hubert.reinterpretcast added inline comments.
================ Comment at: clang/lib/Sema/SemaExprCXX.cpp:8901-8914 + // [P2718R0] Lifetime extension in range-based for loops. + // + // 6.7.7 [class.temporary] p5: + // There are four contexts in which temporaries are destroyed at a different + // point than the end of the full-expression. + // + // 6.7.7 [class.temporary] p6: ---------------- yronglin wrote: > hubert.reinterpretcast wrote: > > yronglin wrote: > > > hubert.reinterpretcast wrote: > > > > yronglin wrote: > > > > > yronglin wrote: > > > > > > hubert.reinterpretcast wrote: > > > > > > > yronglin wrote: > > > > > > > > rsmith wrote: > > > > > > > > > This isn't the right way to model the behavior here -- the > > > > > > > > > presence or absence of an `ExprWithCleanups` is just a > > > > > > > > > convenience to tell consumers of the AST whether they should > > > > > > > > > expect to see cleanups later or not, and doesn't carry an > > > > > > > > > implication of affecting the actual temporary lifetimes and > > > > > > > > > storage durations. > > > > > > > > > > > > > > > > > > The outcome that we should be aiming to reach is that all > > > > > > > > > `MaterializeTemporaryExpr`s created as part of processing the > > > > > > > > > for-range-initializer are marked as being lifetime-extended > > > > > > > > > by the for-range variable. Probably the simplest way to > > > > > > > > > handle that would be to track the current enclosing > > > > > > > > > for-range-initializer variable in the > > > > > > > > > `ExpressionEvaluationContextRecord`, and whenever a > > > > > > > > > `MaterializeTemporaryExpr` is created, if there is a current > > > > > > > > > enclosing for-range-initializer, mark that > > > > > > > > > `MaterializeTemporaryExpr` as being lifetime-extended by it. > > > > > > > > Awesome! Thanks a lot for your advice, this is very helpful! I > > > > > > > > want to take a longer look at it. > > > > > > > As mentioned in D139586, `CXXDefaultArgExpr`s may need additional > > > > > > > handling. Similarly for `CXXDefaultInitExpr`s. > > > > > > Thanks for your tips! I have a question that what's the correct way > > > > > > to extent the lifetime of `CXXBindTemporaryExpr`? Can I just > > > > > > `materialize` the temporary? It may replaced by > > > > > > `MaterializeTemporaryExpr`, and then I can mark it as being > > > > > > lifetime-extended by the for-range variable. > > > > > Eg. > > > > > ``` > > > > > void f() { > > > > > int v[] = {42, 17, 13}; > > > > > Mutex m; > > > > > for (int x : static_cast<void>(LockGuard(m)), v) // lock released > > > > > in C++ 2020 > > > > > { > > > > > LockGuard guard(m); // OK in C++ 2020, now deadlocks > > > > > } > > > > > } > > > > > ``` > > > > > ``` > > > > > BinaryOperator 0x135036220 'int[3]' lvalue ',' > > > > > |-CXXStaticCastExpr 0x1350361d0 'void' static_cast<void> <ToVoid> > > > > > | `-CXXFunctionalCastExpr 0x135036198 'LockGuard':'struct LockGuard' > > > > > functional cast to LockGuard <ConstructorConversion> > > > > > | `-CXXBindTemporaryExpr 0x135036178 'LockGuard':'struct LockGuard' > > > > > (CXXTemporary 