Alias must point to a definition

[Quuxplusone]

Bugzilla Link	PR30341
Status	RESOLVED FIXED
Importance	P normal
Reported by	Mike Aizatsky (aizatsky@chromium.org)
Reported on	2016-09-09 16:33:45 -0700
Last modified on	2016-10-11 09:41:28 -0700
Version	trunk
Hardware	PC Linux
CC	eric@efcs.ca, eugeni.stepanov@gmail.com, hiraditya@msn.com, ismail@i10z.com, laxmansole@gmail.com, llvm-bugs@lists.llvm.org, mclow.lists@gmail.com, rafael@espindo.la, willdtz@gmail.com
Fixed by commit(s)
Attachments	NeonEmitter-2e0fb6.sh (6035 bytes, application/x-shellscript) NeonEmitter-2e0fb6.cpp (4387716 bytes, text/x-c++src)
Blocks
Blocked by
See also

Created attachment 17232
Reproducer script

This happens on the sanitizer-x86_64-linux-bootstrap bot. Reproducer attached.

http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap/builds/12886

[255/2434] Building CXX object tools/clang/utils/TableGen/CMakeFiles/obj.clang-
tblgen.dir/NeonEmitter.cpp.o
FAILED: tools/clang/utils/TableGen/CMakeFiles/obj.clang-
tblgen.dir/NeonEmitter.cpp.o
/mnt/b/sanitizer-buildbot2/sanitizer-x86_64-linux-
bootstrap/build/llvm_build0/bin/clang++   -DCLANG_ENABLE_ARCMT -
DCLANG_ENABLE_OBJC_REWRITER -DCLANG_ENABLE_STATIC_ANALYZER -DGTEST_HAS_RTTI=0 -
D_DEBUG -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -
D__STDC_LIMIT_MACROS -Itools/clang/utils/TableGen -I/mnt/b/sanitizer-
buildbot2/sanitizer-x86_64-linux-
bootstrap/build/llvm/tools/clang/utils/TableGen -I/mnt/b/sanitizer-
buildbot2/sanitizer-x86_64-linux-bootstrap/build/llvm/tools/clang/include -
Itools/clang/include -Iinclude -I/mnt/b/sanitizer-buildbot2/sanitizer-x86_64-
linux-bootstrap/build/llvm/include -nostdinc++ -isystem /mnt/b/sanitizer-
buildbot2/sanitizer-x86_64-linux-bootstrap/build/libcxx_build_msan/include -
isystem /mnt/b/sanitizer-buildbot2/sanitizer-x86_64-linux-
bootstrap/build/libcxx_build_msan/include/c++/v1  -lc++abi -Wl,--
rpath=/mnt/b/sanitizer-buildbot2/sanitizer-x86_64-linux-
bootstrap/build/libcxx_build_msan/lib -L/mnt/b/sanitizer-buildbot2/sanitizer-
x86_64-linux-bootstrap/build/libcxx_build_msan/lib -fsanitize=memory -w -
stdlib=libc++ -fPIC -fvisibility-inlines-hidden -Wall -W -Wno-unused-parameter -
Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-
long -Wcovered-switch-default -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -
Werror=date-time -std=c++11 -fno-omit-frame-pointer -gline-tables-only -
fsanitize=memory -fcolor-diagnostics -ffunction-sections -fdata-sections -fno-
common -Woverloaded-virtual -Wno-nested-anon-types -O3    -UNDEBUG  -fno-
exceptions -fno-rtti -MMD -MT tools/clang/utils/TableGen/CMakeFiles/obj.clang-
tblgen.dir/NeonEmitter.cpp.o -MF
tools/clang/utils/TableGen/CMakeFiles/obj.clang-tblgen.dir/NeonEmitter.cpp.o.d -
o tools/clang/utils/TableGen/CMakeFiles/obj.clang-tblgen.dir/NeonEmitter.cpp.o -
c /mnt/b/sanitizer-buildbot2/sanitizer-x86_64-linux-
bootstrap/build/llvm/tools/clang/utils/TableGen/NeonEmitter.cpp
Alias must point to a definition
void (%"class.(anonymous namespace)::TypeSpec"*)*
@_ZN12_GLOBAL__N_18TypeSpecD2Ev
fatal error: error in backend: Broken module found, compilation aborted!
clang-3.9: error: clang frontend command failed with exit code 70 (use -v to
see invocation)
clang version 4.0.0 (trunk 280954)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /mnt/b/sanitizer-buildbot2/sanitizer-x86_64-linux-
bootstrap/build/llvm_build0/bin
clang-3.9: note: diagnostic msg: PLEASE submit a bug report to
http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and
associated run script.
clang-3.9: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang-3.9: note: diagnostic msg: /tmp/NeonEmitter-5211df.cpp
clang-3.9: note: diagnostic msg: /tmp/NeonEmitter-5211df.sh
clang-3.9: note: diagnostic msg:

********************

[Quuxplusone]

Attached NeonEmitter-2e0fb6.sh (6035 bytes, application/x-shellscript): Reproducer script

[Quuxplusone]

Attached NeonEmitter-2e0fb6.cpp (4387716 bytes, text/x-c++src): reproducer source

[Quuxplusone]

r280908 was fine, r280974 is the first failure I saw. sanitizer-x86_64-linux-bootstrap shows r280957 as broken it's something in between.

