nickdesaulniers 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() :
----------------
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?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D151587/new/
https://reviews.llvm.org/D151587
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