vsk added a subscriber: klimek.
vsk added inline comments.
================
Comment at: lib/CodeGen/CGExprScalar.cpp:1694
// handle things like function to ptr-to-function decay etc.
Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
Expr *E = CE->getSubExpr();
----------------
lebedev.ri wrote:
> vsk wrote:
> > lebedev.ri wrote:
> > > vsk wrote:
> > > > I think maintaining a stack of visited cast exprs in the emitter be
> > > > cheaper/simpler than using ASTContext::getParents. You could push CE
> > > > here and use a RAII helper to pop it. The 'isCastPartOfExplicitCast'
> > > > check then simplifies to a quick stack traversal.
> > > Hmm, two things come to mind:
> > > 1. This pessimizes the (most popular) case when the sanitizer is disabled.
> > > 2. `ASTContext::getParents()` may return more than one parent. I'm not
> > > sure if that matters here?
> > > I'll take a look..
> > As for (1), it's not necessary to push/pop the stack when this sanitizer is
> > disabled. And for (2), IIUC the only interesting case is "explicit-cast
> > <implicit-cast>+", and none of the implicit casts here have more than one
> > parent.
> > I think maintaining a stack of visited cast exprs in the emitter be
> > cheaper/simpler than using ASTContext::getParents.
>
> {F6660377}
>
> So yeah, this could work.
> Except sadly it breaks in subtle cases like https://godbolt.org/g/5V2czU
> I have added those tests beforehand.
>
> Is `ASTContext::getParents()` really horribly slow so we want to
> duplicate/maintain/track
> the current AST stack ourselves?
> If so, we will need to maintain the *entire* stack, not just `CastExpr''s...
I think the scan in 'IsTopCastPartOfExplicitCast' can be fixed: while
traversing backwards, you'd need to check that the previously-visited cast expr
is the child of the current expr. That should address the false negative you
pointed out in interference1. I don't yet see what the issue is with
interference0.
Could you explain why maintaining a stack of unfinished casts wouldn't work? I
don't understand why you'd need the entire stack. My sense is that it's not
required to match the "explicit-cast <implicit-cast>+" pattern, but I could
easily be missing something here. As for why this might be worth looking into,
I think scanning for an explicit cast is much easier to understand when working
with a stack.
+ @klimek to comment on what to expect in terms of the overhead of
ASTContext::getParents. Regardless of what approach we pick, it would help to
see pre/post-patch compile times for a stage2 build of something like clang or
llc.
Repository:
rC Clang
https://reviews.llvm.org/D48958
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