Commit Graph

192 Commits

Author SHA1 Message Date
Florian Hahn 5bccdde070
[Matrix] Move C++ matrix cast checks to TryStaticCast.
At the moment, the matrix support in CheckCXXCStyleCast (added in
D101696) breaks function-style constructor calls that take a
single matrix value, because it is treated as matrix cast.

Instead, unify the C++ matrix cast handling by moving the logic to
TryStaticCast and only handle the case where both types are matrix
types. Otherwise, fall back to the generic mis-match detection.

Suggested by @rjmccall

Reviewed By: SaurabhJha

Differential Revision: https://reviews.llvm.org/D103163
2021-05-28 13:00:28 +01:00
Ole Strohm 66b112d624 [OpenCL] Fix reinterpret_cast of vectors
Fixes issues with vectors in reinterpret_cast in C++ for OpenCL
and adds tests to make sure they both pass without errors and
generate the correct code.

Fixes: PR47977

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D101519
2021-05-17 12:42:21 +01:00
Saurabh Jha ecb235d940 [Matrix] Implement static cast for matrix types
This patch implements static casts for matrix types. This patch finishes all the work needed for https://bugs.llvm.org/show_bug.cgi?id=47141

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D102125
2021-05-16 08:09:12 +01:00
Saurabh Jha db210bc69b [Matrix] Implement C-style explicit type conversions in CXX for matrix types
This patch implements C-style explicit type conversions in CXX for matrix types. It is part of fixing https://bugs.llvm.org/show_bug.cgi?id=47141

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D101696
2021-05-04 15:27:57 +01:00
Saurabh Jha 71ab6c98a0
[Matrix] Implement C-style explicit type conversions for matrix types.
This implements C-style type conversions for matrix types, as specified
in clang/docs/MatrixTypes.rst.

Fixes PR47141.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D99037
2021-04-10 11:48:41 +01:00
Yuanfang Chen 217f0f735a [Clang][Sema] Implement GCC -Wcast-function-type
```
Warn when a function pointer is cast to an incompatible function
pointer. In a cast involving function types with a variable argument
list only the types of initial arguments that are provided are
considered. Any parameter of pointer-type matches any other
pointer-type. Any benign differences in integral types are ignored, like
int vs. long on ILP32 targets. Likewise type qualifiers are ignored. The
function type void (*) (void) is special and matches everything, which
can be used to suppress this warning. In a cast involving pointer to
member types this warning warns whenever the type cast is changing the
pointer to member type. This warning is enabled by -Wextra.
```

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D97831
2021-03-24 16:04:18 -07:00
Anton Zabaznov 840643bbe1 [OpenCL] Refactor diagnostic for OpenCL extension/feature
There is no need to check for enabled pragma for core or optional core features,
thus this check is removed

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D97058
2021-03-12 11:43:53 +03:00
Joe Ellis 1e2da3839c [AArch64][SVE] Allow C-style casts between fixed-size and scalable vectors
This patch allows C-style casting between fixed-size and scalable
vectors. This kind of cast was previously blocked by the compiler, but
it should be allowed.

Differential Revision: https://reviews.llvm.org/D91262
2020-11-19 11:18:35 +00:00
Haojian Wu a96bcfb196 [AST][RecoveryExpr] Support dependent cast-expr in C for error-recovery.
Suppress spurious "typecheck_cond_expect_scalar_operand" diagnostic.

See whole context: https://reviews.llvm.org/D85025

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D84387
2020-10-08 10:00:29 +02:00
Zequan Wu ebf267b87d [Sema][MSVC] warn at dynamic_cast/typeid when /GR- is given
Differential Revision: https://reviews.llvm.org/D86369
2020-09-16 10:38:52 -07:00
Serge Pavlov f1cd6593da [AST][FPEnv] Keep FP options in trailing storage of CastExpr
This is recommit of 6c8041aa0f, reverted in de044f7562 because of some
fails. Original commit message is below.