[Quuxplusone]

A change might have triggered it, not the change in compiler.

[Quuxplusone]

My build also started failing as of September 9th with a similar error about "Alias must point to definition", and on the same symbol too (@_ZN12_GLOBAL__N_18TypeSpecD2Ev). This only occurs during bootstrapping but I'm not building with sanitizer instrumentation. Will investigate more and report back anything useful.

[Quuxplusone]

I've narrowed the problem down to this commit in libcxx, which when I revert
fixes the problem for me:

https://github.com/llvm-mirror/libcxx/commit/b9e6e8972a521e72192f370bb1ca98cbddee6a00

"TypeSpec" in NeonEmitter is a wrapper for std::string, and this commit
marks its destructor as 'always inline'.

Looks like the GlobalAlias is an optimization to avoid emitting duplicated code:

https://github.com/llvm-mirror/clang/blob/0d7600a4bc4258d5d4157765e1764af0efc82805/lib/CodeGen/CGCXX.cpp#L120

In particular this bit mentions libc++'s use of AlwaysInline and describes what
seems to be the issue encountered here:

https://github.com/llvm-mirror/clang/blob/0d7600a4bc4258d5d4157765e1764af0efc82805/lib/CodeGen/CGCXX.cpp#L167

I'm not quite sure where things go wrong, however.

[Quuxplusone]

Indeed reverting r280944 fixes the problem, great analysis Will! It would be nice if libc++ maintainers can take a look at this.

[Quuxplusone]

Should r280944 be reverted then? The bot has been red for almost a week now.

[Quuxplusone]

(In reply to comment #7)
> Should r280944 be reverted then? The bot has been red for almost a week now.

IMHO yes, this is also breaking our openSUSE trunk builds.

[Quuxplusone]

Looking now.

[Quuxplusone]

Looking now.

[Quuxplusone]

I think the problem comes from the class TypeSpec which derives from std::string. Deriving from std::string is not recommended.

for reference: http://stackoverflow.com/questions/6006860/why-should-one-not-derive-from-c-std-string-class

[Quuxplusone]

FYI I reverted the problem commit in r281562 with the intent of recommitting it once we fix the real underlying problem.

I'm working on reducing the NeonEmitter.cpp right now.

[Quuxplusone]

Still not sure what's going on with Clang. Here is a reduced reproducer:

// clang++ -std=c++11 -O1 test.cpp
template <class CharT>
struct String {
  String() {}
  ~String();
};

template <class CharT>
inline __attribute__((visibility("hidden"), always_inline))
String<CharT>::~String() {}

extern template struct String<char>;

struct Foo : public String<char> { Foo() { String<char> s; } };

Foo f;

[Quuxplusone]

Note that the above reproducer compiles fine at -O0 but not at -O1. Since the crash seems to be in the backend is this an LLVM bug and not a Clang one?

Could somebody with more knowledge re-bin this bug accordingly and CC anyone who might know how to fix this.

[Quuxplusone]

Thanks for reducing the testcase. I was able to figure out that the crash is in llc but the original file was too big to efficiently figure out the exact cause. I'll look into the problem.

[Quuxplusone]

For the reduced example, when compiled at -O1 clang front end does not generate
the definition of the destructor (~Foo), instead it makes ~Foo an alias of
~String.

//Generating IR at O0.
$PATH/llvm-project/build/bin/clang-3.9 -cc1 -O0 -triple x86_64-unknown-linux-
gnu -emit-llvm -emit-llvm-uselists -disable-free -main-file-name reduced.cpp -
mrelocation-model static -mthread-model posix -fmath-errno -masm-verbose -
mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -momit-
leaf-frame-pointer -v -dwarf-column-info -debugger-tuning=gdb -resource-dir
$PATH/llvm-project/build/bin/../lib/clang/4.0.0 -std=c++11 -fdeprecated-macro -
ferror-limit 19 -fmessage-length 146 -fobjc-runtime=gcc -fcxx-exceptions -
fexceptions -fdiagnostics-show-option -fcolor-diagnostics -disable-llvm-passes -
o reduced-clang-O0.ll -x c++-cpp-output reduced.ii

