd8 [NFC][ARM][ParallelDSP] Cleanup isNarrowSequence
Remove unused logic.

llvm-svn: 367099
2019-07-26 10:57:42 +00:00
Carl Ritson
00e89b428b [AMDGPU] Add llvm.amdgcn.softwqm intrinsic
Add llvm.amdgcn.softwqm intrinsic which behaves like llvm.amdgcn.wqm
only if there is other WQM computation in the shader.

Reviewers: nhaehnle, tpr

Reviewed By: nhaehnle

Subscribers: arsenm, kzhuravl, jvesely, wdng, yaxunl, dstuttard, t-tye, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64935

llvm-svn: 367097
2019-07-26 09:54:12 +00:00
Momchil Velikov
898d953693 [AArch64] Define ETE and TRBE system registers
Embedded Trace Extension and Trace Buffer Extension are optional
future architecture extensions.
(cf. https://developer.arm.com/architectures/cpu-architecture/a-profile/exploration-tools)

Their system registers are documented here:
https://developer.arm.com/docs/ddi0601/a

ETE shares register names with ETM. One exception is the ETE
TRCEXTINSELR0 register, which has the same encoding as the ETM
TRCEXTINSELR register (but different semantics). This patch treats
them as aliases: the assembler will accept both names, emitting
identical encoding, and the disassembler will keep disassembling
to TRCEXRINSELR.

Differential Revision: https://reviews.llvm.org/D63707

llvm-svn: 367093
2019-07-26 09:19:08 +00:00
Sam Parker
c760b5da11 [ARM][LowOverheadLoops] Add CPSR defs
Both WhileLoopStart and LoopEnd may get turned into a cmp and br pair,
so add an implicit def to these pseudo instructions in case that WLS
and LE aren't generated.

Differential Revision: https://reviews.llvm.org/D65275

llvm-svn: 367089
2019-07-26 08:15:01 +00:00
Pengfei Wang
9ad565f70e [WinEH] Allocate space in funclets stack to save XMM CSRs
Summary:
This is an alternate approach to D57970.
Currently funclets reuse the same stack slots that are used in the
parent function for saving callee-saved xmm registers. If the parent
function modifies a callee-saved xmm register before an excpetion is
thrown, the catch handler will overwrite the original saved value.

This patch allocates space in funclets stack for saving callee-saved xmm
registers and uses RSP instead RBP to access memory.

Reviewers: andrew.w.kaylor, LuoYuanke, annita.zhang, craig.topper,
RKSimon

Subscribers: rnk, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63396

Signed-off-by: pengfei <pengfei.wang@intel.com>
llvm-svn: 367088
2019-07-26 07:33:15 +00:00
Matt Arsenault
a9ea8a9aae AMDGPU/GlobalISel: Handle most function return types
handleAssignments gives up pretty easily on structs, and i8 values for
some reason. The other case that doesn't work is when an implicit sret
needs to be inserted if the return size exceeds the number of return
registers.

llvm-svn: 367082
2019-07-26 02:36:05 +00:00
Amara Emerson
c07fe307b4 [AArch64][GlobalISel] Simplify zext/sext selection, use MachineIRBuilder. NFC.
llvm-svn: 367075
2019-07-26 00:01:09 +00:00
Yonghong Song
329abf2939 [BPF] fix typedef issue for offset relocation
Currently, the CO-RE offset relocation does not work
if any struct/union member or array element is a typedef.
For example,
  typedef const int arr_t[7];
  struct input {
      arr_t a;
  };
  func(...) {
       struct input *in = ...;
       ... __builtin_preserve_access_index(&in->a[1]) ...
  }
The BPF backend calculated default offset is 0 while
4 is the correct answer. Similar issues exist for struct/union
typedef's.

When getting struct/union member or array element type,
we should trace down to the type by skipping typedef
and qualifiers const/volatile as this is what clang did
to generate getelementptr instructions.
(const/volatile member type qualifiers are already
ignored by clang.)

