wangtao
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity
183,551 Commits 29 Branches 0 Tags 2.2 GiB
Commit Graph

44 Commits

Author SHA1 Message Date
James Molloy a9f47b6bae [ARM] Teach the cost model that cross-class copies are costly. 
Cross-class copies being expensive is actually a trait of the microarchitecture, but as I haven't yet seen an example of a microarchitecture where they're cheap it seems best to just enable this by default, covering the non-mcpu build case.

llvm-svn: 217674
 2014-09-12 13:29:40 +00:00
Sanjay Patel b653de1ada Rename getMaximumUnrollFactor -> getMaxInterleaveFactor; also rename option names controlling this variable. 
"Unroll" is not the appropriate name for this variable. Clang already uses 
the term "interleave" in pragmas and metadata for this.

Differential Revision: http://reviews.llvm.org/D5066

llvm-svn: 217528
 2014-09-10 17:58:16 +00:00
Karthik Bhat 7f33ff7dea Allow vectorization of division by uniform power of 2. 
This patch adds support to recognize division by uniform power of 2 and modifies the cost table to vectorize division by uniform power of 2 whenever possible.
Updates Cost model for Loop and SLP Vectorizer.The cost table is currently only updated for X86 backend.
Thanks to Hal, Andrea, Sanjay for the review. (http://reviews.llvm.org/D4971)

llvm-svn: 216371
 2014-08-25 04:56:54 +00:00
Eric Christopher d913448b38 Remove the TargetMachine forwards for TargetSubtargetInfo based 
information and update all callers. No functional change.

llvm-svn: 214781
 2014-08-04 21:25:23 +00:00
Karthik Bhat e03a25da70 Add Support to Recognize and Vectorize NON SIMD instructions in SLPVectorizer. 
This patch adds support to recognize patterns such as fadd,fsub,fadd,fsub.../add,sub,add,sub... and
vectorizes them as vector shuffles if they are profitable.
These patterns of vector shuffle can later be converted to instructions such as addsubpd etc on X86.
Thanks to Arnold and Hal for the reviews. http://reviews.llvm.org/D4015 

llvm-svn: 211339
 2014-06-20 04:32:48 +00:00
Eric Christopher 89f18805f4 Fix typo. 
llvm-svn: 209377
 2014-05-22 01:21:44 +00:00
Craig Topper 062a2baef0 [C++] Use 'nullptr'. Target edition. 
llvm-svn: 207197
 2014-04-25 05:30:21 +00:00
Chandler Carruth 84e68b2994 [Modules] Fix potential ODR violations by sinking the DEBUG_TYPE 
definition below all of the header #include lines, lib/Target/...
edition.

llvm-svn: 206842
 2014-04-22 02:41:26 +00:00
Chandler Carruth aee3ca6cfd [TTI] There is actually no realistic way to pop TTI implementations off 
the stack of the analysis group because they are all immutable passes.
This is made clear by Craig's recent work to use override
systematically -- we weren't overriding anything for 'finalizePass'
because there is no such thing.

This is kind of a lame restriction on the API -- we can no longer push
and pop things, we just set up the stack and run. However, I'm not
invested in building some better solution on top of the existing
(terrifying) immutable pass and legacy pass manager.

llvm-svn: 203437
 2014-03-10 02:45:14 +00:00
Craig Topper 6bc27bf359 [C++11] Add 'override' keyword to virtual methods that override their base class. 
llvm-svn: 203433
 2014-03-10 02:09:33 +00:00
Duncan P. N. Exon Smith 429d2608f9 Change else if => if after return, after r203265 
llvm-svn: 203347
 2014-03-08 15:15:42 +00:00
Ted Kremenek 0b01471694 Remove dead 'return'. 
llvm-svn: 203265
 2014-03-07 18:51:16 +00:00
Craig Topper 73156025e0 Switch all uses of LLVM_OVERRIDE to just use 'override' directly. 
llvm-svn: 202621
 2014-03-02 09:09:27 +00:00
Craig Topper 77dfe45f81 Switch all uses of LLVM_FINAL to just use 'final', and remove the macro. 
llvm-svn: 202618
 2014-03-02 08:08:51 +00:00
Arnold Schwaighofer 445f7fb064 ARMTTI: We don't have 16 allocatable scalar registers 
This caused an regression on libquantum after enabling the new loop vectorizer
unroll heuristics.

