efriedma added inline comments.
================
Comment at: clang/lib/CodeGen/CGExprConstant.cpp:1324
// This is a string literal initializing an array in an initializer.
- return CGM.GetConstantArrayFromStringLiteral(E);
+ return E->isLValue() ?
+ CGM.GetAddrOfConstantStringFromLiteral(E).getPointer() :
----------------
nickdesaulniers wrote:
> efriedma wrote:
> > efriedma wrote:
> > > efriedma wrote:
> > > > nickdesaulniers wrote:
> > > > > efriedma wrote:
> > > > > > nickdesaulniers wrote:
> > > > > > > nickdesaulniers wrote:
> > > > > > > > efriedma wrote:
> > > > > > > > > nickdesaulniers wrote:
> > > > > > > > > > efriedma wrote:
> > > > > > > > > > > Maybe we should have a separate ConstExprEmitterLValue...
> > > > > > > > > > > trying to handle both LValues and RValues on the same
> > > > > > > > > > > codepath has been problematic in the past. It's very
> > > > > > > > > > > easy for code to get confused what it's actually trying
> > > > > > > > > > > to emit.
> > > > > > > > > > So we'd have a `ConstExprEmitterLValue` class with some
> > > > > > > > > > visitor methods, and a `ConstExprEmitterRValue` with other
> > > > > > > > > > methods implemented?
> > > > > > > > > Something like that.
> > > > > > > > >
> > > > > > > > > Actually thinking about it a bit more, not sure you need to
> > > > > > > > > actually implement ConstExprEmitterLValue for now. You might
> > > > > > > > > just be able to ensure we don't ever call ConstExprEmitter
> > > > > > > > > with an lvalue. The current ConstExprEmitter doesn't expect
> > > > > > > > > lvalues, and shouldn't call itself with lvalues. (We bail on
> > > > > > > > > explicit LValueToRValue conversions.) And Evaluate()
> > > > > > > > > shouldn't actually evaluate the contents of an lvalue if it
> > > > > > > > > isn't dereferenced, so there hopefully aren't any performance
> > > > > > > > > issues using that codepath.
> > > > > > > > >
> > > > > > > > > In terms of implementation, I guess that's basically
> > > > > > > > > restoring the destType->isReferenceType() that got removed?
> > > > > > > > > (I know I suggested it, but I wasn't really thinking about
> > > > > > > > > it...)
> > > > > > > > One thing I think we may need to add to `ConstExprEmitter` is
> > > > > > > > the ability to evaluate `CallExpr`s based on certain test case
> > > > > > > > failures...does that seem right?
> > > > > > > See also the calls to `constexpr f()` in
> > > > > > > clang/test/CodeGenCXX/const-init-cxx1y.cpp
> > > > > > The only things I know of that Evaluate() can't handle are
> > > > > > CK_ToUnion, CK_ReinterpretMemberPointer, and
> > > > > > DesignatedInitUpdateExpr. DesignatedInitUpdateExpr is related to
> > > > > > the failures in test/CodeGenCXX/designated-init.cpp; I don't think
> > > > > > the others show up in any of the testcases you've mentioned.
> > > > > > (CK_ToUnion can't appear in C++ code. CK_ReinterpretMemberPointer
> > > > > > is a `reinterpret_cast<T>` where T is a member pointer type.)
> > > > > >
> > > > > > Given none of those constructs show up in const-init-cxx1y.cpp, the
> > > > > > only reason for it to fail is if we aren't correctly falling back
> > > > > > for a construct the current code doesn't know how to handle. You
> > > > > > shouldn't need to implement any new constructs.