This change allow a CastExpr to have optional FPOptionsOverride object,
stored in trailing storage. Of all cast nodes only ImplicitCastExpr,
CStyleCastExpr, CXXFunctionalCastExpr and CXXStaticCastExpr are allowed
to have FPOptions.

Differential Revision: https://reviews.llvm.org/D85960
2020-09-14 12:15:21 +07:00
Serge Pavlov de044f7562 Revert "[AST][FPEnv] Keep FP options in trailing storage of CastExpr"
This reverts commit 6c8041aa0f.
It caused some fails on buildbots.
2020-09-12 17:06:42 +07:00
Serge Pavlov 6c8041aa0f [AST][FPEnv] Keep FP options in trailing storage of CastExpr
This change allow a CastExpr to have optional FPOptionsOverride object,
stored in trailing storage. Of all cast nodes only ImplicitCastExpr,
CStyleCastExpr, CXXFunctionalCastExpr and CXXStaticCastExpr are allowed
to have FPOptions.

Differential Revision: https://reviews.llvm.org/D85960
2020-09-12 14:30:44 +07:00
Hans Wennborg 2d9d270e77 Revert 3e782bf809 "[Sema][MSVC] warn at dynamic_cast when /GR- is given"
This caused more warnings than expected, see https://crbug.com/1126019

Also reverts the follow-up 7907e5516.

> Differential Revision: https://reviews.llvm.org/D86369
2020-09-08 16:10:18 +02:00
Zequan Wu 3e782bf809 [Sema][MSVC] warn at dynamic_cast when /GR- is given
Differential Revision: https://reviews.llvm.org/D86369
2020-09-07 16:46:58 -07:00
Mark de Wever fef2607124 [Sema] Use the proper cast for a fixed bool enum.
When casting an enumerate with a fixed bool type the casting should use
an IntegralToBoolean instead of an IntegralCast as is required per Core
Issue 2338.

Fixes PR47055: Incorrect codegen for enum with bool underlying type

Differential Revision: https://reviews.llvm.org/D85612
2020-08-16 18:40:08 +02:00
Vince Bridgers 351aac0985 [Sema] Add casting check for fixed to fixed point conversions
This change squelches the warning for a cast from fixed to fixed point
conversions when -Wbad-function-cast is enabled.

Fixes:

cast from function call of type '_Fract' to non-matching type '_Fract'
[-Wbad-function-cast]

Reviewed By: bjope

Differential Revision: https://reviews.llvm.org/D85157
2020-08-07 07:58:53 -05:00
Bruno Ricci eb10b065f2
[clang] Pass the NamedDecl* instead of the DeclarationName into many diagnostics.
Background:
-----------
There are two related argument types which can be sent into a diagnostic to
display the name of an entity: DeclarationName (ak_declarationname) or
NamedDecl* (ak_nameddecl) (there is also ak_identifierinfo for
IdentifierInfo*, but we are not concerned with it here).

A DeclarationName in a diagnostic will just be streamed to the output,
which will directly result in a call to DeclarationName::print.

A NamedDecl* in a diagnostic will also ultimately result in a call to
DeclarationName::print, but with two customisation points along the way:

The first customisation point is NamedDecl::getNameForDiagnostic which is
overloaded by FunctionDecl, ClassTemplateSpecializationDecl and
VarTemplateSpecializationDecl to print the template arguments, if any.

The second customisation point is NamedDecl::printName. By default it just
streams the stored DeclarationName into the output but it can be customised
to provide a user-friendly name for an entity. It is currently overloaded by
DecompositionDecl and MSGuidDecl.

What this patch does:
---------------------
For many diagnostics a DeclarationName is used instead of the NamedDecl*.
This bypasses the two customisation points mentioned above. This patches fix
this for diagnostics in Sema.cpp, SemaCast.cpp, SemaChecking.cpp, SemaDecl.cpp,
SemaDeclAttr.cpp, SemaDecl.cpp, SemaOverload.cpp and SemaStmt.cpp.

I have only modified diagnostics where I could construct a test-case which
demonstrates that the change is appropriate (either with this patch or the next
one).