llvm-svn: 200616
 2014-02-01 18:00:25 +00:00
Juergen Ributzka f26beda7c7 Revert "Revert "Add Constant Hoisting Pass" (r200034)" 
This reverts commit r200058 and adds the using directive for
ARMTargetTransformInfo to silence two g++ overload warnings.

llvm-svn: 200062
 2014-01-25 02:02:55 +00:00
Juergen Ributzka 3e752e7af9 Add final and owerride keywords to TargetTransformInfo's subclasses. 
llvm-svn: 200021
 2014-01-24 18:22:59 +00:00
Alp Toker cb40291100 Fix known typos 
Sweep the codebase for common typos. Includes some changes to visible function
names that were misspelt.

llvm-svn: 200018
 2014-01-24 17:20:08 +00:00
Chandler Carruth 8a8cd2bab9 Re-sort all of the includes with ./utils/sort_includes.py so that 
subsequent changes are easier to review. About to fix some layering
issues, and wanted to separate out the necessary churn.

Also comment and sink the include of "Windows.h" in three .inc files to
match the usage in Memory.inc.

llvm-svn: 198685
 2014-01-07 11:48:04 +00:00
Arnold Schwaighofer 89ae217422 ARM cost model: Unaligned vectorized double stores are expensive 
Updated a test case that assumed that <2 x double> would vectorize to use
<4 x float>.

radar://15338229

llvm-svn: 193574
 2013-10-29 01:33:57 +00:00
Arnold Schwaighofer 77af0f6e82 ARM cost model: Account for zero cost scalar SROA instructions 
By vectorizing a series of srl, or, ... instructions we have obfuscated the
intention so much that the backend does not know how to fold this code away.

radar://15336950

llvm-svn: 193573
 2013-10-29 01:33:53 +00:00
Benjamin Kramer 21585fd9c1 Add a overload to CostTable which allows it to infer the size of the table. 
Use it to avoid repeating ourselves too often. Also store MVT::SimpleValueType
in the TTI tables so they can be statically initialized, MVT's constructors
create bloated initialization code otherwise.

llvm-svn: 188095
 2013-08-09 19:33:32 +00:00
Renato Golin 0178a25fc5 Fixes ARM LNT bot from SLP change in O3 
This patch fixes the multiple breakages on ARM test-suite after the SLP
vectorizer was introduced by default on O3. The problem was an illegal
vector type on ARMTTI::getCmpSelInstrCost() <3 x i1> which is not simple.

The guard protects this code from breaking (cause of the problems) but
doesn't fix the issue that is generating the odd vector in the first
place, which also needs to be investigated.

llvm-svn: 187658
 2013-08-02 17:10:04 +00:00
Arnold Schwaighofer da2b311865 ARM cost model: Add cost for gather/scather 
Fixes a 35% degradation compared to unvectorized code in
MiBench/automotive-susan and an equally serious regression on a private
image processing benchmark.

radar://14351991

llvm-svn: 186188
 2013-07-12 19:16:04 +00:00
Arnold Schwaighofer 9da9a43af8 TargetTransformInfo: address calculation parameter for gather/scather 
Address calculation for gather/scather in vectorized code can incur a
significant cost making vectorization unbeneficial. Add infrastructure to add
cost.
Tests and cost model for targets will be in follow-up commits.

radar://14351991

llvm-svn: 186187
 2013-07-12 19:16:02 +00:00
Arnold Schwaighofer 9881dcf2f2 ARM cost model: Integer div and rem is lowered to a function call 
Reflect this in the cost model. I observed this in MiBench/consumer-lame.

radar://13354716

llvm-svn: 180576
 2013-04-25 21:16:18 +00:00
Jim Grosbach 563983c8a3 Legalize vector truncates by parts rather than just splitting. 
Rather than just splitting the input type and hoping for the best, apply
a bit more cleverness. Just splitting the types until the source is
legal often leads to an illegal result time, which is then widened and a
scalarization step is introduced which leads to truly horrible code
generation. With the loop vectorizer, these sorts of operations are much
more common, and so it's worth extra effort to do them well.

