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Commit Graph

269 Commits

Author SHA1 Message Date
Marcin Koscielnicki
b26ad64ef1 [SystemZ] Implement llvm.get.dynamic.area.offset
To be used for AddressSanitizer.

Differential Revision: http://reviews.llvm.org/D19817

llvm-svn: 268572
2016-05-04 23:31:26 +00:00
Marcin Koscielnicki
237ad4edbd [SystemZ] Implement backchain attribute.
This introduces a SystemZ-specific "backchain" attribute on function, which
enables writing the frame backchain link as specified by the ABI.  This will
be used to implement -mbackchain option in clang.

Differential Revision: http://reviews.llvm.org/D19889

llvm-svn: 268571
2016-05-04 23:31:20 +00:00
Craig Topper
945a4cd524 [CodeGen] Default CTTZ_ZERO_UNDEF/CTLZ_ZERO_UNDEF to Expand in TargetLoweringBase. This is what the majority of the targets want and removes a bunch of code. Set it to Legal explicitly in the few cases where that's the desired behavior.
llvm-svn: 267853
2016-04-28 03:34:31 +00:00
Bryan Chan
2567ab558c [SystemZ] Support Swift Calling Convention
Summary:
Port rL265480, rL264754, rL265997 and rL266252 to SystemZ, in order to enable the Swift port on the architecture. SwiftSelf and SwiftError are assigned to R10 and R9, respectively, which are normally callee-saved registers. For more information, see:

RFC: Implementing the Swift calling convention in LLVM and Clang
https://groups.google.com/forum/#!topic/llvm-dev/epDd2w93kZ0

Reviewers: kbarton, manmanren, rjmccall, uweigand

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19414

llvm-svn: 267823
2016-04-28 00:17:23 +00:00
Ahmed Bougacha
db9e64109d [CodeGen] Add getBuildVector and getSplatBuildVector helpers. NFCI.
Differential Revision: http://reviews.llvm.org/D17176

llvm-svn: 267606
2016-04-26 21:15:30 +00:00
Craig Topper
05d7be2051 [SystemZ] Mark CTTZ_ZERO_UNDEF/CTLZ_ZERO_UNDEF as Expand instead of Custom since the custom logic just did what Expand does when CTTZ/CTLZ are Legal. NFC
llvm-svn: 267109
2016-04-22 05:29:58 +00:00
Marcin Koscielnicki
64bfaf0336 [SystemZ] Add support for llvm.thread.pointer intrinsic.
Differential Revision: http://reviews.llvm.org/D19054

llvm-svn: 266844
2016-04-20 01:03:48 +00:00
JF Bastien
f4f5b32f44 NFC: make AtomicOrdering an enum class
Summary:
In the context of http://wg21.link/lwg2445 C++ uses the concept of
'stronger' ordering but doesn't define it properly. This should be fixed
in C++17 barring a small question that's still open.

The code currently plays fast and loose with the AtomicOrdering
enum. Using an enum class is one step towards tightening things. I later
also want to tighten related enums, such as clang's
AtomicOrderingKind (which should be shared with LLVM as a 'C++ ABI'
enum).

This change touches a few lines of code which can be improved later, I'd
like to keep it as NFC for now as it's already quite complex. I have
related changes for clang.

As a follow-up I'll add:
  bool operator<(AtomicOrdering, AtomicOrdering) = delete;
  bool operator>(AtomicOrdering, AtomicOrdering) = delete;
  bool operator<=(AtomicOrdering, AtomicOrdering) = delete;
  bool operator>=(AtomicOrdering, AtomicOrdering) = delete;
This is separate so that clang and LLVM changes don't need to be in sync.

Reviewers: jyknight, reames

Subscribers: jyknight, llvm-commits

Differential Revision: http://reviews.llvm.org/D18775

llvm-svn: 265602
2016-04-06 21:19:33 +00:00
Ulrich Weigand
ef8c006d34 [SystemZ] Support ATOMIC_FENCE
A cross-thread sequentially consistent fence should be lowered into
z/Architecture's BCR serialization instruction, instead of causing a
fatal error in the back-end.

