[Windows] evaluating a variable from an outer scope in a @finally inside a block causes an internal compiler error

[DHowett]

Bugzilla Link	44483
Version	9.0
OS	Windows NT
Attachments	preprocessed repro, repro.sh
CC	@davidchisnall,@dwblaikie,@zygoloid,@rnk

Extended Description

With the following source:

// clang -emit-llvm -S -fblocks --target=i686-pc-windows-msvc repro.mm
int test() {
    int outer;
    auto b = ^() {
        @​try {
        } @​finally {
            outer;
        }
    };
}

the compiler fails with the following stack dump.

Stack dump:
0.      Program arguments: C:\Program Files\LLVM\bin\clang.exe -cc1 -triple i686-pc-windows-msvc19.11.0 -emit-llvm -disable-free -disable-llvm-verifier -discard-value-names -main-file-name repro.mm -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -target-cpu pentium4 -dwarf-column-info -coverage-notes-file E:\repro.gcno -resource-dir C:\Program Files\LLVM\lib\clang\9.0.0 -internal-isystem C:\Program Files\LLVM\lib\clang\9.0.0\include -internal-isystem C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.23.28105\include -internal-isystem C:\Program Files (x86)\Windows Kits\NETFXSDK\4.8\include\um -internal-isystem C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\ucrt -internal-isystem C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\shared -internal-isystem C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\um -internal-isystem C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\winrt -internal-isystem C:\Program Files (x86)\Windows Kits\10\include\10.0.18362.0\cppwinrt -fdeprecated-macro -fdebug-compilation-dir E:\ -ferror-limit 19 -fmessage-length 158 -fblocks -fno-use-cxa-atexit -fms-extensions -fms-compatibility -fms-compatibility-version=19.11 -std=c++14 -fdelayed-template-parsing -fobjc-runtime=gcc -fobjc-exceptions -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -faddrsig -o repro.ll -x objective-c++ repro.mm
1.      <eof> parser at end of file
2.      repro.mm:1:5: LLVM IR generation of declaration 'test'
3.      repro.mm:1:5: Generating code for declaration 'test'
4.      repro.mm:3:18: LLVM IR generation of compound statement ('{}')
 #&#8203;0 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x19d349a C:\Program Files\LLVM\bin\clang.exe 0x19d4903
 #&#8203;1 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a6eccc C:\Program Files\LLVM\bin\clang.exe 0x18d0c3a
 #&#8203;2 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a710c4 C:\Program Files\LLVM\bin\clang.exe 0x1a79dc0
 #&#8203;3 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a7956b C:\Program Files\LLVM\bin\clang.exe 0x1a71626
 #&#8203;4 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a70b1f C:\Program Files\LLVM\bin\clang.exe 0x1a9ef99
 #&#8203;5 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a9bf22 C:\Program Files\LLVM\bin\clang.exe 0x1a9a6bc
 #&#8203;6 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1acdf90 C:\Program Files\LLVM\bin\clang.exe 0x1ac390e
 #&#8203;7 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a0ffdf C:\Program Files\LLVM\bin\clang.exe 0x1a14ebe
 #&#8203;8 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a12948 C:\Program Files\LLVM\bin\clang.exe 0x1a0e3d6
 #&#8203;9 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a0e063 C:\Program Files\LLVM\bin\clang.exe 0x1a7966c
#&#8203;10 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a716b9 C:\Program Files\LLVM\bin\clang.exe 0x1a70b1f
#&#8203;11 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1a79dc0 C:\Program Files\LLVM\bin\clang.exe 0x1995cae
#&#8203;12 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1996599 C:\Program Files\LLVM\bin\clang.exe 0x181cb1d
#&#8203;13 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x181618c C:\Program Files\LLVM\bin\clang.exe 0x1819d61
#&#8203;14 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x181f55d C:\Program Files\LLVM\bin\clang.exe 0x33cbc3f
#&#8203;15 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x33c9719 C:\Program Files\LLVM\bin\clang.exe 0x275b465
#&#8203;16 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1cb3492 C:\Program Files\LLVM\bin\clang.exe 0x1c771cf
#&#8203;17 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x1d09c87 C:\Program Files\LLVM\bin\clang.exe 0x6e87
#&#8203;18 0x00007ff64b3e349a C:\Program Files\LLVM\bin\clang.exe 0x4564 C:\Program Files\LLVM\bin\clang.exe 0x33dda20

[DHowett]

Additional information from a TRUNK build.

Assertion failed: Val && "isa<> used on a null pointer", file E:\src\llvm\include\llvm/Support/Casting.h, line 105

0x00007FF7C1F41FA8 (0x0000000519382838 0x000001A277CBC860 0x000001A277C59918 0xCCCCCCCCCCCCCCCC), llvm::isa<clang::ImplicitParamDecl,clang::VarDecl const *>() + 0x28 bytes(s), E:\src\llvm\include\llvm\Support\Casting.h, line 144
0x00007FF7C2265484 (0x00000005193830D8 0x0000000519385698 0x000001A2782A24E0 0x000001A2782BE800), clang::CodeGen::CodeGenFunction::EmitCapturedLocals() + 0x4B4 bytes(s), E:\src\llvm\tools\clang\lib\CodeGen\CGException.cpp, line 1798 + 0xD byte(s)
0x00007FF7C226464E (0x00000005193830D8 0x0000000519385698 0xCCCCCCCCCCCCCC00 0x000001A2782A24E0), clang::CodeGen::CodeGenFunction::startOutlinedSEHHelper() + 0x6FE bytes(s), E:\src\llvm\tools\clang\lib\CodeGen\CGException.cpp, line 1889
0x00007FF7C2100A62 (0x000001A277CBE330 0x0000000519385698 0x000001A2782A2538 0x0000000519384970), clang::CodeGen::CGObjCRuntime::EmitTryCatchStmt() + 0x562 bytes(s), E:\src\llvm\tools\clang\lib\CodeGen\CGObjCRuntime.cpp, line 203

[DHowett]

David, it looks like we're trying to capture "CXXAbiThisDecl" somewhere south of CGObjCRuntime::EmitTryCatchStmt() for a funclet finally in SEH mode.

EmitTryCatchStmt -> startOutlinedSEHHelper -> EmitCapturedLocals -> CaptureFinder(CXXABIThisDecl) -> CaptureFinder::VisitDeclRefExpr -> refersToEnclosingVariableOrCapture therefore insert ParentThis.

Problem is, CXXAbiThisDecl is null because there is no this decl and we're not in a C++ context with a this.

[rnk]

I applied a shallow fix, but things just crash later:

diff --git a/clang/lib/CodeGen/CGException.cpp b/clang/lib/CodeGen/CGException.cpp
index 53fafab3e0e..7968e446ec8 100644
--- a/clang/lib/CodeGen/CGException.cpp
+++ b/clang/lib/CodeGen/CGException.cpp
@@ -1675,7 +1675,7 @@ struct CaptureFinder : ConstStmtVisitor {

void VisitDeclRefExpr(const DeclRefExpr *E) {
// If this is already a capture, just make sure we capture 'this'.

• if (E->refersToEnclosingVariableOrCapture()) {

• if (ParentThis && E->refersToEnclosingVariableOrCapture()) {
  Captures.insert(ParentThis);
  return;
  }

I seem to recall assuming somewhere that SEH only happens in FunctionDecls, but BlockDecl doesn't inherit from FunctionDecl.

[triplef]

See also #51899 for an even simpler reproduction without referencing a variable. Basically using @finally will crash the compiler.

[llvmbot]

@llvm/issue-subscribers-clang-frontend