[CGExprConstant] stop calling into ConstExprEmitter for Reference type destinations

[nickdesaulniers]

Fixes a bug introduced by
commit b54294e ("[clang][ConstantEmitter] have tryEmitPrivate[ForVarInit] try ConstExprEmitter fast-path first")

In the added test case, the QualType is a LValueReferenceType.

LValueReferenceType 0x558412998d90 'const char (&)[41]'
`-ParenType 0x558412998d30 'const char[41]' sugar
  `-ConstantArrayType 0x558412998cf0 'const char[41]' 41
    `-QualType 0x55841294c271 'const char' const
      `-BuiltinType 0x55841294c270 'char'

Fixes: #69979

[llvmbot]

@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Nick Desaulniers (nickdesaulniers)

Changes

Fixes a bug introduced by
commit b54294e ("[clang][ConstantEmitter] have tryEmitPrivate[ForVarInit] try ConstExprEmitter fast-path first")

In the added test case, the QualType is a LValueReferenceType.

LValueReferenceType 0x558412998d90 'const char (&)[41]'
`-ParenType 0x558412998d30 'const char[41]' sugar
  `-ConstantArrayType 0x558412998cf0 'const char[41]' 41
    `-QualType 0x55841294c271 'const char' const
      `-BuiltinType 0x55841294c270 'char'

Fixes: #69979

---

Full diff: https://github.com/llvm/llvm-project/pull/70366.diff

2 Files Affected:

• (modified) clang/lib/CodeGen/CGExprConstant.cpp (+3)
• (modified) clang/test/CodeGenCXX/const-init-cxx11.cpp (+6)

diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 9b67a8b3335a165..0706647bf45466d 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -22,6 +22,7 @@
 #include "clang/AST/Attr.h"
 #include "clang/AST/RecordLayout.h"
 #include "clang/AST/StmtVisitor.h"
+#include "clang/AST/Type.h"
 #include "clang/Basic/Builtins.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/Sequence.h"
@@ -1358,6 +1359,8 @@ class ConstExprEmitter :
   }
 
   llvm::Constant *VisitStringLiteral(StringLiteral *E, QualType T) {
+    if (!isa<ConstantArrayType>(T.getDesugaredType(CGM.getContext())))
+      return nullptr;
     // This is a string literal initializing an array in an initializer.
     return CGM.GetConstantArrayFromStringLiteral(E);
   }
diff --git a/clang/test/CodeGenCXX/const-init-cxx11.cpp b/clang/test/CodeGenCXX/const-init-cxx11.cpp
index d22d78d2b94edb5..3a12fe444f137bf 100644
--- a/clang/test/CodeGenCXX/const-init-cxx11.cpp
+++ b/clang/test/CodeGenCXX/const-init-cxx11.cpp
@@ -424,6 +424,8 @@ namespace DR2126 {
 // CHECK: @_ZN33ClassTemplateWithStaticDataMember3useE ={{.*}} constant ptr @_ZGRN33ClassTemplateWithStaticDataMember1SIvE1aE_
 // CHECK: @_ZGRN39ClassTemplateWithHiddenStaticDataMember1SIvE1aE_ = linkonce_odr hidden constant i32 5, comdat
 // CHECK: @_ZN39ClassTemplateWithHiddenStaticDataMember3useE ={{.*}} constant ptr @_ZGRN39ClassTemplateWithHiddenStaticDataMember1SIvE1aE_
+// CHECK: @.str.[[STR:[0-9]+]] ={{.*}} constant [9 x i8] c"12345678\00"
+// CHECK-NEXT: @e = global %struct.PR69979 { ptr @.str.[[STR]] }
 // CHECK: @_ZGRZN20InlineStaticConstRef3funEvE1i_ = linkonce_odr constant i32 10, comdat
 // CHECK20: @_ZZN12LocalVarInit4dtorEvE1a = internal constant {{.*}} i32 103
 
@@ -632,6 +634,10 @@ struct X {
 const char *f() { return &X::p; }
 }
 
+struct PR69979 {
+  const char (&d)[9];
+} e {"12345678"};
+
 // VirtualMembers::TemplateClass::templateMethod() must be defined in this TU,
 // not just declared.
 // CHECK: define linkonce_odr void @_ZN14VirtualMembers13TemplateClassIiE14templateMethodEv(ptr {{[^,]*}} %this)

[nickdesaulniers]

Alternatively this could be:

if (isa<LValueReferenceType>(T))

any preference/thoughts @efriedma-quic ?

[efriedma-quic]

I think I'd prefer to avoid calling into ConstExprEmitter at all for cases involving reference binding. I think we've sort of discussed this before. Maybe add a check to tryEmitPrivate()? (We already have a check in tryEmitPrivateForVarInit().)

[nickdesaulniers]

I think I'd prefer to avoid calling into ConstExprEmitter at all for cases involving reference binding. I think we've sort of discussed this before. Maybe add a check to tryEmitPrivate()? (We already have a check in tryEmitPrivateForVarInit().)

Done in 4cff8f0 PTAL @efriedma-quic

[efriedma-quic]

LGTM

[berolinux]

Might be good to backport this to 17.x, given the regression was introduced on the 17 branch after 17.0.0

[nickdesaulniers]

Might be good to backport this to 17.x, given the regression was introduced on the 17 branch after 17.0.0

llvm/llvm-project-release-prs#765