Reviewed By: erichkeane, aaron.ballman

Differential Revision: https://reviews.llvm.org/D84656
2020-07-28 10:30:35 +01:00
Leonard Chan 2f6bb2a692 [clang][Attribute] Fix noderef attribute false-negatives
`noderef` was failing to trigger warnings in some cases related to c++ style
casting. This patch addresses them.

Differential Revision: https://reviews.llvm.org/D77836
2020-06-10 12:20:54 -07:00
Ties Stuij ecd682bbf5 [ARM] Add __bf16 as new Bfloat16 C Type
Summary:
This patch upstreams support for a new storage only bfloat16 C type.
This type is used to implement primitive support for bfloat16 data, in
line with the Bfloat16 extension of the Armv8.6-a architecture, as
detailed here:

https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/arm-architecture-developments-armv8-6-a

The bfloat type, and its properties are specified in the Arm Architecture
Reference Manual:

https://developer.arm.com/docs/ddi0487/latest/arm-architecture-reference-manual-armv8-for-armv8-a-architecture-profile

In detail this patch:
- introduces an opaque, storage-only C-type __bf16, which introduces a new bfloat IR type.

This is part of a patch series, starting with command-line and Bfloat16
assembly support. The subsequent patches will upstream intrinsics
support for BFloat16, followed by Matrix Multiplication and the
remaining Virtualization features of the armv8.6-a architecture.

The following people contributed to this patch:
- Luke Cheeseman
- Momchil Velikov
- Alexandros Lamprineas
- Luke Geeson
- Simon Tatham
- Ties Stuij

Reviewers: SjoerdMeijer, rjmccall, rsmith, liutianle, RKSimon, craig.topper, jfb, LukeGeeson, fpetrogalli

Reviewed By: SjoerdMeijer

Subscribers: labrinea, majnemer, asmith, dexonsmith, kristof.beyls, arphaman, danielkiss, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76077
2020-06-05 10:32:43 +01:00
Florian Hahn 8f3f88d2f5 [Matrix] Implement matrix index expressions ([][]).
This patch implements matrix index expressions
(matrix[RowIdx][ColumnIdx]).

It does so by introducing a new MatrixSubscriptExpr(Base, RowIdx, ColumnIdx).
MatrixSubscriptExprs are built in 2 steps in ActOnMatrixSubscriptExpr. First,
if the base of a subscript is of matrix type, we create a incomplete
MatrixSubscriptExpr(base, idx, nullptr). Second, if the base is an incomplete
MatrixSubscriptExpr, we create a complete
MatrixSubscriptExpr(base->getBase(), base->getRowIdx(), idx)

Similar to vector elements, it is not possible to take the address of
a MatrixSubscriptExpr.
For CodeGen, a new MatrixElt type is added to LValue, which is very
similar to VectorElt. The only difference is that we may need to cast
the type of the base from an array to a vector type when accessing it.

Reviewers: rjmccall, anemet, Bigcheese, rsmith, martong

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D76791
2020-06-01 20:08:49 +01:00
Alexey Bader e95ee300c0 [SYCL] Prohibit arithmetic operations for incompatible pointers
Summary:
This change enables OpenCL diagnostics for the pointers annotated with
address space attribute SYCL mode.

Move `isAddressSpaceOverlapping` method from PointerType to QualType.

Reviewers: Anastasia, rjmccall

Reviewed By: rjmccall

Subscribers: rjmccall, jeroen.dobbelaere, Fznamznon, yaxunl, ebevhan, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80317
2020-05-22 13:43:24 +03:00
Anastasia Stulova a6a237f204 [OpenCL] Added addrspace_cast operator in C++ mode.
This operator is intended for casting between
pointers to objects in different address spaces
and follows similar logic as const_cast in C++.

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60193
2020-05-18 12:07:54 +01:00
Richard Sandiford 856bdd01fd [Sema][SVE] Allow casting SVE types to themselves in C
Casts from an SVE type to itself aren't very useful, but they are
supposed to be valid, and could occur in things like macro expansions.