This patch fixed this issue, for each access index,
skipping typedef and const/volatile/restrict BTF types.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D65259

llvm-svn: 367062
2019-07-25 21:47:27 +00:00
Amara Emerson
e54dc6b8b5 [AArch64][GlobalISel] Fix G_SELECT legalization fallback after r366943.
Changes the order of legalization of G_ICMP suggested by Petar in D65079.

llvm-svn: 367060
2019-07-25 21:44:52 +00:00
Yonghong Song
d8efec97be [BPF] fix CO-RE incorrect index access string
Currently, we expect the CO-RE offset relocation records
a string encoding the original getelementptr access index,
so kernel bpf loader can decode it correctly.

For example,
  struct s { int a; int b; };
  struct t { int c; int d; };
  #define _(x) (__builtin_preserve_access_index(x))
  int get_value(const void *addr1, const void *addr2);
  int test(struct s *arg1, struct t *arg2) {
    return get_value(_(&arg1->b), _(&arg2->d));
  }

We expect two offset relocations:
  reloc 1: type s, access index 0, 1
  reloc 2: type t, access index 0, 1

Two globals are created to retain access indexes for the
above two relocations with global variable names.
The first global has a name "0:1:". Unfortunately,
the second global has the name "0:1:.1" as the llvm
internals automatically add suffix ".1" to a global
with the same name. Later on, the BPF peels the last
character and record "0:1" and "0:1:." in the
relocation table.

This is not desirable. BPF backend could use the global
variable suffix knowledge to generate correct access str.
This patch rather took an approach not relying on
that knowledge. It generates "s:0:1:" and "t:0:1:" to
avoid global variable suffixes and later on generate
correct index access string "0:1" for both records.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D65258

llvm-svn: 367030
2019-07-25 16:01:26 +00:00
Michael Liao
53f967f2bd [AMDGPU] Run unreachable-mbb-elimination after isel to clean up PHIs.
Summary:
- As LCSSA is turned on just before isel, it may create PHI of the flow,
  which is consumed by pseudo structurized CFG instructions. When that
  PHIs are eliminated in O0, COPY may be placed wrongly as the these
  pseudo structurized CFG instructions are considering prologue of MBB.
- Run extra `unreachable-mbb-elimination` at the end of isel to clean up
  PHIs.

Reviewers: arsenm, rampitec

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64353

llvm-svn: 367023
2019-07-25 14:50:18 +00:00
Momchil Velikov
a655f476b0 [AArch64][SVE] Allow explicit size specifier for predicate operand
... for the vector forms of `{SQ,UQ,}{INC,DEC}P` instructions. Also continue
supporting the exsting behaviour of not requiring an explicit size
specifier. The preferred disasembly is *with* the specifier.

This is implemented by redefining intruction forms to require vector predicates
with explicit size and adding aliases, which allow a predicate with no size.

Differential Revision: https://reviews.llvm.org/D65145

llvm-svn: 367019
2019-07-25 13:56:04 +00:00
Matt Arsenault
a85af76c72 AMDGPU: Don't assert on v4f16 arguments to shader calling conventions
llvm-svn: 367018
2019-07-25 13:55:07 +00:00
Simon Pilgrim
447fe31964 [X86] concatSubVectors - remove unnecessary args. NFCI.
All these args can be cheaply recomputed and it makes it much easier to use the function as a quick helper.

llvm-svn: 367014
2019-07-25 13:05:46 +00:00
Pablo Barrio
275954539d [ARM][AArch64] Support for Cortex-A65 & A65AE, Neoverse E1 & N1
Summary:
Add support for Cortex-A65, Cortex-A65AE, Neoverse E1 and Neoverse N1.
Neoverse E1 and Cortex-A65(&AE) only implement the AArch64 state of the
Arm architecture. Neoverse N1 implements both AArch32 and AArch64.