0x135036178) > > > > > | `-CXXConstructExpr 0x135036140 'LockGuard':'struct LockGuard' > > > > > 'void (Mutex &)' > > > > > | `-DeclRefExpr 0x135035e18 'Mutex':'class Mutex' lvalue Var > > > > > 0x135035b40 'm' 'Mutex':'class Mutex' > > > > > `-DeclRefExpr 0x135036200 'int[3]' lvalue Var 0x135035928 'v' 'int[3]' > > > > > ``` > > > > If `MaterializeTemporaryExpr` represents a "temporary materialization > > > > conversion", then the above should already have one just under the > > > > `static_cast` to `void` (since the cast operand would be a > > > > discarded-value expression). > > > > > > > > There may be unfortunate effects from materializing temporaries for > > > > discarded-value expressions though: Technically, temporaries are also > > > > created for objects having scalar type. > > > > > > > > Currently, `MaterializeTemporaryExpr` is documented as being tied to > > > > reference binding, but that is not correct: for example, > > > > `MaterializeTemporaryExpr` also appears when a member access is made on > > > > a temporary of class type. > > > http://eel.is/c++draft/class.temporary says: > > > ``` > > > [Note 3: Temporary objects are materialized: > > > ....... > > > (2.6) > > > when a prvalue that has type other than cv void appears as a > > > discarded-value expression ([expr.context]). > > > — end note] > > > ``` > > > Seems we should materialized the discard-value expression in this case, > > > WDYT? > > I think we should, but what is the codegen fallout? Would no-opt builds > > start writing `42` into allocated memory for `static_cast<void>(42)`? > Thanks for your confirm @hubert.reinterpretcast ! > > I have tried locally, the generated IR of `void f()` is: > ``` > define void @f()() { > entry: > %v = alloca [3 x i32], align 4 > %m = alloca %class.Mutex, align 8 > %__range1 = alloca ptr, align 8 > %ref.tmp = alloca %struct.LockGuard, align 8 > %__begin1 = alloca ptr, align 8 > %__end1 = alloca ptr, align 8 > %x = alloca i32, align 4 > %guard = alloca %struct.LockGuard, align 8 > call void @llvm.memcpy.p0.p0.i64(ptr align 4 %v, ptr align 4 > @__const.f().v, i64 12, i1 false) > %call = call noundef ptr @Mutex::Mutex()(ptr noundef nonnull align 8 > dereferenceable(64) %m) > %call1 = call noundef ptr @LockGuard::LockGuard(Mutex&)(ptr noundef nonnull > align 8 dereferenceable(8) %ref.tmp, ptr noundef nonnull align 8 > dereferenceable(64) %m) > store ptr %v, ptr %__range1, align 8 > %0 = load ptr, ptr %__range1, align 8 > %arraydecay = getelementptr inbounds [3 x i32], ptr %0, i64 0, i64 0 > store ptr %arraydecay, ptr %__begin1, align 8 > %1 = load ptr, ptr %__range1, align 8 > %arraydecay2 = getelementptr inbounds [3 x i32], ptr %1, i64 0, i64 0 > %add.ptr = getelementptr inbounds i32, ptr %arraydecay2, i64 3 > store ptr %add.ptr, ptr %__end1, align 8 > br label %for.cond > > for.cond: ; preds = %for.inc, %entry > %2 = load ptr, ptr %__begin1, align 8 > %3 = load ptr, ptr %__end1, align 8 > %cmp = icmp ne ptr %2, %3 > br i1 %cmp, label %for.body, label %for.cond.cleanup > > for.cond.cleanup: ; preds = %for.cond > %call5 = call noundef ptr @LockGuard::~LockGuard()(ptr noundef nonnull > align 8 