Author: bryanpkc
Differential Revision: http://reviews.llvm.org/D18644

llvm-svn: 265292
2016-04-04 12:45:44 +00:00
Ulrich Weigand
c0457f68d4 [SystemZ] Support llvm.frameaddress/llvm.returnaddress intrinsics
Enable the SystemZ back-end to lower FRAMEADDR and RETURNADDR, which
previously would cause the back-end to crash.  Currently, only a
frame count of zero is supported.

Author: bryanpkc
Differential Revision: http://reviews.llvm.org/D18514

llvm-svn: 265291
2016-04-04 12:44:55 +00:00
Sanjay Patel
f7ad46820f [DAG] use !isUndef() ; NFCI
llvm-svn: 263453
2016-03-14 18:09:43 +00:00
Sanjay Patel
f22bc14a47 [DAG] use isUndef() ; NFCI
llvm-svn: 263448
2016-03-14 17:28:46 +00:00
Ulrich Weigand
849b3a7299 [SystemZ] Fix ABI for i128 argument and return types
According to the SystemZ ABI, 128-bit integer types should be
passed and returned via implicit reference.  However, this is
not currently implemented at the LLVM IR level for the i128
type.  This does not matter when compiling C/C++ code, since
clang will implement the implicit reference itself.

However, it turns out that when calling libgcc helper routines
operating on 128-bit integers, LLVM will use i128 argument and
return value types; the resulting code is not compatible with
the ABI used in libgcc, leading to crashes (see PR26559).

This should be simple to fix, except that i128 currently is not
even a legal type for the SystemZ back end.  Therefore, common
code will already split arguments and return values into multiple
parts.  The bulk of this patch therefore consists of detecting
such parts, and correctly handling passing via implicit reference
of a value split into multiple parts.  If at some time in the
future, i128 becomes a legal type, this code can be removed again.

This fixes PR26559.

llvm-svn: 261325
2016-02-19 14:10:21 +00:00
Ahmed Bougacha
f23bc5a4de [CodeGen] Prefer "if (SDValue R = ...)" to "if (R.getNode())". NFCI.
llvm-svn: 260316
2016-02-09 22:54:12 +00:00
Ulrich Weigand
c791b42454 [SystemZ] Fix wrong-code generation for certain always-false conditions
We've found another bug in the code generation logic conditions for a
certain class of always-false conditions, those of the form
   if ((a & 1) < 0)

These only reach the back end when compiling without optimization.

The bug was introduced by the choice of using TEST UNDER MASK
to implement a check for
   if ((a & MASK) < VAL)
as
   if ((a & MASK) == 0)

where VAL is less than the the lowest bit of MASK.  This is correct
in all cases except for VAL == 0, in which case the original
condition is always false, but the replacement isn't.

Fixed by excluding that particular case.

llvm-svn: 259381
2016-02-01 18:31:19 +00:00
Ulrich Weigand
e19c3de669 [SystemZ] Fix assertion failure in adjustSubwordCmp
When comparing a zero-extended value against a constant small enough to
be in range of the inner type, it doesn't matter whether a signed or
unsigned compare operation (for the outer type) is being used.  This is
why the code in adjustSubwordCmp had this assertion:

    assert(C.ICmpType == SystemZICMP::Any &&
           "Signedness shouldn't matter here.");

assuming the the caller had already detected that fact.  However, it
turns out that there cases, in particular with always-true or always-
false conditions that have not been eliminated when compiling at -O0,
where this is not true.

Instead of failing an assertion if C.ICmpType is not SystemZICMP::Any
here, we can simply *set* it safely to SystemZICMP::Any, however.

llvm-svn: 255786
2015-12-16 18:04:06 +00:00
Jonas Paulsson
4e06f54193 [Stack realignment] Handling of aligned allocas.
This patch implements dynamic realignment of stack objects for targets
with a non-realigned stack pointer. Behaviour in FunctionLoweringInfo
is changed so that for a target that has StackRealignable set to
false, over-aligned static allocas are considered to be variable-sized
objects and are handled with DYNAMIC_STACKALLOC nodes.

It would be good to group aligned allocas into a single big alloca as
an optimization, but this is yet todo.

SystemZ benefits from this, due to its stack frame layout.

New tests SystemZ/alloca-03.ll for aligned allocas, and
SystemZ/alloca-04.ll for "no-realign-stack" attribute on functions.