Such casts already work for C++ and are tested by sizeless-1.cpp.
This patch makes them work for C too.

Differential Revision: https://reviews.llvm.org/D76694
2020-03-25 10:52:43 +00:00
Arthur Eubanks 51d7f64544 Don't emit pointer to int cast warnings under -Wmicrosoft-cast
Summary:
MSVC also warns on this:
$ cat /tmp/a.c
int f(void* p) { return (int) p; }

$ cl /c /tmp/a.c
C:/src/tmp/a.c(1): warning C4311: 'type cast': pointer truncation from
'void *' to 'int'

Warnings originally added in https://reviews.llvm.org/D72231.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75643
2020-03-09 13:27:46 -07:00
Nathan Chancellor 2a41b31fcd [Sema] Add -Wpointer-to-enum-cast and -Wvoid-pointer-to-enum-cast
GCC does not warn on casts from pointers to enumerators, while clang
currently does: https://godbolt.org/z/3DFDVG

This causes a bunch of extra warnings in the Linux kernel, where
certain structs contain a void pointer to avoid using a gigantic
union for all of the various types of driver data, such as
versions.

Add a diagnostic that allows certain projects like the kernel to
disable the warning just for enums, which allows those projects to
keep full compatibility with GCC but keeps the intention of treating
casts to integers and enumerators the same by default so that other
projects have the opportunity to catch issues not noticed before (or
follow suite and disable the warning).

Link: https://github.com/ClangBuiltLinux/linux/issues/887

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D75758
2020-03-07 16:43:39 -08:00
Arthur Eubanks cfff4851ac Add warnings for casting ptr -> smaller int for C++ in Microsoft mode
Adds warnings to groups recently added in https://reviews.llvm.org/D72231.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D75708
2020-03-05 15:17:06 -08:00
Mark de Wever 56eb15a1c7 [Sema] Fix pointer-to-int-cast diagnostic for _Bool
The diagnostic added in D72231 also shows a diagnostic when casting to a
_Bool. This is unwanted. This patch removes the diagnostic for _Bool types.

Differential Revision: https://reviews.llvm.org/D74860
2020-02-22 19:39:49 +01:00
Mark de Wever 9658d895c8 [Sema] Adds the pointer-to-int-cast diagnostic
Converting a pointer to an integer whose result cannot represented in the
integer type is undefined behavior is C and prohibited in C++. C++ already
has a diagnostic when casting. This adds a diagnostic for C.

Since this diagnostic uses the range of the conversion it also modifies
int-to-pointer-cast diagnostic to use a range.

Fixes PR8718: No warning on casting between pointer and non-pointer-sized int

Differential Revision: https://reviews.llvm.org/D72231
2020-02-16 15:38:25 +01:00
Anastasia Stulova 6064f426a1 [OpenCL] Restrict addr space conversions in nested pointers
Address space conversion changes pointer representation.
This commit disallows such conversions when they are not
legal i.e. for the nested pointers even with compatible
address spaces. Because the address space conversion in
the nested levels can't be generated to modify the pointers
correctly. The behavior implemented is as follows:

- Any implicit conversions of nested pointers with different
  address spaces is rejected.
- Any conversion of address spaces in nested pointers in safe
  casts (e.g. const_cast or static_cast) is rejected.
- Conversion in low level C-style or reinterpret_cast is accepted
  but with a warning (this aligns with OpenCL C behavior).

Fixes PR39674

Differential Revision: https://reviews.llvm.org/D73360
2020-02-07 12:04:35 +00:00
Saar Raz b65b1f322b [Concepts] Function trailing requires clauses
Function trailing requires clauses now parsed, supported in overload resolution and when calling, referencing and taking the address of functions or function templates.

Differential Revision: https://reviews.llvm.org/D43357
2020-01-09 15:07:51 +02:00
Richard Smith 3ced23976a Refactor CompareReferenceRelationship and its callers in preparation for
implementing the resolution of CWG2352.

No functionality change, except that we now convert the referent of a
reference binding to the underlying type of the reference in more cases;
we used to happen to preserve the type sugar from the referent if the
only type change was in the cv-qualifiers.