Add a legalization hook for the operands of a TRUNCATE node, which will
be encountered after the result type has been legalized, but if the
operand type is still illegal. If simple splitting of both types
ends up with the result type of each half still being legal, just
do that (v16i16 -> v16i8 on ARM, for example). If, however, that would
result in an illegal result type (v8i32 -> v8i8 on ARM, for example),
we can get more clever with power-two vectors. Specifically,
split the input type, but also widen the result element size, then
concatenate the halves and truncate again.  For example on ARM,
To perform a "%res = v8i8 trunc v8i32 %in" we transform to:
  %inlo = v4i32 extract_subvector %in, 0
  %inhi = v4i32 extract_subvector %in, 4
  %lo16 = v4i16 trunc v4i32 %inlo
  %hi16 = v4i16 trunc v4i32 %inhi
  %in16 = v8i16 concat_vectors v4i16 %lo16, v4i16 %hi16
  %res = v8i8 trunc v8i16 %in16

This allows instruction selection to generate three VMOVN instructions
instead of a sequences of moves, stores and loads.

Update the ARMTargetTransformInfo to take this improved legalization
into account.

Consider the simplified IR:

define <16 x i8> @test1(<16 x i32>* %ap) {
  %a = load <16 x i32>* %ap
  %tmp = trunc <16 x i32> %a to <16 x i8>
  ret <16 x i8> %tmp
}

define <8 x i8> @test2(<8 x i32>* %ap) {
  %a = load <8 x i32>* %ap
  %tmp = trunc <8 x i32> %a to <8 x i8>
  ret <8 x i8> %tmp
}

Previously, we would generate the truly hideous:
	.syntax unified
	.section	__TEXT,__text,regular,pure_instructions
	.globl	_test1
	.align	2
_test1:                                 @ @test1
@ BB#0:
	push	{r7}
	mov	r7, sp
	sub	sp, sp, #20
	bic	sp, sp, #7
	add	r1, r0, #48
	add	r2, r0, #32
	vld1.64	{d24, d25}, [r0:128]
	vld1.64	{d16, d17}, [r1:128]
	vld1.64	{d18, d19}, [r2:128]
	add	r1, r0, #16
	vmovn.i32	d22, q8
	vld1.64	{d16, d17}, [r1:128]
	vmovn.i32	d20, q9
	vmovn.i32	d18, q12
	vmov.u16	r0, d22[3]
	strb	r0, [sp, #15]
	vmov.u16	r0, d22[2]
	strb	r0, [sp, #14]
	vmov.u16	r0, d22[1]
	strb	r0, [sp, #13]
	vmov.u16	r0, d22[0]
	vmovn.i32	d16, q8
	strb	r0, [sp, #12]
	vmov.u16	r0, d20[3]
	strb	r0, [sp, #11]
	vmov.u16	r0, d20[2]
	strb	r0, [sp, #10]
	vmov.u16	r0, d20[1]
	strb	r0, [sp, #9]
	vmov.u16	r0, d20[0]
	strb	r0, [sp, #8]
	vmov.u16	r0, d18[3]
	strb	r0, [sp, #3]
	vmov.u16	r0, d18[2]
	strb	r0, [sp, #2]
	vmov.u16	r0, d18[1]
	strb	r0, [sp, #1]
	vmov.u16	r0, d18[0]
	strb	r0, [sp]
	vmov.u16	r0, d16[3]
	strb	r0, [sp, #7]
	vmov.u16	r0, d16[2]
	strb	r0, [sp, #6]
	vmov.u16	r0, d16[1]
	strb	r0, [sp, #5]
	vmov.u16	r0, d16[0]
	strb	r0, [sp, #4]
	vldmia	sp, {d16, d17}
	vmov	r0, r1, d16
	vmov	r2, r3, d17
	mov	sp, r7
	pop	{r7}
	bx	lr

	.globl	_test2
	.align	2
_test2:                                 @ @test2
@ BB#0:
	push	{r7}
	mov	r7, sp
	sub	sp, sp, #12
	bic	sp, sp, #7
	vld1.64	{d16, d17}, [r0:128]
	add	r0, r0, #16
	vld1.64	{d20, d21}, [r0:128]
	vmovn.i32	d18, q8
	vmov.u16	r0, d18[3]
	vmovn.i32	d16, q10
	strb	r0, [sp, #3]
	vmov.u16	r0, d18[2]
	strb	r0, [sp, #2]
	vmov.u16	r0, d18[1]
	strb	r0, [sp, #1]
	vmov.u16	r0, d18[0]
	strb	r0, [sp]
	vmov.u16	r0, d16[3]
	strb	r0, [sp, #7]
	vmov.u16	r0, d16[2]
	strb	r0, [sp, #6]
	vmov.u16	r0, d16[1]
	strb	r0, [sp, #5]
	vmov.u16	r0, d16[0]
	strb	r0, [sp, #4]
	ldm	sp, {r0, r1}
	mov	sp, r7
	pop	{r7}
	bx	lr