Cortex-A65:
https://developer.arm.com/ip-products/processors/cortex-a/cortex-a65

Cortex-A65AE:
https://developer.arm.com/ip-products/processors/cortex-a/cortex-a65ae

Neoverse E1:
https://developer.arm.com/ip-products/processors/neoverse/neoverse-e1

Neoverse N1:
https://developer.arm.com/ip-products/processors/neoverse/neoverse-n1

Patch by Diogo Sampaio and Pablo Barrio

Reviewers: samparker, LukeCheeseman, sbaranga, ostannard

Reviewed By: ostannard

Subscribers: ostannard, javed.absar, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64406

llvm-svn: 367007
2019-07-25 10:59:45 +00:00
Kai Luo
985e52a4c1 [PowerPC][NFC] Make getDefMIPostRA public
llvm-svn: 366995
2019-07-25 08:36:44 +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
Seiya Nuta
21277e3ec2 [MC] Add MCInstrAnalysis::evaluateMemoryOperandAddress
Summary:
Add a new method which tries to compute the target address referenced by an operand.

This patch supports x86_64 RIP-relative addressing for now.

It is necessary to print referenced symbol names in llvm-objdump.

Reviewers: andreadb, MaskRay, grosbach, jgalenson, craig.topper

Reviewed By: MaskRay, craig.topper

Subscribers: bcain, rupprecht, jhenderson, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63847

llvm-svn: 366987
2019-07-25 06:57:09 +00:00
Eli Friedman
82e109279d [ARM] Remove dead code from ARMConstantIslands.
tLDRHi is not a pc-relative load; it can't directly refer to a
constant pool or jump table.

llvm-svn: 366963
2019-07-24 23:36:14 +00:00
Jessica Paquette
728b18f29f [AArch64][GlobalISel] Select immediate modes for ADD when selecting G_GEP
Before, we weren't able to select things like this for G_GEP:

add	x0, x8, #8

And instead we'd materialize the 8.

This teaches GISel to do that. It gives some considerable code size savings
on 252.eon-- about 4%!

Differential Revision: https://reviews.llvm.org/D65248

llvm-svn: 366959
2019-07-24 23:11:01 +00:00
Amara Emerson
de81bd0faa [AArch64][GlobalISel] Don't try to use GISel if subtarget doesn't have neon or fp.
Throughout the legalizerinfo we currently make the assumption that the target
has neon and FP target features available. Fixing it will require a refactor of
the whole thing, so until then make sure we fall back.

Works around PR42734

Differential Revision: https://reviews.llvm.org/D65244

llvm-svn: 366957
2019-07-24 23:00:04 +00:00
Roman Lebedev
017e272c3a [Codegen] (X & (C l>>/<< Y)) ==/!= 0 --> ((X <</l>> Y) & C) ==/!= 0 fold
Summary:
This was originally reported in D62818.
https://rise4fun.com/Alive/oPH

InstCombine does the opposite fold, in hope that `C l>>/<< Y` expression
will be hoisted out of a loop if `Y` is invariant and `X` is not.
But as it is seen from the diffs here, if it didn't get hoisted,
the produced assembly is almost universally worse.

Much like with my recent "hoist add/sub by/from const" patches,
we should get almost universal win if we hoist constant,
there is almost always an "and/test by imm" instruction,
but "shift of imm" not so much, so we may avoid having to
materialize the immediate, and thus need one less register.
And since we now shift not by constant, but by something else,
the live-range of that something else may reduce.

Special care needs to be applied not to disturb x86 `BT` / hexagon `tstbit`
instruction pattern. And to not get into endless combine loop.