This exposed a bug in how we generate code for trivial assignment
operators: if the type sugar (particularly the may_alias attribute)
got lost during reference binding, we'd use the "wrong" TBAA information
for the load during the assignment.
2019-12-18 14:05:57 -08:00
Richard Smith eea8ba097c Check whether the destination is a complete type in a static_cast (or
C-style cast) to an enumeration type.

We previously forgot to check this, and happened to get away with it
(with bad diagnostics) only because we misclassified incomplete
enumeration types as not being unscoped enumeration types. This also
fixes the misclassification.
2019-12-16 18:33:35 -08:00
Dávid Bolvanský 55507110b9 [Diagnostics] Improve some error messages related to bad use of dynamic_cast 2019-11-04 16:26:43 +01:00
Richard Smith 70f59b5bbc When diagnosing an ambiguity, only note the candidates that contribute
to the ambiguity, rather than noting all viable candidates.
2019-10-24 14:58:29 -07:00
Peter Collingbourne 766f15814a Sema: Create a no-op implicit cast for lvalue function conversions.
This fixes an assertion failure in the case where an implicit conversion for a
function call involves an lvalue function conversion, and makes the AST for
initializations involving implicit lvalue function conversions more accurate.

Differential Revision: https://reviews.llvm.org/D66437

llvm-svn: 375313
2019-10-19 00:34:54 +00:00
Richard Smith 74ce7112c3 Fix behavior of __builtin_bit_cast when the From and To types are the
same.

We were missing the lvalue-to-rvalue conversion entirely in this case,
and in fact still need the full CK_LValueToRValueBitCast conversion to
perform a load with no TBAA.

llvm-svn: 373874
2019-10-07 02:45:12 +00:00
Erik Pilkington 055fcec78c [Sema] Check __builtin_bit_cast operand for completeness before materializing it.
This shouldn't be observable, but it doesn't make sense to materialize an
incomplete type.

llvm-svn: 368610
2019-08-12 19:29:43 +00:00
Erik Pilkington 0a223d981e [Sema] Require a complete type for __builtin_bit_cast operands
Fixes llvm.org/PR42936

llvm-svn: 368600
2019-08-12 18:31:27 +00:00
Anastasia Stulova 46b55fa58d [OpenCL] Update comments/diagnostics to refer to C++ for OpenCL
Clang doesn't implement OpenCL C++, change the comments to
reflect that.

Differential Revision: https://reviews.llvm.org/D64867

llvm-svn: 366421
2019-07-18 10:02:35 +00:00
Rui Ueyama 49a3ad21d6 Fix parameter name comments using clang-tidy. NFC.
This patch applies clang-tidy's bugprone-argument-comment tool
to LLVM, clang and lld source trees. Here is how I created this
patch:

$ git clone https://github.com/llvm/llvm-project.git
$ cd llvm-project
$ mkdir build
$ cd build
$ cmake -GNinja -DCMAKE_BUILD_TYPE=Debug \
    -DLLVM_ENABLE_PROJECTS='clang;lld;clang-tools-extra' \
    -DCMAKE_EXPORT_COMPILE_COMMANDS=On -DLLVM_ENABLE_LLD=On \
    -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ ../llvm
$ ninja
$ parallel clang-tidy -checks='-*,bugprone-argument-comment' \
    -config='{CheckOptions: [{key: StrictMode, value: 1}]}' -fix \
    ::: ../llvm/lib/**/*.{cpp,h} ../clang/lib/**/*.{cpp,h} ../lld/**/*.{cpp,h}

llvm-svn: 366177
2019-07-16 04:46:31 +00:00
Erik Pilkington eee944e7f9 [C++2a] Add __builtin_bit_cast, used to implement std::bit_cast
This commit adds a new builtin, __builtin_bit_cast(T, v), which performs a
bit_cast from a value v to a type T. This expression can be evaluated at
compile time under specific circumstances.