Now, however, we generate the much more straightforward:
	.syntax unified
	.section	__TEXT,__text,regular,pure_instructions
	.globl	_test1
	.align	2
_test1:                                 @ @test1
@ BB#0:
	add	r1, r0, #48
	add	r2, r0, #32
	vld1.64	{d20, d21}, [r0:128]
	vld1.64	{d16, d17}, [r1:128]
	add	r1, r0, #16
	vld1.64	{d18, d19}, [r2:128]
	vld1.64	{d22, d23}, [r1:128]
	vmovn.i32	d17, q8
	vmovn.i32	d16, q9
	vmovn.i32	d18, q10
	vmovn.i32	d19, q11
	vmovn.i16	d17, q8
	vmovn.i16	d16, q9
	vmov	r0, r1, d16
	vmov	r2, r3, d17
	bx	lr

	.globl	_test2
	.align	2
_test2:                                 @ @test2
@ BB#0:
	vld1.64	{d16, d17}, [r0:128]
	add	r0, r0, #16
	vld1.64	{d18, d19}, [r0:128]
	vmovn.i32	d16, q8
	vmovn.i32	d17, q9
	vmovn.i16	d16, q8
	vmov	r0, r1, d16
	bx	lr

llvm-svn: 179989
 2013-04-21 23:47:41 +00:00
Renato Golin 227eb6fc5f Improve long vector sext/zext lowering on ARM 
The ARM backend currently has poor codegen for long sext/zext
operations, such as v8i8 -> v8i32. This patch addresses this
by performing a custom expansion in ARMISelLowering. It also
adds/changes the cost of such lowering in ARMTTI.

This partially addresses PR14867.

Patch by Pete Couperus

llvm-svn: 177380
 2013-03-19 08:15:38 +00:00
Arnold Schwaighofer ae0052f114 ARM cost model: Make some vector integer to float casts cheaper 
The default logic marks them as too expensive.

For example, before this patch we estimated:
  cost of 16 for instruction:   %r = uitofp <4 x i16> %v0 to <4 x float>

While this translates to:
  vmovl.u16 q8, d16
  vcvt.f32.u32  q8, q8

All other costs are left to the values assigned by the fallback logic. Theses
costs are mostly reasonable in the sense that they get progressively more
expensive as the instruction sequences emitted get longer.

radar://13445992

llvm-svn: 177334
 2013-03-18 22:47:09 +00:00
Arnold Schwaighofer 6c9c3a8b99 ARM cost model: Correct cost for some cheap float to integer conversions 
Fix cost of some "cheap" cast instructions. Before this patch we used to
estimate for example:
  cost of 16 for instruction:   %r = fptoui <4 x float> %v0 to <4 x i16>

While we would emit:
  vcvt.s32.f32  q8, q8
  vmovn.i32 d16, q8
  vuzp.8  d16, d17

All other costs are left to the values assigned by the fallback logic. Theses
costs are mostly reasonable in the sense that they get progressively more
expensive as the instruction sequences emitted get longer.

radar://13434072

llvm-svn: 177333
 2013-03-18 22:47:06 +00:00
Arnold Schwaighofer 9d7a3827e4 ARM cost model: Fix costs for some vector selects 
I was too pessimistic in r177105. Vector selects that fit into a legal register
type lower just fine. I was mislead by the code fragment that I was using. The
stores/loads that I saw in those cases came from lowering the conditional off
an address.