> > > > > in clang/test/CodeGenCXX/designated-init.cpp we have:
> > > > > ```
> > > > > >> 22 namespace ModifyStaticTemporary {
> > > > > >>
> > > > > 23 struct A { int &&temporary; int x; };
> > > > >
> > > > > 24 constexpr int f(int &r) { r *= 9; return r - 12; }
> > > > >
> > > > > 25 A a = { 6, f(a.temporary) };
> > > > > ```
> > > > > In the AST, that looks like:
> > > > > ```
> > > > > | |-VarDecl 0x562b77df39b0 <line:25:3, col:29> col:5 used a
> > > > > 'A':'ModifyStaticTemporary::A' cinit
> > > > > | | `-ExprWithCleanups 0x562b77df3c68 <col:9, col:29>
> > > > > 'A':'ModifyStaticTemporary::A'
> > > > > | | `-InitListExpr 0x562b77df3bb8 <col:9, col:29>
> > > > > 'A':'ModifyStaticTemporary::A'
> > > > > | | |-MaterializeTemporaryExpr 0x562b77df3c08 <col:11> 'int'
> > > > > xvalue extended by Var 0x562b77df39b0 'a'
> > > > > 'A':'ModifyStaticTemporary::A'
> > > > > | | | `-IntegerLiteral 0x562b77df3a18 <col:11> 'int' 6
> > > > > | | `-CallExpr 0x562b77df3b30 <col:14, col:27> 'int'
> > > > > | | |-ImplicitCastExpr 0x562b77df3b18 <col:14> 'int (*)(int &)'
> > > > > <FunctionToPointerDecay>
> > > > > | | | `-DeclRefExpr 0x562b77df3ad0 <col:14> 'int (int &)'
> > > > > lvalue Function 0x562b77df37a0 'f' 'int (int &)'
> > > > > | | `-MemberExpr 0x562b77df3aa0 <col:16, col:18> 'int' lvalue
> > > > > .temporary 0x562b77df35c0
> > > > > | | `-DeclRefExpr 0x562b77df3a80 <col:16>
> > > > > 'A':'ModifyStaticTemporary::A' lvalue Var 0x562b77df39b0 'a'
> > > > > 'A':'ModifyStaticTemporary::A'
> > > > > ```
> > > > > (So, indeed no `DesignatedInitUpdateExpr`) but the call to the
> > > > > `constexpr` `f()` updates the reference (to `54`). If I remove the
> > > > > visitor for `MaterializeTemporaryExpr`, we fail to evaluate `f` and
> > > > > end up emitting `6` rather than `54`. Doesn't that mean that the
> > > > > fast path (`ConstExprEmitter`) needs to be able to evaluate
> > > > > `CallExpr`?
> > > > >
> > > > > Or should `VisitInitListExpr` bail if any of the inits
> > > > > `isa<MaterializeTemporaryExpr>` (or perhaps `isa<CallExpr>`)?
> > > > There are a few related cases here.
> > > >
> > > > Case number one is when you have something like `int z(); A a = { z(),
> > > > z() };`. There's no constant evaluation going on: you just emit two
> > > > zero-initialized variables, and the runtime init initializes both of
> > > > them.
> > > >
> > > > Case number two is when everything is obviously constant: something
> > > > like `A a = { 1, 2 };`
> > > >
> > > > Case number three is when there are simple side-effects, and the
> > > > standard requires we evaluate them at compile-time. Something like `A
> > > > a = { 1, ++a.temporary };`. In this case, we need to ensure that we
> > > > use Evaluate() to compute the value of both the temporary and the
> > > > variable. The literal "1" is not the correct value to use.
> > > > CodeGenModule::GetAddrOfGlobalTemporary is supposed to ensure we use
> > > > the value from the evaluation of the variable as a whole (see comment
> > > > "If the initializer of the extending declaration").
> > > >
> > > > Case number four is when we can't constant-evaluate a variable as a
> > > > whole, but we do evaluate some of the temporaries involved. Something
> > > > like `int z(); A a = { 1, a.temporary += z() };` In this case, we
> > > > constant-evaluate the temporary using the initial value, then emit
> > > > runtime initialization to finish computing the value of the variable as
> > > > a whole.
> > > >
> > > > You example should fall under case three. Both the temporary and the
> > > > variable should be evaluated by Evaluate().
> > > >
> > > > I'm not sure how the code ends up emitting the value 6, but hopefully
> > > > that helps?
> > > Oh, I think I see what's happening; the code that looks for the temporary
> > > in GetAddrOfGlobalTemporary isn't reliable if the whole variable isn't
> > > evaluated first. It ends up pulling out the result of a partial
> > > evaluation, or something like that.
> > >
> > > Making EmitArrayInitialization/EmitRecordInitialization bail if they see
> > > a MaterializeTemporaryExpr should deal with the issue, I think? Not sure
> > > if you'd need to recursively visit all the initializers (I don't remember
> > > what constructs allow lifetime extension off the top of my head).
> > To be more precise, what happens is that calling EvaluateAsLValue on the
> > MaterializeTemporaryExpr actually *corrupts* the computed value of the
> > temporary: the complete variable is evaluated earlier for other reasons,
> > then EvaluateAsLValue overwrites the correct value we computed earlier with
> > the wrong value.
> > In this case, we need to ensure that we use Evaluate() to compute the value
> > of both the temporary and the variable.
>
> Just triple checking, `Evaluate` is the "slow path" (i.e.