Review and help from Ulrich Weigand and Hal Finkel.

llvm-svn: 254227
2015-11-28 11:02:32 +00:00
Ulrich Weigand
2b0ba63e17 [SystemZ] Simplify boolean conditional return statements
Use clang-tidy to simplify conditonal return statements.

Author: LegalizeAdulthood
Differential Revision: http://reviews.llvm.org/D9986

llvm-svn: 253038
2015-11-13 13:00:27 +00:00
Joseph Tremoulet
6a3a04f00f [WinEH] Update exception pointer registers
Summary:
The CLR's personality routine passes these in rdx/edx, not rax/eax.

Make getExceptionPointerRegister a virtual method parameterized by
personality function to allow making this distinction.

Similarly make getExceptionSelectorRegister a virtual method parameterized
by personality function, for symmetry.


Reviewers: pgavlin, majnemer, rnk

Subscribers: jyknight, dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D14344

llvm-svn: 252383
2015-11-07 01:11:31 +00:00
NAKAMURA Takumi
5ce9110622 Untabify.
llvm-svn: 251769
2015-11-02 01:38:12 +00:00
Ulrich Weigand
5d4e78925f [SystemZ] Fix another assertion failure in tryBuildVectorShuffle
This fixes yet another scenario where tryBuildVectorShuffle would
attempt to create a BUILD_VECTOR node with an invalid combination
of types.  This can happen if the incoming BUILD_VECTOR has elements
of a type different from the vector element type, which is allowed
in certain cases as long as they are all the same type.

When one of these elements is used in the residual vector, and
UNDEF elements are added to fill up the residual vector, those
UNDEFs then have to use the type of the original element, not
the vector element type, or else the resulting BUILD_VECTOR
will have an invalid type combination.

llvm-svn: 249706
2015-10-08 17:46:59 +00:00
Jonas Paulsson
6bdd2eb055 [SystemZ] Use load-and-test for fp compare with 0 if vector support is present.
Since the LTxBRCompare instructions can't be used with vector registers, a
normal load-and-test instruction (with a modelled def operand) is used instead.

Reviewed by Ulrich Weigand.

llvm-svn: 249664
2015-10-08 07:40:16 +00:00
NAKAMURA Takumi
162c68483b Prune trailing whitespaces.
llvm-svn: 248265
2015-09-22 11:19:03 +00:00
NAKAMURA Takumi
dd4e00ed1e Untabify.
llvm-svn: 248264
2015-09-22 11:15:07 +00:00
Ulrich Weigand
ac3a1a8604 [SystemZ] Fix expansion of ISD::FPOW and ISD::FSINCOS
The ISD::FPOW and ISD::FSINCOS opcodes default to Legal, but there
is no legal instruction for those on SystemZ.  This could cause
LLVM internal errors.  Fixed by setting the operation action to
Expand for those opcodes.

Also added test cases for all other LLVM IR intrinsics that should
generate a library call.  (Those already work correctly since the
default operation action is fine.)

llvm-svn: 248180
2015-09-21 17:35:45 +00:00
Ulrich Weigand
3f6dcea99f [SystemZ] Fix assertion failure in tryBuildVectorShuffle
Under certain circumstances, tryBuildVectorShuffle would attempt to
create a BUILD_VECTOR node with an invalid combination of types.
This happened when one of the components of the original BUILD_VECTOR
was itself a TRUNCATE node.  That TRUNCATE was stripped off during
intermediate processing to simplify code, but when adding the node
back to the result vector, we still need it to get the type right.

llvm-svn: 247694
2015-09-15 14:27:46 +00:00
Ulrich Weigand
6643dc8666 [SystemZ] Support large LLVM IR struct return values
Recent mesa/llvmpipe crashes on SystemZ due to a failed assertion when
attempting to compile a routine with a return type of
  { <4 x float>, <4 x float>, <4 x float>, <4 x float> }
on a system without vector instruction support.

This is because after legalizing the vector type, we get a return value
consisting of 16 floats, which cannot all be returned in registers.

Usually, what should happen in this case is that the target's CanLowerReturn
routine rejects the return type, in which case SelectionDAG falls back to
implementing a structure return in memory via implicit reference.

However, the SystemZ target never actually implemented any CanLowerReturn
routine, and thus would accept any struct return type.

