ilya-biryukov added inline comments.
================
Comment at: clang/lib/CodeGen/CGExprAgg.cpp:581
+ Expr *filler = nullptr;
+ if (auto *ILE = dyn_cast<InitListExpr>(ExprToVisit))
+ filler = ILE->getArrayFiller();
----------------
ilya-biryukov wrote:
> ayzhao wrote:
> > ilya-biryukov wrote:
> > > - Should we have a filler for `CXXParenInitListExpr` too?
> > > It seems like an important optimization and could have large effect on
> > > compile times if we don't
> > >
> > > - Same question for semantic and syntactic-form (similar to
> > > `InitListExpr`): should we have it here?
> > > I am not sure if it's semantically required (probably not?), but that's
> > > definitely something that `clang-tidy` and other source tools will rely
> > > on.
> > >
> > > We should probably share the code there, I suggest moving it to a shared
> > > base class and using it where appropriate instead of the derived classes.
> > > Should we have a filler for CXXParenInitListExpr too? It seems like an
> > > important optimization and could have large effect on compile times if we
> > > don't
> >
> > This feels like premature optimization - presumably, wouldn't this only be
> > an issue with extraordinarily large (say, O(1000)) arrays?
> >
> > > Same question for semantic and syntactic-form (similar to InitListExpr):
> > > should we have it here? I am not sure if it's semantically required
> > > (probably not?), but that's definitely something that clang-tidy and
> > > other source tools will rely on
> >
> > IIRC this doesn't apply to paren list aggregate expressions, as the
> > syntatic form would be the enclosing `ParenListExpr`.
> > This feels like premature optimization - presumably, wouldn't this only be
> > an issue with extraordinarily large (say, O(1000)) arrays?
> Yes, this should only happen with large arrays. Normally I would agree, but
> it's surprising that changing `{}` to `()` in the compiler would lead to
> performance degradation.
> In that sense, this premature optimization is already implemented, we are
> rather degrading performance for a different syntax to do the same thing.
>
> Although we could also land without it, but in that case could you open a GH
> issue and add a FIXME to track the implementation of this particular
> optimization?
> This should increase the chances of users finding the root cause of the issue
> if they happen to hit it.
>
> > IIRC this doesn't apply to paren list aggregate expressions, as the
> > syntatic form would be the enclosing ParenListExpr.
> Do we even have the enclosing `ParenListExpr`? If I dump the AST with `clang
> -fsyntax-only -Xclang=-ast-dump ...` for the following code:
> ```
> struct pair { int a; int b = 2; };
> int main() {
> pair(1); pair p(1);
> pair b{1}; pair{1};
> }
> ```
>
> I get
> ```
> `-FunctionDecl 0x557d79717e98 <line:2:1, line:5:1> line:2:5 main 'int ()'
> `-CompoundStmt 0x557d797369d0 <col:12, line:5:1>
> |-CXXFunctionalCastExpr 0x557d79718528 <line:3:3, col:9> 'pair':'pair'
> functional cast to pair <NoOp>
> | `-CXXParenListInitExpr 0x557d79718500 <col:3> 'pair':'pair'
> | |-IntegerLiteral 0x557d79718010 <col:8> 'int' 1
> | `-IntegerLiteral 0x557d79717e18 <line:1:30> 'int' 2
> |-DeclStmt 0x557d79718650 <line:3:12, col:21>
> | `-VarDecl 0x557d79718568 <col:12, col:17> col:17 p 'pair':'pair'
> parenlistinit
> | `-CXXParenListInitExpr 0x557d79718610 <col:17> 'pair':'pair'
> | |-IntegerLiteral 0x557d797185d0 <col:19> 'int' 1
> | `-IntegerLiteral 0x557d79717e18 <line:1:30> 'int' 2
> |-DeclStmt 0x557d797187d8 <line:4:3, col:12>
> | `-VarDecl 0x557d79718680 <col:3, col:11> col:8 b 'pair':'pair' listinit
> | `-InitListExpr 0x557d79718750 <col:9, col:11> 'pair':'pair'
> | |-IntegerLiteral 0x557d797186e8 <col:10> 'int' 1
> | `-CXXDefaultInitExpr 0x557d797187a0 <col:11> 'int'
> `-CXXFunctionalCastExpr 0x557d797369a8 <col:14, col:20> 'pair':'pair'
> functional cast to pair <NoOp>
> `-InitListExpr 0x557d79718868 <col:18, col:20> 'pair':'pair'
> |-IntegerLiteral 0x557d79718800 <col:19> 'int' 1
> `-CXXDefaultInitExpr 0x557d797188b8 <col:20> 'int'
> ```
> It feels like the `ParentListExpr` is replaced during semantic analysis and
> there is no way to get it back. I also tried running `clang-query` and trying
> to `match parenListExpr()` and go 0 results. Is it just missing in the AST
> dump and I run `clang-query` incorrectly or do we actually not have the
> syntactic form of this expression after all?
Another important thing to address from the dump: notice how braced
initialization creates `CXXDefaultInitExpr` and `CXXParenListInitExpr` copies
the default argument expression directly. It's really important to use the
former form, here's the example that currently breaks:
```
#include <iostream>
struct func_init { int some_int; const char* fn = __builtin_FUNCTION(); };
int main() {
func_init a(10);
std::cout << "From paren init:" << a.fn << std::endl;
func_init b{10};
std::cout << "From braced init: " << b.fn << std::endl;
}
```
The program above is expected to report `main` for both cases, but instead we
get:
```
From paren init:
From braced init: main
```
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D129531/new/
https://reviews.llvm.org/D129531
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