r285643 - A compound literal within a global lambda or block is still within

[John McCall via cfe-commits]

Author: rjmccall
Date: Mon Oct 31 16:56:26 2016
New Revision: 285643

URL: http://llvm.org/viewvc/llvm-project?rev=285643&view=rev
Log:
A compound literal within a global lambda or block is still within
the body of a function for the purposes of computing its storage
duration and deciding whether its initializer must be constant.

There are a number of problems in our current treatment of compound
literals.  C specifies that a compound literal yields an l-value
referring to an object with either static or automatic storage
duration, depending on where it was written; in the latter case,
the literal object has a lifetime tied to the enclosing scope (much
like an ObjC block), not the enclosing full-expression.  To get these
semantics fully correct in our current design, we would need to
collect compound literals on the ExprWithCleanups, just like we do
with ObjC blocks; we would probably also want to identify literals
like we do with materialized temporaries.  But it gets stranger;
GCC adds compound literals to C++ as an extension, but makes them
r-values, which are generally assumed to have temporary storage
duration.  Ignoring destructor ordering, the difference only matters
if the object's address escapes the full-expression, which for an
r-value can only happen with reference binding (which extends
temporaries) or array-to-pointer decay (which does not).  GCC then
attempts to lock down on array-to-pointer decay in ad hoc ways.
Arguably a far superior language solution for C++ (and perhaps even
array r-values in C, which can occur in other ways) would be to
propagate lifetime extension through array-to-pointer decay, so
that initializing a pointer object to a decayed r-value array
extends the lifetime of the complete object containing the array.
But this would be a major change in semantics which arguably ought
to be blessed by the committee(s).

Anyway, I'm not fixing any of that in this patch; I did try, but
it got out of hand.

Fixes rdar://28949016.

Modified:
    cfe/trunk/lib/Sema/SemaExpr.cpp
    cfe/trunk/test/CodeGenCXX/compound-literals.cpp
    cfe/trunk/test/Sema/compound-literal.c
    cfe/trunk/test/SemaCXX/compound-literal.cpp

Modified: cfe/trunk/lib/Sema/SemaExpr.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaExpr.cpp?rev=285643&r1=285642&r2=285643&view=diff
==============================================================================
--- cfe/trunk/lib/Sema/SemaExpr.cpp (original)
+++ cfe/trunk/lib/Sema/SemaExpr.cpp Mon Oct 31 16:56:26 2016
@@ -5572,7 +5572,7 @@ Sema::BuildCompoundLiteralExpr(SourceLoc
     return ExprError();
   LiteralExpr = Result.get();
 
-  bool isFileScope = getCurFunctionOrMethodDecl() == nullptr;
+  bool isFileScope = !CurContext->isFunctionOrMethod();
   if (isFileScope &&
       !LiteralExpr->isTypeDependent() &&
       !LiteralExpr->isValueDependent() &&

Modified: cfe/trunk/test/CodeGenCXX/compound-literals.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGenCXX/compound-literals.cpp?rev=285643&r1=285642&r2=285643&view=diff
==============================================================================
--- cfe/trunk/test/CodeGenCXX/compound-literals.cpp (original)
+++ cfe/trunk/test/CodeGenCXX/compound-literals.cpp Mon Oct 31 16:56:26 2016
@@ -55,3 +55,11 @@ union PR21912Ty {
 union PR21912Ty *PR21912_2 = (union PR21912Ty[]){{.d = 2.0}, {.l = 3}};
 // CHECK-LABEL: define {{.*}}__cxx_global_var_init.3()
 // CHECK: store %union.PR21912Ty* getelementptr inbounds ([2 x %union.PR21912Ty], [2 x %union.PR21912Ty]* bitcast (<{ { double }, %union.PR21912Ty }>* @.compoundliteral.4 to [2 x %union.PR21912Ty]*), i32 0, i32 0), %union.PR21912Ty** @PR21912_2
+
+// This compound literal should have local scope.
+int computed_with_lambda = [] {
+  int *array = (int[]) { 1, 3, 5, 7 };
+  return array[0];
+}();
+// CHECK-LABEL: define internal i32 @{{.*}}clEv
+// CHECK:         alloca [4 x i32]

Modified: cfe/trunk/test/Sema/compound-literal.c
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Sema/compound-literal.c?rev=285643&r1=285642&r2=285643&view=diff
==============================================================================
--- cfe/trunk/test/Sema/compound-literal.c (original)
+++ cfe/trunk/test/Sema/compound-literal.c Mon Oct 31 16:56:26 2016
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -pedantic %s
+// RUN: %clang_cc1 -fsyntax-only -verify -fblocks -pedantic %s
 // REQUIRES: LP64
 
 struct foo { int a, b; };
@@ -36,3 +36,9 @@ void IncompleteFunc(unsigned x) {
 
 // PR6080
 int array[(sizeof(int[3]) == sizeof( (int[]) {0,1,2} )) ? 1 : -1];
+
+// rdar://28949016 - Constant restriction should not apply to compound literals in blocks
+int (^block)(int) = ^(int i) {
+  int *array = (int[]) {i, i + 2, i + 4};
+  return array[i];
+};

Modified: cfe/trunk/test/SemaCXX/compound-literal.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/compound-literal.cpp?rev=285643&r1=285642&r2=285643&view=diff
==============================================================================
--- cfe/trunk/test/SemaCXX/compound-literal.cpp (original)
+++ cfe/trunk/test/SemaCXX/compound-literal.cpp Mon Oct 31 16:56:26 2016
@@ -86,3 +86,13 @@ int PR17415 = (int){PR17415}; // expecte
 template<unsigned> struct Value { };
 template<typename T>
 int &check_narrowed(Value<sizeof((T){1.1})>);
+
+#if __cplusplus >= 201103L
+// Compound literals in global lambdas have automatic storage duration
+// and are not subject to the constant-initialization rules.
+int computed_with_lambda = [] {
+  int x = 5;
+  int result = ((int[]) { x, x + 2, x + 4, x + 6 })[0];
+  return result;
+}();
+#endif