> `VarDecl::evaluateValue`, `Expr::EvaluateAsLValue`, and
> `Expr::EvaluateAsRValue`?
>
> > To be more precise, what happens is that calling EvaluateAsLValue on the
> > MaterializeTemporaryExpr actually *corrupts* the computed value of the
> > temporary
>
> So the slow path gets it wrong? But prior to this patch, that's was used
> first before ConstExprEmitter? (Maybe I should add more comments about fast
> vs slow path)
>
> ---
>
> > Not sure if you'd need to recursively visit all the initializers (I don't
> > remember what constructs allow lifetime extension off the top of my head).
>
> I think I would; the last test case currently failing is
> clang/test/CodeGenCXX/atomicinit.cpp:
> ```
> struct X {
> constexpr X(int n) : n(n) {}
> short n;
> char c = 6;
> };
>
> struct Y {
> _Atomic(X) a;
> _Atomic(int) b;
> };
> Y y = { X(4), 5 };
> ```
> The AST for `y` looks like:
> ```
> `-VarDecl 0x562bba0cad00 <line:11:1, col:17> col:3 y 'Y':'Y' cinit
> `-ExprWithCleanups 0x562bba0e9f28 <col:7, col:17> 'Y':'Y'
> `-InitListExpr 0x562bba0e9c20 <col:7, col:17> 'Y':'Y'
> |-ImplicitCastExpr 0x562bba0e9ef8 <col:9, col:12> '_Atomic(X)'
> <NonAtomicToAtomic>
> | `-ImplicitCastExpr 0x562bba0e9ee0 <col:9, col:12> 'X':'X'
> <ConstructorConversion>
> | `-CXXConstructExpr 0x562bba0e9eb0 <col:9, col:12> 'X':'X' 'void (X
> &&) noexcept' elidable
> | `-MaterializeTemporaryExpr 0x562bba0e9c70 <col:9, col:12> 'X':'X'
> xvalue
> | `-CXXFunctionalCastExpr 0x562bba0e9b88 <col:9, col:12> 'X':'X'
> functional cast to X <ConstructorConversion>
> | `-CXXConstructExpr 0x562bba0e9a10 <col:9, col:12> 'X':'X'
> 'void (int)'
> | `-IntegerLiteral 0x562bba0cadc0 <col:11> 'int' 4
> `-ImplicitCastExpr 0x562bba0e9f10 <col:15> '_Atomic(int)'
> <NonAtomicToAtomic>
> `-IntegerLiteral 0x562bba0e9bb0 <col:15> 'int' 5
> ```
> so we'd need to peek through the casts to find that there was a
> `MaterializeTemporaryExpr` in there, then bail?
> Just triple checking, Evaluate is the "slow path" (i.e.
> VarDecl::evaluateValue, Expr::EvaluateAsLValue, and Expr::EvaluateAsRValue?
Yes.
>> To be more precise, what happens is that calling EvaluateAsLValue on the
>> MaterializeTemporaryExpr actually *corrupts* the computed value of the
>> temporary
> So the slow path gets it wrong? But prior to this patch, that's was used
> first before ConstExprEmitter? (Maybe I should add more comments about fast
> vs slow path)
The EvaluateAsLValue bug only shows up if you EvaluateAsLValue pieces of the
initializer. If you use the slow path first, we never EvaluateAsLValue pieces
of the initializer; we just evaluate the whole variable initializer in one
evaluation. (Depending on the construct, we may have to evaluate it during
semantic analysis; if we do, the evaluation is cached.)
--------
> I think I would; the last test case currently failing is
> clang/test/CodeGenCXX/atomicinit.cpp:
I don't think that's the same issue? There, the MaterializeTemporaryExpr is
immediately passed to a CXXConstructExpr, which returns an rvalue, so the final
IR shouldn't actually reference the temporary. It looks like the issue is that
VisitCXXConstructExpr is broken; it tries to look through a trivial move
constructor, but the the operand of a move constructor is an lvalue, so the
recursive visit doesn't work correctly. The following crashes even without
your patch:
```
struct X {
constexpr X(int n) : n(n) {}
short n;
char c = 6;
};
struct Y {
_Atomic(X) a;
int b;
};
int z;
Y y = { X(4), z };
```
You can probably just kill off the VisitCXXConstructExpr codepath... or if you
want to try to repair it, I guess you can teach it to specifically handle only
trivial constructors where the operand is a MaterializeTemporaryExpr.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D151587/new/
https://reviews.llvm.org/D151587
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