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[flang] Don't duplicate impure function call for UBOUND() #153648

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Merged
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Aug 18, 2025
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[flang] Don't duplicate impure function call for UBOUND() #153648

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27 changes: 17 additions & 10 deletions flang/include/flang/Evaluate/tools.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1144,15 +1144,14 @@ std::optional<std::string> FindImpureCall(
std::optional<std::string> FindImpureCall(
FoldingContext &, const ProcedureRef &);

// Predicate: is a scalar expression suitable for naive scalar expansion
// in the flattening of an array expression?
// TODO: capture such scalar expansions in temporaries, flatten everything
class UnexpandabilityFindingVisitor
: public AnyTraverse<UnexpandabilityFindingVisitor> {
// Predicate: does an expression contain anything that would prevent it from
// being duplicated so that two instances of it then appear in the same
// expression?
class UnsafeToCopyVisitor : public AnyTraverse<UnsafeToCopyVisitor> {
public:
using Base = AnyTraverse<UnexpandabilityFindingVisitor>;
using Base = AnyTraverse<UnsafeToCopyVisitor>;
using Base::operator();
explicit UnexpandabilityFindingVisitor(bool admitPureCall)
explicit UnsafeToCopyVisitor(bool admitPureCall)
: Base{*this}, admitPureCall_{admitPureCall} {}
template <typename T> bool operator()(const FunctionRef<T> &procRef) {
return !admitPureCall_ || !procRef.proc().IsPure();
Expand All @@ -1163,14 +1162,22 @@ class UnexpandabilityFindingVisitor
bool admitPureCall_{false};
};

template <typename A>
bool IsSafelyCopyable(const A &x, bool admitPureCall = false) {
return !UnsafeToCopyVisitor{admitPureCall}(x);
}

// Predicate: is a scalar expression suitable for naive scalar expansion
// in the flattening of an array expression?
// TODO: capture such scalar expansions in temporaries, flatten everything
template <typename T>
bool IsExpandableScalar(const Expr<T> &expr, FoldingContext &context,
const Shape &shape, bool admitPureCall = false) {
if (UnexpandabilityFindingVisitor{admitPureCall}(expr)) {
if (IsSafelyCopyable(expr, admitPureCall)) {
return true;
} else {
auto extents{AsConstantExtents(context, shape)};
return extents && !HasNegativeExtent(*extents) && GetSize(*extents) == 1;
} else {
return true;
}
}

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10 changes: 6 additions & 4 deletions flang/lib/Evaluate/shape.cpp
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Expand Up @@ -623,7 +623,7 @@ MaybeExtentExpr GetRawUpperBound(
} else if (semantics::IsAssumedSizeArray(symbol) &&
dimension + 1 == symbol.Rank()) {
return std::nullopt;
} else {
} else if (IsSafelyCopyable(base, /*admitPureCall=*/true)) {
return ComputeUpperBound(
GetRawLowerBound(base, dimension), GetExtent(base, dimension));
}
Expand Down Expand Up @@ -678,9 +678,11 @@ static MaybeExtentExpr GetUBOUND(FoldingContext *context,
} else if (semantics::IsAssumedSizeArray(symbol) &&
dimension + 1 == symbol.Rank()) {
return std::nullopt; // UBOUND() folding replaces with -1
} else if (auto lb{GetLBOUND(base, dimension, invariantOnly)}) {
return ComputeUpperBound(
std::move(*lb), GetExtent(base, dimension, invariantOnly));
} else if (IsSafelyCopyable(base, /*admitPureCall=*/true)) {
if (auto lb{GetLBOUND(base, dimension, invariantOnly)}) {
return ComputeUpperBound(
std::move(*lb), GetExtent(base, dimension, invariantOnly));
}
}
}
} else if (const auto *assoc{
Expand Down
18 changes: 18 additions & 0 deletions flang/test/Evaluate/bug153031.f90
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@@ -0,0 +1,18 @@
! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
! Ensure that UBOUND() calculation from LBOUND()+SIZE() isn't applied to
! variables containing references to impure functions.
type t
real, allocatable :: a(:)
end type
interface
pure integer function pure(n)
integer, intent(in) :: n
end
end interface
type(t) :: x(10)
allocate(x(1)%a(2))
!CHECK: PRINT *, ubound(x(int(impure(1_4),kind=8))%a,dim=1_4)
print *, ubound(x(impure(1))%a, dim=1)
!CHECK: PRINT *, int(size(x(int(pure(1_4),kind=8))%a,dim=1,kind=8)+lbound(x(int(pure(1_4),kind=8))%a,dim=1,kind=8)-1_8,kind=4)
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@eugeneepshteyn eugeneepshteyn Aug 14, 2025

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Ok, so because it's PURE, it's ok to call it multiple times with the same value, because it's supposed to produce the same results on the same inputs. (And there's no way in Fortran to check how many times it's called without making it impure :-) )

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@DanielCChen DanielCChen Aug 14, 2025

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Should Flang also pull out the evaluation of the PURE function to get better runtime performance even though it has no side-effect being evaluated multiple times?

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@klausler klausler Aug 14, 2025

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CSE can do so in optimization if it wants.

print *, ubound(x(pure(1))%a, dim=1)
end
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