[PATCH] Implements DR 712 (odr-use, potential results of an expression)
Richard Smith
richard at metafoo.co.uk
Wed May 29 15:01:59 PDT 2013
On Wed, May 29, 2013 at 11:19 AM, Faisal Vali <faisalv at gmail.com> wrote:
> Attached is an attempt to implement DR712 - please see the copied post at
> the end of the message for the general implementation strategy, and Richard
> smith's initial guidance.
>
> In regards to the testing, I test odr use through lambda capturing - I am
> not sure if this is the best way to test it - any guidance will be
> appreciated.
>
I think that's the most reliable way to test it.
> In my ODR use visitor:
> - am i correct in having the relevant traverses return false? I felt it
> was safe to abort, but am not certain.
> - do i need the empty VisitStmt?
>
Yes, these were correct, but see below.
> - do i need the constexpr check of the second operator of (s.*cexpr)?
>
Yes.
> Also, as a caveat, I feel especially unsure patching codegen - so will
> appreciate an especially careful review of those changes.
> -I have left some fixme comments in there as I copied code - not sure if
> that was the
> right thing to do.
>
I assume you didn't mean to include the changes to ParseAST.cpp?
> - do I need to check for IsLambdaCallOperator in CGExpr.cpp
> EmitDeclRefValue?
> - should I refactor out the block of code that emits a constant in
> EmitDeclRefValue, EmitMemberExpr and EmitPointerToData .. into
> CodeGenFunction?
>
> The regression tests seem to work.
>
Index: include/clang/Sema/Sema.h
===================================================================
--- include/clang/Sema/Sema.h (revision 182855)
+++ include/clang/Sema/Sema.h (working copy)
@@ -3016,6 +3016,9 @@
void MarkMemberReferenced(MemberExpr *E);
void UpdateMarkingForLValueToRValue(Expr *E);
+ // Assess the expression for any references to a variable
+ // that do not entail an odr-use of that variable.
+ void UpdateNonODRUsedVariableReferences(Expr *E);
Isn't this function the same thing as UpdateMarkingForLValueToRValue? Maybe
merge them into a single function? (FWIW, I prefer the old name.)
@@ -11508,13 +11508,23 @@
// Note that we use the C++11 definition everywhere because nothing in
// C++03 depends on whether we get the C++03 version correct. The second
// part does not apply to references, since they are not objects.
+ // Per DR: 712 constant expressions in discarded value expressions
+ // are not odr-used either.
The mention of discarded value expressions seems out of place here.
+ // http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3383.html#712
We usually don't include such links in comments.
+ //
const VarDecl *DefVD;
- if (E && !isa<ParmVarDecl>(Var) &&
+ if (E && SemaRef.getLangOpts().CPlusPlus11 &&
+ E->isCXX11ConstantExpr(SemaRef.Context)) {
+ if (!Var->getType()->isReferenceType())
+ SemaRef.MaybeODRUseExprs.insert(E);
Hah, this is a literal interpretation of the standard text, but isn't what
was intended. It allows DeclRefExprs referring to globals, for instance.
What was *intended* was to look for objects on which the lvalue-to-rvalue
conversion could be applied in a constant expression, that is, what the
existing code does:
+ }
+ else if (E && !isa<ParmVarDecl>(Var) &&
Var->isUsableInConstantExpressions(SemaRef.Context) &&
Var->getAnyInitializer(DefVD) && DefVD->checkInitIsICE()) {
Index: lib/Sema/SemaExprCXX.cpp
===================================================================
--- lib/Sema/SemaExprCXX.cpp (revision 182855)
+++ lib/Sema/SemaExprCXX.cpp (working copy)
@@ -5518,6 +5518,78 @@
return false;
}
+namespace {
+
+ struct ConstantExpressionODRCleaner : RecursiveASTVisitor<
+ ConstantExpressionODRCleaner> {
+
We normally don't include blank lines in these cases. Also, maybe split
this line after the : instead of after the < ?
A RecursiveASTVisitor seems overkill here, since you're neither using the
recursive walk, nor dispatch on anything other than Stmts. Maybe this
should be a ConstStmtVisitor instead?
Maybe name this something like PotentialResultsSetFinder, since that's
closer to what it does.
