rsmith added inline comments.
================
Comment at: lib/CodeGen/CGExprConstant.cpp:1413
+ } else if (!Init->isEvaluatable(CE.CGM.getContext())) {
+ return false;
+ } else if (InitTy->hasPointerRepresentation()) {
----------------
sepavloff wrote:
> rjmccall wrote:
> > Aren't the null-pointer and integer-constant-expression checks below
> > already checking this? Also, `isEvaluatable` actually computes the full
> > value internally (as an `APValue`), so if you're worried about the memory
> > and compile-time effects of producing such a value, you really shouldn't
> > call it.
> >
> > You could reasonably move this entire function to be a method on `Expr`
> > that takes an `ASTContext`.
> Comment for `EvaluateAsRValue` says that it tries calculate expression
> agressively. Indeed, for the code:
> ```
> decltype(nullptr) null();
> int *p = null();
> ```
> compiler ignores potential side effect of `null()` and removes the call,
> leaving only zero initialization. `isNullPointerConstant` behaves similarly.
Nonetheless, it looks like this function could evaluate `Init` up to three
times, which seems unreasonable. Instead of the checks based on trying to
evaluate the initializer (`isNullPointerConstant` + `isIntegerConstantExpr`),
how about calling `VarDecl::evaluateValue()` (which will return a potentially
pre-computed and cached initializer value) and checking if the result is a zero
constant?
In fact, `tryEmitPrivateForVarInit` already does most of that for you, and the
right place to make this change is probably in `tryEmitPrivateForMemory`, where
you can test to see if the `APValue` is zero-initialized and produce a
`zeroinitializer` if so. As a side-benefit, putting the change there will mean
we'll also start using `zeroinitializer` for zero-initialized subobjects of
objects that have non-zero pieces.
Repository:
rC Clang
https://reviews.llvm.org/D46241
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