nickdesaulniers added inline comments.
================
Comment at: clang/lib/AST/ExprConstant.cpp:8360
+ // Do not constant fold an R-value.
+ if (Info.EvalMode == EvalInfo::EM_ConstantFold && !E->isLValue())
+ return false;
----------------
efriedma wrote:
> nickdesaulniers wrote:
> > efriedma wrote:
> > > Checking isLValue() doesn't make sense; consider:
> > >
> > > ```
> > > struct R { mutable long x; };
> > > struct Z { const R &x, y; };
> > > Z z = { R{1}, z.x.x=10 };
> > > ```
> > >
> > > Maybe also want to check for EM_IgnoreSideEffects? Not sure what cases,
> > > if any, that would affect.
> > >
> > > We should probably check `E->getStorageDuration() == SD_Static`. The
> > > cases where it's a local temporary don't hit the getOrCreateValue()
> > > codepath, so the evaluated value should be handled correctly.
> > >
> > > Checking EvalMode feels a little weird, but I guess it should do the
> > > right thing in the cases I can think of? I'd like a second opinion on
> > > this.
> > Changing this condition to:
> > ```
> > if (E->getStorageDuration() == SD_Static &&
> >
> > Info.EvalMode == EvalInfo::EM_ConstantFold &&
> >
> > E->isXValue())
> >
> > return false;
> > ```
> > allows all tests in tree to pass, but messes up the test case you posted
> > above. I'm trying to sus out what else might be different about that test
> > case...we should return `false` for that, but I'm not sure what's different
> > about that case.
> >
> > In particular, I was playing with `E->isUsableInConstantExpressions` and
> > `E->getLifetimeExtendedTemporaryDecl()`, but getting your case to work, I
> > end up regressing
> > clang/test/SemaCXX/attr-require-constant-initialization.cpp...argh!!
> Shouldn't that just be the following?
>
> ```
> if (E->getStorageDuration() == SD_Static &&
> Info.EvalMode == EvalInfo::EM_ConstantFold)
>
> return false;
> ```
>
> A materialized temporary is always going to be either an LValue or an XValue,
> and the difference between the two isn't relevant here.
I wish it were that simple. Checking those two alone will produce failures in
the following tests:
Failed Tests (2):
Clang :: CodeGenCXX/mangle-ms.cpp
Clang :: SemaCXX/attr-require-constant-initialization.cpp
error: 'error' diagnostics seen but not expected:
File
/android0/llvm-project/clang/test/SemaCXX/attr-require-constant-initialization.cpp
Line 92: variable does not have a constant initializer
as an example of one failure, which is basically:
```
void foo(void) {
__attribute__((require_constant_initialization)) static const int &temp_init
= 42;
}
```
specifically, `-std=c++03` is the only language version that fails.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D151587/new/
https://reviews.llvm.org/D151587
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