+ llvm::SmallPtrSet<Expr*, 2>& MaybeODRUseExprs;
" &", not "& ", please. I have other formatting-related comments on this
patch, but clang-format can tell you what they are. :-)
+ typedef RecursiveASTVisitor<ConstantExpressionODRCleaner> inherited;
Should be "Inherited", bu just remove this since you're not actually using
it.
+ // C++14 CD, DR 712: 3.2 para 2
Please use "C++1y" here so that we can more easily find it with a grep
later.
+ // normally, we don't need to do any additional conversions to handle
it,
+ // but if it is a volatile lvalue with a special form, we perform an
+ // lvalue-to-rvalue conversion. Additionally, if it was a c++11
constant
+ // expression.
The added comment here doesn't look right; I think you should say something
like "Even if we do not perform an lvalue-to-rvalue conversion, we pretend
that one was performed when checking for odr-uses of variables."
Index: lib/CodeGen/CGExpr.cpp
===================================================================
You have CodeGen changes but no tests for them, please add some, covering
all the ways in which a variable can be referenced without being odr-used.
--- lib/CodeGen/CGExpr.cpp (revision 182855)
+++ lib/CodeGen/CGExpr.cpp (working copy)
@@ -1800,6 +1800,13 @@
return CGF.EmitLValueForField(LV, FD);
}
+
+inline bool isLambdaCallOperator(const Decl *D) {
This seems generally useful; perhaps it should be a member of
CXXMethodDecl? If not, s/inline/static/, please.
+ if (!ND->isUsed(false) || (E->refersToEnclosingLocal() &&
+ isLambdaCallOperator(CurCodeDecl))) {
+ if (const VarDecl *VD = dyn_cast<VarDecl>(ND)) {
Please flip these two 'if's around, and merge this into the preceding code
(the end result should just be removing the isReferenceType() check from
the preceding code block and adding the lambda-enclosing-local check).
@@ -2476,7 +2497,23 @@
LValue CodeGenFunction::EmitMemberExpr(const MemberExpr *E) {
Expr *BaseExpr = E->getBase();
+ ASTContext &Context = CGM.getContext();
+ APValue ConstExprResult;
+ // Emit a field member access that evaluates to a constexpr s.x
+ if (Context.getLangOpts().CPlusPlus11 && E->isCXX11ConstantExpr(Context,
+ &ConstExprResult)) {
+ if (FieldDecl *FD = dyn_cast<FieldDecl>(E->getMemberDecl())) {
+ CharUnits Alignment = Context.getDeclAlign(FD);
+ llvm::Constant *Val =
+ CGM.EmitConstantValue(ConstExprResult, FD->getType(), this);
+ assert(Val && "failed to emit C++11 constant member expression");
+ // FIXME: Eventually we will want to emit vector element references.
+ QualType T = E->getType();
+ return MakeAddrLValue(Val, T, Alignment);
+ }
+ }
I don't think this approach can work. Consider this:
struct S { int x; };
constexpr S a = { 1 };
S b = { 2 };
bool k;
return (k ? a : b).x;
This does not odr-use 'a', but it does odr-use 'b'. The MemberExpr is not a
constant expression here. I think instead what you should do is to teach
AggExprEmitter (and friends) to directly emit a constant value when they
visit a DeclRefExpr which is not an odr-use
@@ -3239,6 +3276,23 @@
LValue CodeGenFunction::
EmitPointerToDataMemberBinaryExpr(const BinaryOperator *E) {
llvm::Value *BaseV;
+
+ Expr *ObjExpr = E->getLHS();
+ ASTContext &Context = CGM.getContext();
+ APValue ConstExprResult;
+ // Emit a field member access that evaluates to a constexpr s.*memfn
+ if (Context.getLangOpts().CPlusPlus11 && E->isCXX11ConstantExpr(Context,
+ &ConstExprResult)) {
+ const MemberPointerType *MPT
+ = E->getRHS()->getType()->getAs<MemberPointerType>();
+ llvm::Constant *Val =
+ CGM.EmitConstantValue(ConstExprResult, MPT->getPointeeType(), this);
+ assert(Val && "failed to emit C++11 constant member expression");
+ // FIXME: Eventually we will want to emit vector element references.
+ QualType T = E->getType();
+ return MakeAddrLValue(Val, T);
+ }
In this case, you should constant-evaluate the RHS but not the LHS. Also,
use EvaluateAsRValue here; it doesn't matter whether we need to perform
non-standard constant folding. And there's no need to check for C++11 mode.