This patch fixes the crash by implementing CanLowerReturn.  As a side effect,
this also handles fp128 return values, fixing a todo that was noted in
SystemZCallingConv.td.

llvm-svn: 244889
2015-08-13 13:37:06 +00:00
Alex Lorenz
7b1d22a17d PseudoSourceValue: Replace global manager with a manager in a machine function.
This commit removes the global manager variable which is responsible for
storing and allocating pseudo source values and instead it introduces a new
manager class named 'PseudoSourceValueManager'. Machine functions now own an
instance of the pseudo source value manager class.

This commit also modifies the 'get...' methods in the 'MachinePointerInfo'
class to construct pseudo source values using the instance of the pseudo
source value manager object from the machine function.

This commit updates calls to the 'get...' methods from the 'MachinePointerInfo'
class in a lot of different files because those calls now need to pass in a
reference to a machine function to those methods.

This change will make it easier to serialize pseudo source values as it will
enable me to transform the mips specific MipsCallEntry PseudoSourceValue
subclass into two target independent subclasses.

Reviewers: Akira Hatanaka
llvm-svn: 244693
2015-08-11 23:09:45 +00:00
Chih-Hung Hsieh
17e97fbf13 Implement target independent TLS compatible with glibc's emutls.c.
The 'common' section TLS is not implemented.
Current C/C++ TLS variables are not placed in common section.
DWARF debug info to get the address of TLS variables is not generated yet.

clang and driver changes in http://reviews.llvm.org/D10524

  Added -femulated-tls flag to select the emulated TLS model,
  which will be used for old targets like Android that do not
  support ELF TLS models.

Added TargetLowering::LowerToTLSEmulatedModel as a target-independent
function to convert a SDNode of TLS variable address to a function call
to __emutls_get_address.

Added into lib/Target/*/*ISelLowering.cpp to call LowerToTLSEmulatedModel
for TLSModel::Emulated. Although all targets supporting ELF TLS models are
enhanced, emulated TLS model has been tested only for Android ELF targets.
Modified AsmPrinter.cpp to print the emutls_v.* and emutls_t.* variables for
emulated TLS variables.
Modified DwarfCompileUnit.cpp to skip some DIE for emulated TLS variabls.

TODO: Add proper DIE for emulated TLS variables.
      Added new unit tests with emulated TLS.

Differential Revision: http://reviews.llvm.org/D10522

llvm-svn: 243438
2015-07-28 16:24:05 +00:00
Mehdi Amini
9f0c09e5bf Remove access to the DataLayout in the TargetMachine
Summary:
Replace getDataLayout() with a createDataLayout() method to make
explicit that it is intended to create a DataLayout only and not
accessing it for other purpose.

This change is the last of a series of commits dedicated to have a
single DataLayout during compilation by using always the one owned
by the module.

Reviewers: echristo

Subscribers: jholewinski, llvm-commits, rafael, yaron.keren

Differential Revision: http://reviews.llvm.org/D11103

(cherry picked from commit 5609fc56bca971e5a7efeaa6ca4676638eaec5ea)

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 243114
2015-07-24 16:04:22 +00:00
Mehdi Amini
3a4451076e Revert "Remove access to the DataLayout in the TargetMachine"
This reverts commit 0f720d984f419c747709462f7476dff962c0bc41.

It breaks clang too badly, I need to prepare a proper patch for clang
first.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 243089
2015-07-24 03:36:55 +00:00
Mehdi Amini
30b6ee8541 Remove access to the DataLayout in the TargetMachine
Summary:
Replace getDataLayout() with a createDataLayout() method to make
explicit that it is intended to create a DataLayout only and not
accessing it for other purpose.

This change is the last of a series of commits dedicated to have a
single DataLayout during compilation by using always the one owned
by the module.

Reviewers: echristo

Subscribers: jholewinski, llvm-commits, rafael, yaron.keren

Differential Revision: http://reviews.llvm.org/D11103

(cherry picked from commit 5609fc56bca971e5a7efeaa6ca4676638eaec5ea)

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 243083
2015-07-24 01:44:39 +00:00
Mehdi Amini
761f22ac31 Make isLegalAddressingMode() taking DataLayout as an argument
Summary:
This change is part of a series of commits dedicated to have a single
DataLayout during compilation by using always the one owned by the
module.