dereferenceable(8) %ref.tmp) #6 > br label %for.end > > for.body: ; preds = %for.cond > %4 = load ptr, ptr %__begin1, align 8 > %5 = load i32, ptr %4, align 4 > store i32 %5, ptr %x, align 4 > %call3 = call noundef ptr @LockGuard::LockGuard(Mutex&)(ptr noundef nonnull > align 8 dereferenceable(8) %guard, ptr noundef nonnull align 8 > dereferenceable(64) %m) > %call4 = call noundef ptr @LockGuard::~LockGuard()(ptr noundef nonnull > align 8 dereferenceable(8) %guard) #6 > br label %for.inc > > for.inc: ; preds = %for.body > %6 = load ptr, ptr %__begin1, align 8 > %incdec.ptr = getelementptr inbounds i32, ptr %6, i32 1 > store ptr %incdec.ptr, ptr %__begin1, align 8 > br label %for.cond > > for.end: ; preds = %for.cond.cleanup > %call6 = call noundef ptr @Mutex::~Mutex()(ptr noundef nonnull align 8 > dereferenceable(64) %m) #6 > ret void > } > ``` > > The AST of `void f()` is: > ``` > |-FunctionDecl 0x14311d408 <line:62:1, line:69:1> line:62:6 used f 'void ()' > | `-CompoundStmt 0x143808898 <col:10, line:69:1> > | |-DeclStmt 0x14311d710 <line:63:3, col:25> > | | `-VarDecl 0x14311d528 <col:3, col:24> col:7 used v 'int[3]' cinit > | | `-InitListExpr 0x14311d648 <col:13, col:24> 'int[3]' > | | |-IntegerLiteral 0x14311d590 <col:14> 'int' 42 > | | |-IntegerLiteral 0x14311d5b0 <col:18> 'int' 17 > | | `-IntegerLiteral 0x14311d5d0 <col:22> 'int' 13 > | |-DeclStmt 0x14311d9f0 <line:64:3, col:10> > | | `-VarDecl 0x14311d740 <col:3, col:9> col:9 used m 'Mutex':'Mutex' > callinit destroyed > | | `-CXXConstructExpr 0x14311d9b0 <col:9> 'Mutex':'Mutex' 'void ()' > | `-CXXForRangeStmt 0x143808700 <line:65:3, line:68:3> > | |-<<<NULL>>> > | |-DeclStmt 0x14311e2c0 <line:65:16> > | | `-VarDecl 0x14311dfa8 <col:16, col:49> col:16 implicit used __range1 > 'int (&)[3]' cinit > | | `-ExprWithCleanups 0x14311e238 <col:16, col:49> 'int[3]' lvalue > | | `-BinaryOperator 0x14311de38 <col:16, col:49> 'int[3]' lvalue ',' > | | |-CXXStaticCastExpr 0x14311dde8 <col:16, col:46> 'void' > static_cast<void> <ToVoid> > | | | `-MaterializeTemporaryExpr 0x14311ddd0 <col:34, col:45> > 'LockGuard':'LockGuard' xvalue extended by Var 0x14311dfa8 '__range1' 'int > (&)[3]' > | | | `-CXXFunctionalCastExpr 0x14311dd98 <col:34, col:45> > 'LockGuard':'LockGuard' functional cast to LockGuard <ConstructorConversion> > | | | `-CXXBindTemporaryExpr 0x14311dd78 <col:34, col:45> > 'LockGuard':'LockGuard' (CXXTemporary 0x14311dd78) > | | | `-CXXConstructExpr 0x14311dd40 <col:34, col:45> > 'LockGuard':'LockGuard' 'void (Mutex &)' > | | | `-DeclRefExpr 0x14311da18 <col:44> 'Mutex':'Mutex' > lvalue Var 0x14311d740 'm' 'Mutex':'Mutex' > | | `-DeclRefExpr 0x14311de18 <col:49> 'int[3]' lvalue Var > 0x14311d528 'v' 'int[3]' > | |-DeclStmt 0x143808588 <col:14> > | | `-VarDecl 0x14311e360 <col:14> col:14 implicit used __begin1 'int > *':'int *' cinit > | | `-ImplicitCastExpr 0x143808400 <col:14> 'int *' > <ArrayToPointerDecay> > | | `-DeclRefExpr 0x14311e2d8 <col:14> 'int[3]' lvalue Var > 0x14311dfa8 '__range1' 'int (&)[3]' > | |-DeclStmt 0x1438085a0 <col:14> > | | `-VarDecl 0x143808248 <col:14, col:16> col:14 implicit used __end1 > 'int *':'int *' cinit > | | `-BinaryOperator 0x143808468 <col:14, col:16> 'int *' '+' > | | |-ImplicitCastExpr 0x143808450 <col:14> 'int *' > <ArrayToPointerDecay> > | | | `-DeclRefExpr 0x14311e2f8 <col:14> 'int[3]' lvalue Var > 0x14311dfa8 '__range1' 'int (&)[3]' > | | `-IntegerLiteral 0x143808430 <col:16> 'long' 3 > | |-BinaryOperator 0x143808628 <col:14> 'bool' '!