Index: test/CXX/basic/basic.def.odr/p2-DR712-C++14-CD.cpp
===================================================================
--- test/CXX/basic/basic.def.odr/p2-DR712-C++14-CD.cpp (revision 0)
+++ test/CXX/basic/basic.def.odr/p2-DR712-C++14-CD.cpp (working copy)
Maybe just call this .../p2-potential-results.cpp -- this isn't
C++1y-specific. This file contains both UTF-8 characters in comments and
\r\n line separators, please fix.
> Also, the patch (
> http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20130506/079656.html)
> was LGTM'd by Doug, anyway we can get that one committed please - thank you!
>
>
>
>
> On Sat, May 18, 2013 at 1:42 PM, Richard Smith <richard at metafoo.co.uk>wrote:
>
>> On Sat, May 18, 2013 at 9:22 AM, Faisal Vali <faisalv at gmail.com> wrote:
>>
>>> Before I return to working on generic lambdas, I thought it might be
>>> useful to nail down DR 712 which reworded the definition of odr-use in
>>> terms of the potential results of an expression (in order to avoid
>>> odr-using a variable that represents a constant expression whenever we can
>>> get away with it). This is relevant to lambdas, because it affects which
>>> variables get captured and which don't.
>>>
>>>
>>> Let me review how I think clang currently handles this (pre-712
>>> resolution - please correct me where I err). While parsing and
>>> semanalyzing expressions, all variables that are usable in constant
>>> expressions (and so might not need to be odr-used) are stored in
>>> MaybeODRUseExprs (a set container) as DeclRefExprs (DRE). Then, if an
>>> Lvalue-to-Rvalue conversion is performed on the DRE, it is removed from the
>>> set above (In UpdateMarkingForLValueToRValue). Anything left in that set
>>> is marked as odr-used by ActOnFinishFullExpr.
>>>
>>>
>>> OK, with the above in mind, these are my initial thoughts on how to
>>> implement it so that not only do we handle those constant expressions that
>>> are lvalue-to-rvalue converted, but also those that are discarded, and also
>>> handle the conditional expressions and member-access expressions.
>>>
>>> The relevant hooks/callbacks/functions within clang are:
>>> - UpdateMarkingForLValueToRValue
>>> - IgnoredValueConversions
>>>
>>> A function i intend to write is GetPotentialResultsOfExpression: when
>>> passed an Expr* E, returns a vector of its potential results as
>>> DeclRefExpr's (and MemberExpr's??); this function should follow from 3.2
>>> para 2.
>>>
>>> Now in each hook, GetPotentialResults shall be called, and those
>>> expressions shall be removed from the MaybeODRUseExpr.
>>>
>>
>> This seems like the right approach, although you presumably don't
>> actually need to build the vector and could just remove the odr-use marker
>> directly.
>>
>>
>>> What are your thoughts on the above preliminary strategy? I have this
>>> sense that Member expressions are going to require some more work, but
>>> we'll see...
>>>
>>> Also before I proceed any further, I was hoping to get some more clarity
>>> on 3.2 para 2 and para 3 - please see my queries below:
>>>
>>>
>>> Consider the following code:
>>>
>>>
>>> struct S {
>>> const int mi;
>>> constexpr S(int i) : mi(i) { }
>>> };
>>>
>>> int f(const int& );
>>> void g(int);
>>> int main() {
>>> constexpr S r{5};
>>> constexpr S s{r};
>>> [](char c) {
>>> c = s.mi; // #1 does this odr-use 's'
>>>
>>
>> No. Here, e = 's.mi', ex = 's' (a member of the set of potential results
>> of e), and e is subject to an lvalue-to-rvalue conversion.
>>
>>
>>> g(s.mi); // #2 what about this?
>>>
>>
>> Also no, because an lvalue-to-rvalue conversion is applied to s.mi.
>>
>>
>>> s.mi; // #3 and this?
>>>
>>
>> Also no, because this is a discarded-value expression.
>>
>>
>>> c ? s.mi : r.mi; // #4 and this?
>>>
>>
>> Also no.
>>
>>
>>> g( c ? s.mi : r.mi ); // #5
>>>
>>
>> Also no.