Reviewers: RKSimon, efriedma, t.p.northover, craig.topper, spatel, arsenm

Reviewed By: spatel

Subscribers: hiraditya, MaskRay, wuzish, xbolva00, nikic, nemanjai, jvesely, wdng, nhaehnle, javed.absar, tpr, kristof.beyls, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62871

llvm-svn: 366955
2019-07-24 22:57:22 +00:00
Jessica Paquette
68499112cf [AArch64][GlobalISel] Fold G_MUL into XRO load addressing mode when possible
If we have a G_MUL, and either the LHS or the RHS of that mul is the legal
shift value for a load addressing mode, we can fold it into the load.

This gives some code size savings on some SPEC tests. The best are around 2%
on 300.twolf and 3% on 254.gap.

Differential Revision: https://reviews.llvm.org/D65173

llvm-svn: 366954
2019-07-24 22:49:42 +00:00
Amara Emerson
13af1ed8e3 [GlobalISel] Support for inlining memcpy, memset and memmove calls.
This introduces a new family of combiner helper routines that re-use the
target specific cost model from SelectionDAG, and generate inline implementations
of the memcpy family of intrinsics.

The combines are only enabled at optimization levels higher than -O0, and give
very substantial performance improvements.

Differential Revision: https://reviews.llvm.org/D65167

llvm-svn: 366951
2019-07-24 22:17:31 +00:00
Stanislav Mekhanoshin
c43784ff26 [AMDGPU] Increase kernel padding
To support prefetch mode 3 we need to pad current
cacheline and fill 3 cachelines after. Current padding
is only sufficient for mode 2.

Differential Revision: https://reviews.llvm.org/D65236

llvm-svn: 366938
2019-07-24 19:40:13 +00:00
David Green
cd7a6fa314 [ARM] Rewrite how VCMP are lowered, using a single node
This removes the VCEQ/VCNE/VCGE/VCEQZ/etc nodes, just using two called VCMP and
VCMPZ with an extra operand as the condition code. I believe this will make
some combines simpler, allowing us to just look at these codes and not the
operands. It also helps fill in a missing VCGTUZ MVE selection without adding
extra nodes for it.

Differential Revision: https://reviews.llvm.org/D65072

llvm-svn: 366934
2019-07-24 17:36:47 +00:00
Simon Pilgrim
7d318b2bb1 [DAGCombine] matchBinOpReduction - add partial reduction matching
This patch adds support for recognizing cases where a larger vector type is being used to reduce just the elements in the lower subvector:

e.g. <8 x i32> reduction pattern in a <16 x i32> vector:

<4,5,6,7,u,u,u,u,u,u,u,u,u,u,u,u>
<2,3,u,u,u,u,u,u,u,u,u,u,u,u,u,u>
<1,u,u,u,u,u,u,u,u,u,u,u,u,u,u,u>

matchBinOpReduction returns the lower extracted subvector in such cases, assuming isExtractSubvectorCheap accepts the extraction.

I've only enabled it for X86 reduction sums so far. I intend to enable it for the bitop/minmax cases in future patches, and eventually I think its worth turning it on all the time. This is mainly just a case of ensuring calls to matchBinOpReduction don't make assumptions on the vector width based on the original vector extraction.

Fixes the x86 partial reduction sum cases in PR33758 and PR42023.

Differential Revision: https://reviews.llvm.org/D65047

llvm-svn: 366933
2019-07-24 17:29:56 +00:00
David Green
047a0b6575 [ARM] Disable MVE fptosi and friends
The prevents us from trying to convert an i1 predicate vector to a float, or
vice-versa. Better patterns are possible, which will follow in a subsequent
commit. For now we just expand them.

Differential Revision: https://reviews.llvm.org/D65066

llvm-svn: 366931
2019-07-24 17:26:26 +00:00
Jessica Paquette
c19c30776a [AArch64][GlobalISel] Make vector dup optimization look at last elt of ZeroVec
Fix an off-by-one error which made us not look at the last element of the
zero vector. This caused a miscompile in 188.ammp.

Differential Revision: https://reviews.llvm.org/D65168

llvm-svn: 366930
2019-07-24 17:18:51 +00:00