Changing the code fragment to:

%T0_3 = type <8 x i18>
%T1_3 = type <8 x i1>

define void @func_blend3(%T0_3* %loadaddr, %T0_3* %loadaddr2,
                         %T1_3* %blend, %T0_3* %storeaddr) {
  %v0 = load %T0_3* %loadaddr
  %v1 = load %T0_3* %loadaddr2
==> FROM:
  ;%c = load %T1_3* %blend
==> TO:
  %c = icmp slt %T0_3 %v0, %v1
==> USE:
  %r = select %T1_3 %c, %T0_3 %v0, %T0_3 %v1

  store %T0_3 %r, %T0_3* %storeaddr
  ret void
}

revealed this mistake.

radar://13403975

llvm-svn: 177170
 2013-03-15 18:31:01 +00:00
Arnold Schwaighofer f5284ff61f ARM cost model: Fix cost of fptrunc and fpext instructions 
A vector fptrunc and fpext simply gets split into scalar instructions.

radar://13192358

llvm-svn: 177159
 2013-03-15 15:10:47 +00:00
Arnold Schwaighofer 8070b382ec ARM cost model: Increase cost of some vector selects we do terrible on 
By terrible I mean we store/load from the stack.

This matters on PAQp8 in _Z5trainPsS_ii (which is inlined into Mixer::update)
where we decide to vectorize a loop with a VF of 8 resulting in a 25%
degradation on a cortex-a8.

LV: Found an estimated cost of 2 for VF 8 For instruction:   icmp slt i32
LV: Found an estimated cost of 2 for VF 8 For instruction:   select i1, i32, i32

The bug that tracks the CodeGen part is PR14868.

radar://13403975

llvm-svn: 177105
 2013-03-14 19:17:02 +00:00
Arnold Schwaighofer 90774f3c8f ARM cost model: Increase the cost for vector casts that use the stack 
Increase the cost of v8/v16-i8 to v8/v16-i32 casts and truncates as the backend
currently lowers those using stack accesses.

This was responsible for a significant degradation on
MultiSource/Benchmarks/Trimaran/enc-pc1/enc-pc1
where we vectorize one loop to a vector factor of 16. After this patch we select
a vector factor of 4 which will generate reasonable code.

unsigned char cle[32];

void test(short c) {
  unsigned short compte;
  for (compte = 0; compte <= 31; compte++) {
    cle[compte] = cle[compte] ^ c;
  }
}

radar://13220512

llvm-svn: 176898
 2013-03-12 21:19:22 +00:00
Arnold Schwaighofer 89aef93841 ARM cost model: Add vector reverse shuffle costs 
A reverse shuffle is lowered to a vrev and possibly a vext instruction (quad
word).

radar://13171406

llvm-svn: 174933
 2013-02-12 02:40:39 +00:00
Arnold Schwaighofer 594fa2dc2b ARM cost model: Address computation in vector mem ops not free 
Adds a function to target transform info to query for the cost of address
computation. The cost model analysis pass now also queries this interface.
The code in LoopVectorize adds the cost of address computation as part of the
memory instruction cost calculation. Only there, we know whether the instruction
will be scalarized or not.
Increase the penality for inserting in to D registers on swift. This becomes
necessary because we now always assume that address computation has a cost and
three is a closer value to the architecture.

radar://13097204

llvm-svn: 174713
 2013-02-08 14:50:48 +00:00
Arnold Schwaighofer 213fced704 ARM cost model: Add costs for vector selects 
Vector selects are cheap on NEON. They get lowered to a vbsl instruction.

radar://13158753

llvm-svn: 174631
 2013-02-07 16:10:15 +00:00
Arnold Schwaighofer a804bbee9b ARM cost model: Cost for scalar integer casts and floating point conversions 
Also adds some costs for vector integer float conversions.

llvm-svn: 174371
 2013-02-05 14:05:55 +00:00
Arnold Schwaighofer 98f1012f9b ARM cost model: Penalize insertelement into D subregisters 
Swift has a renaming dependency if we load into D subregisters. We don't have a
way of distinguishing between insertelement operations of values from loads and
other values. Therefore, we are pessimistic for now (The performance problem
showed up in example 14 of gcc-loops).

radar://13096933

llvm-svn: 174300
 2013-02-04 02:52:05 +00:00
Renato Golin 5e9d55eca0 Adding simple cast cost to ARM 
Changing ARMBaseTargetMachine to return ARMTargetLowering intead of
the generic one (similar to x86 code).

