efriedma added reviewers: efriedma, rsmith.
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() :
----------------
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).
Repository:
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CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D151587/new/
https://reviews.llvm.org/D151587
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