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Handle constant "pointers" for __atomic_always_lock_free/__atomic_is_lock_free. #99340

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merged 5 commits into from
Jul 22, 2024
Merged

Handle constant "pointers" for __atomic_always_lock_free/__atomic_is_lock_free. #99340

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2cb327b
Handle constant "pointers" for `__atomic_always_lock_free`/`__atomic_…
jyknight Jul 17, 2024
3e6440c
Fix bug with implicit casts from non-pointer types.
jyknight Jul 19, 2024
69e4fa4
fix clang-format issue.
jyknight Jul 19, 2024
9ea6910
Merge remote-tracking branch 'origin/main' into atomic-lock-free-intptr
jyknight Jul 22, 2024
4e3c491
Add release note
jyknight Jul 22, 2024
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7 changes: 7 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -440,6 +440,13 @@ Non-comprehensive list of changes in this release
pointers, enabling more powerful alias analysis when accessing pointer types.
The new behavior can be enabled using ``-fpointer-tbaa``.

- The ``__atomic_always_lock_free`` and ``__atomic_is_lock_free``
builtins may now return true if the pointer argument is a
compile-time constant (e.g. ``(void*)4``), and constant pointer is
sufficiently-aligned for the access requested. Previously, only the
type of the pointer was taken into account. This improves
compatibility with GCC's libstdc++.

New Compiler Flags
------------------
- ``-fsanitize=implicit-bitfield-conversion`` checks implicit truncation and
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36 changes: 26 additions & 10 deletions clang/lib/AST/ExprConstant.cpp
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Expand Up @@ -12949,19 +12949,35 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
Info.Ctx.getTargetInfo().getMaxAtomicInlineWidth();
if (Size <= Info.Ctx.toCharUnitsFromBits(InlineWidthBits)) {
if (BuiltinOp == Builtin::BI__c11_atomic_is_lock_free ||
Size == CharUnits::One() ||
E->getArg(1)->isNullPointerConstant(Info.Ctx,
Expr::NPC_NeverValueDependent))
// OK, we will inline appropriately-aligned operations of this size,
// and _Atomic(T) is appropriately-aligned.
Size == CharUnits::One())
return Success(1, E);

QualType PointeeType = E->getArg(1)->IgnoreImpCasts()->getType()->
castAs<PointerType>()->getPointeeType();
if (!PointeeType->isIncompleteType() &&
Info.Ctx.getTypeAlignInChars(PointeeType) >= Size) {
// OK, we will inline operations on this object.
// If the pointer argument can be evaluated to a compile-time constant
// integer (or nullptr), check if that value is appropriately aligned.
const Expr *PtrArg = E->getArg(1);
Expr::EvalResult ExprResult;
APSInt IntResult;
if (PtrArg->EvaluateAsRValue(ExprResult, Info.Ctx) &&
ExprResult.Val.toIntegralConstant(IntResult, PtrArg->getType(),
Info.Ctx) &&
IntResult.isAligned(Size.getAsAlign()))
return Success(1, E);

// Otherwise, check if the type's alignment against Size.
if (auto *ICE = dyn_cast<ImplicitCastExpr>(PtrArg)) {
// Drop the potential implicit-cast to 'const volatile void*', getting
// the underlying type.
if (ICE->getCastKind() == CK_BitCast)
PtrArg = ICE->getSubExpr();
}

if (auto PtrTy = PtrArg->getType()->getAs<PointerType>()) {
QualType PointeeType = PtrTy->getPointeeType();
if (!PointeeType->isIncompleteType() &&
Info.Ctx.getTypeAlignInChars(PointeeType) >= Size) {
// OK, we will inline operations on this object.
return Success(1, E);
}
}
}
}
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25 changes: 25 additions & 0 deletions clang/test/Sema/atomic-ops.c
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Expand Up @@ -124,6 +124,31 @@ _Static_assert(__atomic_always_lock_free(4, &i64), "");
_Static_assert(!__atomic_always_lock_free(8, &i32), "");
_Static_assert(__atomic_always_lock_free(8, &i64), "");

// Validate use with fake pointers constants. This mechanism is used to allow
// validating atomicity of a given size and alignment.
_Static_assert(__atomic_is_lock_free(1, (void*)1), "");
_Static_assert(__atomic_is_lock_free(1, (void*)-1), "");
_Static_assert(__atomic_is_lock_free(4, (void*)2), ""); // expected-error {{not an integral constant expression}}
_Static_assert(__atomic_is_lock_free(4, (void*)-2), ""); // expected-error {{not an integral constant expression}}
_Static_assert(__atomic_is_lock_free(4, (void*)4), "");
_Static_assert(__atomic_is_lock_free(4, (void*)-4), "");

_Static_assert(__atomic_always_lock_free(1, (void*)1), "");
_Static_assert(__atomic_always_lock_free(1, (void*)-1), "");
_Static_assert(!__atomic_always_lock_free(4, (void*)2), "");
_Static_assert(!__atomic_always_lock_free(4, (void*)-2), "");
_Static_assert(__atomic_always_lock_free(4, (void*)4), "");
_Static_assert(__atomic_always_lock_free(4, (void*)-4), "");
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Should we also add some test coverage where the fake pointer is a constant expression but utter nonsense? e.g.,

__atomic_always_lock_free(1, (void*)"well would you look at this")
__atomic_always_lock_free(1, (void*)42.0f)

(Just to make sure that the conversion correctly ignores these as valid hints.)

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Neat, found a pre-existing bug: _Static_assert(__atomic_always_lock_free(2, "string"), ""); triggers an assertion failure in clang at head because the type of "string" is ConstantArrayType not PointerType, in this cast:

llvm-project/clang/lib/AST/ExprConstant.cpp

Line 12960 in 83fbd79

castAs<PointerType>()->getPointeeType();

The argument AST looks like:

(gdb) p E->getArg(1)->dump()
ImplicitCastExpr 0x1463b528 'const volatile void *' <BitCast>
`-ImplicitCastExpr 0x1463b510 'char *' <ArrayToPointerDecay>
  `-StringLiteral 0x1463b440 'char[7]' lvalue "string"

I suppose it shouldn't do "IgnoreImpCasts", and instead only ignore a single level of BitCast ImplicitCastExpr?

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I think it should undergo an actual lvalue to rvalue conversion on the operand so that we get all the expected conversions like decay. We have the same issue with: https://godbolt.org/z/P69vx5W8a or with the even-more-cursed: https://godbolt.org/z/zxTbbWbdc

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It does undergo such conversions already -- it's just that we were stripping them all off with the IgnoreImpCasts call. I've pushed what I think is a valid fix, PTAL.

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Ah, you're right, sorry for that!


// Ensure that "weird" constants don't cause trouble.
_Static_assert(__atomic_always_lock_free(1, "string"), "");
_Static_assert(!__atomic_always_lock_free(2, "string"), "");
_Static_assert(__atomic_always_lock_free(2, (int[2]){}), "");
void dummyfn();
_Static_assert(__atomic_always_lock_free(2, dummyfn) || 1, "");



#define _AS1 __attribute__((address_space(1)))
#define _AS2 __attribute__((address_space(2)))

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