//Generating IR at O1.
$PATH/llvm-project/build/bin/clang-3.9 -cc1 -O1 -triple x86_64-unknown-linux-
gnu -emit-llvm -emit-llvm-uselists -disable-free -main-file-name reduced.cpp -
mrelocation-model static -mthread-model posix -fmath-errno -masm-verbose -
mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -momit-
leaf-frame-pointer -v -dwarf-column-info -debugger-tuning=gdb -resource-dir
$PATH/llvm-project/build/bin/../lib/clang/4.0.0 -std=c++11 -fdeprecated-macro -
ferror-limit 19 -fmessage-length 146 -fobjc-runtime=gcc -fcxx-exceptions -
fexceptions -fdiagnostics-show-option -fcolor-diagnostics -disable-llvm-passes -
o reduced-clang-O1.ll -x c++-cpp-output reduced.ii

Specifically, "define linkonce_odr void @_ZN3FooD2Ev(%struct.Foo* %this)"
is not there when compiled at -O1. But it generates,

@_ZN3FooD2Ev = linkonce_odr alias void (%struct.Foo*), bitcast (void
(%struct.String*)* @_ZN6StringIcED2Ev to void (%struct.Foo*)*)

Still debugging...

$ diff reduced-clang-O0.ll reduced-clang-O1.ll

11,12d10
< $_ZN3FooD2Ev = comdat any
<
16a15,16
> @_ZN3FooD2Ev = linkonce_odr alias void (%struct.Foo*), bitcast (void
(%struct.String*)* @_ZN6StringIcED2Ev to void (%struct.Foo*)*)
>
21c21
<   %0 = call i32 @__cxa_atexit(void (i8*)* bitcast (void (%struct.Foo*)*
@_ZN3FooD2Ev to void (i8*)*), i8* getelementptr inbounds (%struct.Foo,
%struct.Foo* @f, i32 0, i32 0), i8* @__dso_handle) #2
---
>   %0 = call i32 @__cxa_atexit(void (i8*)* bitcast (void (%struct.Foo*)*
@_ZN3FooD2Ev to void (i8*)*), i8* getelementptr inbounds (%struct.Foo,
%struct.Foo* @f, i32 0, i32 0), i8* @__dso_handle) #1
32c32
<   store %struct.Foo* %this, %struct.Foo** %this.addr, align 8
---
>   store %struct.Foo* %this, %struct.Foo** %this.addr, align 8, !tbaa !1
36c36,38
<   invoke void @_ZN6StringIcEC1Ev(%struct.String* %s)
---
>   %1 = bitcast %struct.String* %s to i8*
>   call void @llvm.lifetime.start(i64 1, i8* %1) #1
>   invoke void @_ZN6StringIcEC2Ev(%struct.String* %s)
40c42,44
<   call void @_ZN6StringIcED2Ev(%struct.String* %s) #2
---
>   call void @_ZN6StringIcED2Ev(%struct.String* %s) #1
>   %2 = bitcast %struct.String* %s to i8*
>   call void @llvm.lifetime.end(i64 1, i8* %2) #1
44c48
<   %1 = landingpad { i8*, i32 }
---
>   %3 = landingpad { i8*, i32 }
46,51c50,55
<   %2 = extractvalue { i8*, i32 } %1, 0
<   store i8* %2, i8** %exn.slot, align 8
<   %3 = extractvalue { i8*, i32 } %1, 1
<   store i32 %3, i32* %ehselector.slot, align 4
<   %4 = bitcast %struct.Foo* %this1 to %struct.String*
<   call void @_ZN6StringIcED2Ev(%struct.String* %4) #2
---
>   %4 = extractvalue { i8*, i32 } %3, 0
>   store i8* %4, i8** %exn.slot, align 8
>   %5 = extractvalue { i8*, i32 } %3, 1
>   store i32 %5, i32* %ehselector.slot, align 4
>   %6 = bitcast %struct.Foo* %this1 to %struct.String*
>   call void @_ZN6StringIcED2Ev(%struct.String* %6) #1
60,70d63
< }
<
< ; Function Attrs: inlinehint nounwind uwtable
< define linkonce_odr void @_ZN3FooD2Ev(%struct.Foo* %this) unnamed_addr #1
comdat align 2 {
< entry:
<   %this.addr = alloca %struct.Foo*, align 8
<   store %struct.Foo* %this, %struct.Foo** %this.addr, align 8
<   %this1 = load %struct.Foo*, %struct.Foo** %this.addr, align 8
<   %0 = bitcast %struct.Foo* %this1 to %struct.String*
<   call void @_ZN6StringIcED2Ev(%struct.String* %0) #2
<   ret void
73c66
<   uselistorder void (%struct.String*)* @_ZN6StringIcED2Ev, { 1, 2, 0 }
---
>   uselistorder void (%struct.String*)* @_ZN6StringIcED2Ev, { 2, 0, 1 }
77c70
< declare i32 @__cxa_atexit(void (i8*)*, i8*, i8*) #2
---
> declare i32 @__cxa_atexit(void (i8*)*, i8*, i8*) #1
79c72,73
< declare void @_ZN6StringIcEC2Ev(%struct.String*) unnamed_addr #3
---
> ; Function Attrs: argmemonly nounwind
> declare void @llvm.lifetime.start(i64, i8* nocapture) #2
81c75,82
< declare void @_ZN6StringIcEC1Ev(%struct.String*) unnamed_addr #3
---
> ; Function Attrs: nounwind uwtable
> define available_externally void @_ZN6StringIcEC2Ev(%struct.String* %this)
unnamed_addr #3 align 2 {
> entry:
>   %this.addr = alloca %struct.String*, align 8
>   store %struct.String* %this, %struct.String** %this.addr, align 8, !tbaa !1
>   %this1 = load %struct.String*, %struct.String** %this.addr, align 8
>   ret void
> }
89c90
<   store %struct.String* %this, %struct.String** %this.addr, align 8
---
>   store %struct.String* %this, %struct.String** %this.addr, align 8, !tbaa !1
92,94d92
<
< ; uselistorder directives
<   uselistorder i32 1, { 1, 0, 2, 3, 4, 5 }
96a95,97
> ; Function Attrs: argmemonly nounwind
> declare void @llvm.lifetime.end(i64, i8* nocapture) #2
>
105,107c106,108
< attributes #1 = { inlinehint nounwind uwtable "correctly-rounded-divide-sqrt-
fp-math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-
frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false"
"no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-
math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-
features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-
float"="false" }
< attributes #2 = { nounwind }
< attributes #3 = { "correctly-rounded-divide-sqrt-fp-math"="false" "disable-
tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-
elim"="false" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "no-signed-
zeros-fp-math"="false" "no-trapping-math"="false" "stack-protector-buffer-
size"="8" "target-cpu"="x86-64" "target-features"="+fxsr,+mmx,+sse,+sse2,+x87"
"unsafe-fp-math"="false" "use-soft-float"="false" }
---
> attributes #1 = { nounwind }
> attributes #2 = { argmemonly nounwind }
> attributes #3 = { nounwind uwtable "correctly-rounded-divide-sqrt-fp-
math"="false" "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-
frame-pointer-elim"="false" "no-infs-fp-math"="false" "no-jump-tables"="false"
"no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-
math"="false" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-
features"="+fxsr,+mmx,+sse,+sse2,+x87" "unsafe-fp-math"="false" "use-soft-
float"="false" }
112a114,117
> !1 = !{!2, !2, i64 0}
> !2 = !{!"any pointer", !3, i64 0}
> !3 = !{!"omnipotent char", !4, i64 0}
> !4 = !{!"Simple C++ TBAA"}