Tests showing which instructions were added to cast when necessary
or cost zero when not. Downcast to 16 bits are not lowered in NEON,
so costs are not there yet.

llvm-svn: 173849
 2013-01-29 23:31:38 +00:00
Nadav Rotem b1791a75cd ARM Cost model: Use the size of vector registers and widest vectorizable instruction to determine the max vectorization factor. 
llvm-svn: 172010
 2013-01-09 22:29:00 +00:00
Nadav Rotem b696c36fcd Cost Model: Move the 'max unroll factor' variable to the TTI and add initial Cost Model support on ARM. 
llvm-svn: 171928
 2013-01-09 01:15:42 +00:00
Chandler Carruth d3e73556d6 Move TargetTransformInfo to live under the Analysis library. This no 
longer would violate any dependency layering and it is in fact an
analysis. =]

llvm-svn: 171686
 2013-01-07 03:08:10 +00:00
Chandler Carruth 664e354de7 Switch TargetTransformInfo from an immutable analysis pass that requires 
a TargetMachine to construct (and thus isn't always available), to an
analysis group that supports layered implementations much like
AliasAnalysis does. This is a pretty massive change, with a few parts
that I was unable to easily separate (sorry), so I'll walk through it.

The first step of this conversion was to make TargetTransformInfo an
analysis group, and to sink the nonce implementations in
ScalarTargetTransformInfo and VectorTargetTranformInfo into
a NoTargetTransformInfo pass. This allows other passes to add a hard
requirement on TTI, and assume they will always get at least on
implementation.

The TargetTransformInfo analysis group leverages the delegation chaining
trick that AliasAnalysis uses, where the base class for the analysis
group delegates to the previous analysis *pass*, allowing all but tho
NoFoo analysis passes to only implement the parts of the interfaces they
support. It also introduces a new trick where each pass in the group
retains a pointer to the top-most pass that has been initialized. This
allows passes to implement one API in terms of another API and benefit
when some other pass above them in the stack has more precise results
for the second API.

The second step of this conversion is to create a pass that implements
the TargetTransformInfo analysis using the target-independent
abstractions in the code generator. This replaces the
ScalarTargetTransformImpl and VectorTargetTransformImpl classes in
lib/Target with a single pass in lib/CodeGen called
BasicTargetTransformInfo. This class actually provides most of the TTI
functionality, basing it upon the TargetLowering abstraction and other
information in the target independent code generator.

The third step of the conversion adds support to all TargetMachines to
register custom analysis passes. This allows building those passes with
access to TargetLowering or other target-specific classes, and it also
allows each target to customize the set of analysis passes desired in
the pass manager. The baseline LLVMTargetMachine implements this
interface to add the BasicTTI pass to the pass manager, and all of the
tools that want to support target-aware TTI passes call this routine on
whatever target machine they end up with to add the appropriate passes.

The fourth step of the conversion created target-specific TTI analysis
passes for the X86 and ARM backends. These passes contain the custom
logic that was previously in their extensions of the
ScalarTargetTransformInfo and VectorTargetTransformInfo interfaces.
I separated them into their own file, as now all of the interface bits
are private and they just expose a function to create the pass itself.
Then I extended these target machines to set up a custom set of analysis
passes, first adding BasicTTI as a fallback, and then adding their
customized TTI implementations.

The fourth step required logic that was shared between the target
independent layer and the specific targets to move to a different
interface, as they no longer derive from each other. As a consequence,
a helper functions were added to TargetLowering representing the common
logic needed both in the target implementation and the codegen
implementation of the TTI pass. While technically this is the only
change that could have been committed separately, it would have been
a nightmare to extract.

The final step of the conversion was just to delete all the old
boilerplate. This got rid of the ScalarTargetTransformInfo and
VectorTargetTransformInfo classes, all of the support in all of the
targets for producing instances of them, and all of the support in the
tools for manually constructing a pass based around them.

Now that TTI is a relatively normal analysis group, two things become
straightforward. First, we can sink it into lib/Analysis which is a more
natural layer for it to live. Second, clients of this interface can
depend on it *always* being available which will simplify their code and
behavior. These (and other) simplifications will follow in subsequent
commits, this one is clearly big enough.

Finally, I'm very aware that much of the comments and documentation
needs to be updated. As soon as I had this working, and plausibly well
commented, I wanted to get it committed and in front of the build bots.
I'll be doing a few passes over documentation later if it sticks.

Commits to update DragonEgg and Clang will be made presently.

llvm-svn: 171681
 2013-01-07 01:37:14 +00:00