Reviewers: echristo

Subscribers: jholewinski, llvm-commits, rafael, yaron.keren

Differential Revision: http://reviews.llvm.org/D11040

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 241778
2015-07-09 02:09:40 +00:00
Mehdi Amini
abf873c623 Make TargetLowering::getPointerTy() taking DataLayout as an argument
Summary:
This change is part of a series of commits dedicated to have a single
DataLayout during compilation by using always the one owned by the
module.

Reviewers: echristo

Subscribers: jholewinski, ted, yaron.keren, rafael, llvm-commits

Differential Revision: http://reviews.llvm.org/D11028

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 241775
2015-07-09 02:09:04 +00:00
Benjamin Kramer
89b0e53c15 [TargetLowering] StringRefize asm constraint getters.
There is some functional change here because it changes target code from
atoi(3) to StringRef::getAsInteger which has error checking. For valid
constraints there should be no difference.

llvm-svn: 241411
2015-07-05 19:29:18 +00:00
Justin Bogner
df8438db3e SystemZ: Avoid left shifting negative values (it's UB)
Found by ubsan.

llvm-svn: 240420
2015-06-23 15:38:24 +00:00
Alexander Kornienko
f993659b8f Revert r240137 (Fixed/added namespace ending comments using clang-tidy. NFC)
Apparently, the style needs to be agreed upon first.

llvm-svn: 240390
2015-06-23 09:49:53 +00:00
Alexander Kornienko
40cb19d802 Fixed/added namespace ending comments using clang-tidy. NFC
The patch is generated using this command:

tools/clang/tools/extra/clang-tidy/tool/run-clang-tidy.py -fix \
  -checks=-*,llvm-namespace-comment -header-filter='llvm/.*|clang/.*' \
  llvm/lib/


Thanks to Eugene Kosov for the original patch!

llvm-svn: 240137
2015-06-19 15:57:42 +00:00
Matt Arsenault
b0334192af Add address space argument to isLegalAddressingMode
This is important because of different addressing modes
depending on the address space for GPU targets.

This only adds the argument, and does not update
any of the uses to provide the correct address space.

llvm-svn: 238723
2015-06-01 05:31:59 +00:00
Aaron Ballman
0bbb494eec Silencing an MSVC warning: '<<' : result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?); NFC
llvm-svn: 236987
2015-05-11 12:45:53 +00:00
Saleem Abdulrasool
09a1c9ca34 SystemZ: silence a GCC warning
warning: enumeral and non-enumeral type in conditional expression

Cast the 0 to the appropriate type.  NFC.  Identified by GCC 4.9.2

llvm-svn: 236942
2015-05-10 00:53:41 +00:00
Matthias Braun
3b3ecc12b2 Change getTargetNodeName() to produce compiler warnings for missing cases, fix them
llvm-svn: 236775
2015-05-07 21:33:59 +00:00
Ulrich Weigand
cd1cd8d125 [SystemZ] Add vector intrinsics
This adds intrinsics to allow access to all of the z13 vector instructions.
Note that instructions whose semantics can be described by standard LLVM IR
do not get any intrinsics.

For each instructions whose semantics *cannot* (fully) be described, we
define an LLVM IR target-specific intrinsic that directly maps to this
instruction.

For instructions that also set the condition code, the LLVM IR intrinsic
returns the post-instruction CC value as a second result.  Instruction
selection will attempt to detect code that compares that CC value against
constants and use the condition code directly instead.

Based on a patch by Richard Sandiford.

llvm-svn: 236527
2015-05-05 19:31:09 +00:00
Ulrich Weigand
f07c2265fd [SystemZ] Mark v1i128 and v1f128 as unsupported
The ABI specifies that <1 x i128> and <1 x fp128> are supposed to be
passed in vector registers.  We do not yet support those types, and
some infrastructure is missing before we can do so.

In order to prevent accidentally generating code violating the ABI,
this patch adds checks to detect those types and error out if user
code attempts to use them.

llvm-svn: 236526
2015-05-05 19:30:05 +00:00
Ulrich Weigand
096deccae0 [SystemZ] Handle sub-128 vectors
The ABI allows sub-128 vectors to be passed and returned in registers,
with the vector occupying the upper part of a register.  We therefore
want to legalize those types by widening the vector rather than promoting
the elements.