[Quuxplusone]

Since the base class String<char> is 'available_externally', the verifier does
not count these definitions as available and aborts. I'll add Rafael for his
insights.

commit 8ca97044ad94547f4e3740515a4c483824422813
Author: Rafael Espindola <rafael.espindola@gmail.com>, Thu Nov 26 19:22:59 2015
+0000 (10 months ago)
Committer: Rafael Espindola <rafael.espindola@gmail.com>, Thu Nov 26 19:22:59
2015 +0000 (10 months ago)
Branches: <Expand>

Disallow aliases to available_externally.

[Quuxplusone]

(In reply to comment #17)
> Since the base class String<char> is 'available_externally', the verifier
> does not count these definitions as available and aborts. I'll add Rafael
> for his insights.
>
> commit 8ca97044ad94547f4e3740515a4c483824422813
> Author: Rafael Espindola <rafael.espindola@gmail.com>, Thu Nov 26 19:22:59
> 2015 +0000 (10 months ago)
> Committer: Rafael Espindola <rafael.espindola@gmail.com>, Thu Nov 26
> 19:22:59 2015 +0000 (10 months ago)
> Branches: <Expand>
>
> Disallow aliases to available_externally.

That is correct. The available_externally will not be there in the end to allow
the creation of the alias. The bug seems to be in clang trying to create such
an alias.

[Quuxplusone]

Patch for review: https://reviews.llvm.org/D24682

[Quuxplusone]

https://reviews.llvm.org/rL283063