>>
>>
>>> };
>>> }
>>>
>>> My intial suspicion was that neither s nor r were odr-used in the lambda
>>> (because the compiler can figure out the value of 'mi' at its point of use,
>>> and use it or discard it) - but then based on Richard's analysis of the
>>> example below where he states that s.operator int() is an odr-use of 's', i
>>> am now unsure.
>>>
>>
>> Note that member function calls aren't listed in the rules of 3.2/2. 's'
>> is not in the set of potential results of 's.operator int()'.
>>
>>
>>> Here is my attempt at trying to figure this out (e refers to expression,
>>> ex as a potentially-evaluated expression, using the language of 3.2 para 2
>>> and 3)
>>>
>>> In #1,
>>> the variable 's' appears as a potentially evaluated expression and
>>> satisfies the requirements for appearing in a constant expression, and it
>>> is an object, and it is an element of the set of potential results of an
>>> expression 'e' (s.mi) and the lvalue-to-rvalue conversion is applied to
>>> that expression so it is not odr-used? But what if 'e' is deemed to be 's'
>>> in 's.mi' - no lvalue-to-rvalue conversion is applied so it should be
>>> captured?
>>>
>>
>> The rule says: "x [...] is odr-used unless [...] ex is is an element of
>> the set of potential results of an expression e, where [...]"
>>
>> That is, if there *exists* such an 'e', then 'x' is not odr-used.
>>
>> Hope that helps!
>>
>>
>>> OK, I'm going to give up on analyzing the rest, because I feel I'm
>>> missing something and am unable to think about this clearly.
>>>
>>> Any help or guidance will be appreciated!
>>>
>>>
>>>
>>> On Wed, May 15, 2013 at 3:21 PM, Richard Smith <richard at metafoo.co.uk>wrote:
>>>
>>>> On Tue, May 14, 2013 at 9:48 PM, Faisal Vali <faisalv at gmail.com> wrote:
>>>>
>>>>>
>>>>>
>>>>> While we're on the topic, can i ask you to clarify a few capture and
>>>>> constexpr
>>>>> questions below:
>>>>>
>>>>
>>>> OK, the relevant context is [basic.def.odr]p2 and p3.
>>>>
>>>>
>>>>> struct S {
>>>>> constexpr S() {}
>>>>> constexpr operator int() const { return 0; }
>>>>> };
>>>>> void fooS(S s) { }
>>>>>
>>>>> void fooInt(int i) { }
>>>>>
>>>>> void f() {
>>>>> constexpr S s {};
>>>>> auto L = [] (int x) {
>>>>> (void) s; // 1
>>>>>
>>>>
>>>> Here, the 's' expression is a discarded-value expression, and the 's'
>>>> variable is in the set of potential results of the 's' expression, and 's'
>>>> satisfies the constraints for appearing in a constant expression, so 's' is
>>>> not odr-used, so is not captured. We should not require a capture-default
>>>> here.
>>>>
>>>>
>>>>> fooS(s); // 2
>>>>>
>>>>
>>>> Here, 's' is implicitly passed to the implicit S::S(const S&)
>>>> constructor. This is neither an lvalue-to-rvalue conversion nor a
>>>> discarded-value expression, so 's' is odr-used and we have an error due to
>>>> the missing capture-default.
>>>>
>>>>
>>>>> fooInt(s); // 3
>>>>>
>>>>
>>>> This is equivalent to 's.operator int()', which again odr-uses 's', so
>>>> requires a capture-default.
>>>>
>>>>
>>>>> };
>>>>> }
>>>>>
>>>>> Should the above be ok in spite of L not having a default capture?
>>>>> Currently clang errors on all of them individually - is that the right
>>>>> behavior?
>>>>>
>>>>> Also, how would you want me to start submitting patches for commit -
>>>>> do you want
>>>>> me to break up the patch into smaller patches? - and if so, do you
>>>>> have any thoughts
>>>>> on how I might want to break up the functionality per patch?
>>>>>
>>>>
>>>> Smaller patches are definitely better, if you can decompose the
>>>> functionality into coherent smaller chunks. There are some hints on how to
>>>> decompose the functionality here (but this division may or may not work for
>>>> you):
>>>>
>>>>
>>>> http://clang.llvm.org/docs/InternalsManual.html#how-to-add-an-expression-or-statement
>>>>
>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20130529/bd86a6d9/attachment.html>
More information about the cfe-commits
mailing list