The patch includes some simple tests for sub-128 vectors and also tests
that we can recognize various pack sequences, some of which use sub-128
vectors as temporary results.  One of these forms is based on the pack
sequences generated by llvmpipe when no intrinsics are used.

Signed unpacks are recognized as BUILD_VECTORs whose elements are
individually sign-extended.  Unsigned unpacks can have the equivalent
form with zero extension, but they also occur as shuffles in which some
elements are zero.

Based on a patch by Richard Sandiford.

llvm-svn: 236525
2015-05-05 19:29:21 +00:00
Ulrich Weigand
73b770d782 [SystemZ] Add CodeGen support for scalar f64 ops in vector registers
The z13 vector facility includes some instructions that operate only on the
high f64 in a v2f64, effectively extending the FP register set from 16
to 32 registers.  It's still better to use the old instructions if the
operands happen to fit though, since the older instructions have a shorter
encoding.

Based on a patch by Richard Sandiford.

llvm-svn: 236524
2015-05-05 19:28:34 +00:00
Ulrich Weigand
119ff7dab3 [SystemZ] Add CodeGen support for v4f32
The architecture doesn't really have any native v4f32 operations except
v4f32->v2f64 and v2f64->v4f32 conversions, with only half of the v4f32
elements being used.  Even so, using vector registers for <4 x float>
and scalarising individual operations is much better than generating
completely scalar code, since there's much less register pressure.
It's also more efficient to do v4f32 comparisons by extending to 2
v2f64s, comparing those, then packing the result.

This particularly helps with llvmpipe.

Based on a patch by Richard Sandiford.

llvm-svn: 236523
2015-05-05 19:27:45 +00:00
Ulrich Weigand
2413036cdd [SystemZ] Add CodeGen support for v2f64
This adds ABI and CodeGen support for the v2f64 type, which is natively
supported by z13 instructions.

Based on a patch by Richard Sandiford.

llvm-svn: 236522
2015-05-05 19:26:48 +00:00
Ulrich Weigand
938ff50eda [SystemZ] Add CodeGen support for integer vector types
This the first of a series of patches to add CodeGen support exploiting
the instructions of the z13 vector facility.  This patch adds support
for the native integer vector types (v16i8, v8i16, v4i32, v2i64).

When the vector facility is present, we default to the new vector ABI.
This is characterized by two major differences:
- Vector types are passed/returned in vector registers
  (except for unnamed arguments of a variable-argument list function).
- Vector types are at most 8-byte aligned.

The reason for the choice of 8-byte vector alignment is that the hardware
is able to efficiently load vectors at 8-byte alignment, and the ABI only
guarantees 8-byte alignment of the stack pointer, so requiring any higher
alignment for vectors would require dynamic stack re-alignment code.

However, for compatibility with old code that may use vector types, when
*not* using the vector facility, the old alignment rules (vector types
are naturally aligned) remain in use.

These alignment rules are not only implemented at the C language level
(implemented in clang), but also at the LLVM IR level.  This is done
by selecting a different DataLayout string depending on whether the
vector ABI is in effect or not.

Based on a patch by Richard Sandiford.

llvm-svn: 236521
2015-05-05 19:25:42 +00:00
Ulrich Weigand
b59883a864 [SystemZ] Reclassify f32 subregs of f64 registers
At the moment, all subregs defined by the SystemZ target can be modified
independently of the wider register.  E.g. writing to a GR32 does not
change the upper 32 bits of the GR64.  Writing to an FP32 does not change
the lower 32 bits of the FP64.

Hoewver, the upcoming support for the vector extension redefines FP64 as
one half of a V128.  Floating-point operations leave the other half of
a V128 in an unpredictable state, so it's no longer the case that writing
to an FP32 leaves the bits of the underlying register (the V128) alone.
I'd prefer to have separate subreg_ names for this situation, so that
it's obvious at a glance whether we're talking about a subreg that leaves
the other parts of the register alone.

No behavioral change intended.

Patch originally by Richard Sandiford.

llvm-svn: 236433
2015-05-04 17:41:22 +00:00