The compile time evaluation currently doesn't support bit-fields, but I'm
planning on fixing this in a follow up (some of the logic for figuring this out
is in CodeGen). I'm also planning follow-ups for supporting some more esoteric
types that the constexpr evaluator supports, as well as extending
__builtin_memcpy constexpr evaluation to use the same infrastructure.

rdar://44987528

Differential revision: https://reviews.llvm.org/D62825

llvm-svn: 364954
2019-07-02 18:28:13 +00:00
Anastasia Stulova 5099aef869 [PR41567][Sema] Fixed cast kind in addr space conversions
This change sets missing cast kind correctly in the address
space conversion case.

Differential Revision: https://reviews.llvm.org/D62299

llvm-svn: 362409
2019-06-03 15:42:36 +00:00
Anastasia Stulova 5b6dda33d1 [Sema][OpenCL] Make address space conversions a bit stricter.
The semantics for converting nested pointers between address
spaces are not very well defined. Some conversions which do not
really carry any meaning only produce warnings, and in some cases
warnings hide invalid conversions, such as 'global int*' to
'local float*'!

This patch changes the logic in checkPointerTypesForAssignment
and checkAddressSpaceCast to fail properly on implicit conversions
that should definitely not be permitted. We also dig deeper into the
pointer types and warn on explicit conversions where the address
space in a nested pointer changes, regardless of whether the address
space is compatible with the corresponding pointer nesting level
on the destination type.

Fixes PR39674!

Patch by ebevhan (Bevin Hansson)!

Differential Revision: https://reviews.llvm.org/D58236

llvm-svn: 360258
2019-05-08 14:23:49 +00:00
David Blaikie 5e32805050 SemaOverload: Complete candidates before emitting the error, to ensure diagnostics emitted (or suppressed) during completion don't interfere with the overload notes
Because diagnostics and their notes are not connected at the API level,
if the error message for an overload is emitted, then the overload
candidates are completed - if a diagnostic is emitted during that work,
the notes related to overload candidates would be attached to the latter
diagnostic, not the original error. Sort of worse, if the latter
diagnostic was disabled, the notes are disabled.

Reviewers: rsmith

Differential Revision: https://reviews.llvm.org/D61357

llvm-svn: 359854
2019-05-03 00:44:50 +00:00
Anastasia Stulova 6f7c536e08 [Sema] Change addr space diagnostics in casts to follow C++ style.
This change adds a new diagnostic for mismatching address spaces
to be used for C++ casts (only enabled in C style cast for now,
the rest will follow!).

The change extends C-style cast rules to account for address spaces.
It also adds a separate function for address space cast checking that
can be used to map from a separate address space cast operator
addrspace_cast (to be added as a follow up patch).

Note, that after this change clang will no longer allows arbitrary
address space conversions in reinterpret_casts because they can lead
to accidental errors. The implicit safe conversions would still be
allowed.

Differential Revision: https://reviews.llvm.org/D58346

llvm-svn: 355609
2019-03-07 17:06:30 +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
Fangrui Song 407659ab0a Revert "Revert r347417 "Re-Reinstate 347294 with a fix for the failures.""
It seems the two failing tests can be simply fixed after r348037

Fix 3 cases in Analysis/builtin-functions.cpp
Delete the bad CodeGen/builtin-constant-p.c for now

llvm-svn: 348053
2018-11-30 23:41:18 +00:00
Fangrui Song f5d3335d75 Revert r347417 "Re-Reinstate 347294 with a fix for the failures."
Kept the "indirect_builtin_constant_p" test case in test/SemaCXX/constant-expression-cxx1y.cpp
while we are investigating why the following snippet fails:

  extern char extern_var;
  struct { int a; } a = {__builtin_constant_p(extern_var)};

llvm-svn: 348039
2018-11-30 21:26:09 +00:00
Hans Wennborg 48ee4ad325 Re-commit r347417 "Re-Reinstate 347294 with a fix for the failures."
This was reverted in r347656 due to me thinking it caused a miscompile of
Chromium. Turns out it was the Chromium code that was broken.

llvm-svn: 347756
2018-11-28 14:04:12 +00:00