=' > | | |-ImplicitCastExpr 0x1438085f8 <col:14> 'int *':'int *' > <LValueToRValue> > | | | `-DeclRefExpr 0x1438085b8 <col:14> 'int *':'int *' lvalue Var > 0x14311e360 '__begin1' 'int *':'int *' > | | `-ImplicitCastExpr 0x143808610 <col:14> 'int *':'int *' > <LValueToRValue> > | | `-DeclRefExpr 0x1438085d8 <col:14> 'int *':'int *' lvalue Var > 0x143808248 '__end1' 'int *':'int *' > | |-UnaryOperator 0x143808668 <col:14> 'int *':'int *' lvalue prefix '++' > | | `-DeclRefExpr 0x143808648 <col:14> 'int *':'int *' lvalue Var > 0x14311e360 '__begin1' 'int *':'int *' > | |-DeclStmt 0x14311dee0 <col:8, col:50> > | | `-VarDecl 0x14311de78 <col:8, col:14> col:12 x 'int' cinit > | | `-ImplicitCastExpr 0x1438086d0 <col:14> 'int' <LValueToRValue> > | | `-UnaryOperator 0x1438086b8 <col:14> 'int' lvalue prefix '*' > cannot overflow > | | `-ImplicitCastExpr 0x1438086a0 <col:14> 'int *':'int *' > <LValueToRValue> > | | `-DeclRefExpr 0x143808680 <col:14> 'int *':'int *' lvalue Var > 0x14311e360 '__begin1' 'int *':'int *' > | `-CompoundStmt 0x143808880 <line:66:3, line:68:3> > | `-DeclStmt 0x143808868 <line:67:5, col:23> > | `-VarDecl 0x143808778 <col:5, col:22> col:15 guard > 'LockGuard':'LockGuard' callinit destroyed > | `-CXXConstructExpr 0x143808820 <col:15, col:22> > 'LockGuard':'LockGuard' 'void (Mutex &)' > | `-DeclRefExpr 0x1438087e0 <col:21> 'Mutex':'Mutex' lvalue Var > 0x14311d740 'm' 'Mutex':'Mutex' > ``` > > > Would no-opt builds start writing 42 into allocated memory for > > static_cast<void>(42)? > I have got this result with my locally build clang > > ``` > define noundef i32 @main() { > entry: > %retval = alloca i32, align 4 > %ref.tmp = alloca i32, align 4 > store i32 0, ptr %retval, align 4 > store i32 42, ptr %ref.tmp, align 4 > call void @f()() > ret i32 0 > } > ``` > > Would no-opt builds start writing 42 into allocated memory for > > static_cast<void>(42)? > I have got this result with my locally build clang > > ``` > define noundef i32 @main() { > entry: > %retval = alloca i32, align 4 > %ref.tmp = alloca i32, align 4 > store i32 0, ptr %retval, align 4 > store i32 42, ptr %ref.tmp, align 4 > call void @f()() > ret i32 0 > } > ``` @rsmith, in terms of the AST, do you have advice on the approach we should take in generating `MaterializeTemporaryExpr`s for discarded-value expressions? Do we go with a language-specification purest direction and generate these for all prvalue object-typed discarded-value expressions? Do we limit generating them to range-for initializers? Do we further specialize and limit to non-scalar types or, even more specifically, to cases involving constructors or destructors? Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D153701/new/ https://reviews.llvm.org/D153701 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits