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[Clang] enhance loop analysis to handle variable changes inside lambdas #135573

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93c8fc7
[Clang] enhance loop analysis to handle variable changes inside lambdas
a-tarasyuk Apr 17, 2025
e92de51
Merge branch 'main' into fix/132038
a-tarasyuk Apr 18, 2025
541df49
Merge branch 'main' into fix/132038
a-tarasyuk Apr 18, 2025
0005379
add FIXME test case
a-tarasyuk Apr 18, 2025
b75af8f
Merge branch 'fix/132038' of https://github.com/a-tarasyuk/llvm-proje…
a-tarasyuk Apr 18, 2025
169172f
Merge branch 'main' of https://github.com/llvm/llvm-project into fix/…
a-tarasyuk Apr 18, 2025
f29ad4f
Merge branch 'main' into fix/132038
a-tarasyuk Apr 21, 2025
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1 change: 1 addition & 0 deletions clang/docs/ReleaseNotes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -405,6 +405,7 @@ Improvements to Clang's diagnostics
they're only triggered if the authors are already making the choice to use
``preferred_type`` attribute.

- The ``-Wloop-analysis`` warning now handles variable modifications inside lambda expressions (#GH132038).

Improvements to Clang's time-trace
----------------------------------
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19 changes: 17 additions & 2 deletions clang/lib/Sema/SemaStmt.cpp
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Expand Up @@ -2000,9 +2000,24 @@ namespace {
}

void VisitDeclRefExpr(DeclRefExpr *E) {
if (VarDecl *VD = dyn_cast<VarDecl>(E->getDecl()))
if (VarDecl *VD = dyn_cast<VarDecl>(E->getDecl())) {
if (Decls.count(VD))
FoundDecl = true;
} else if (CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(E->getDecl());
MD && isLambdaCallOperator(MD)) {
for (const auto &Capture : MD->getParent()->captures()) {
if (!Capture.capturesVariable())
continue;

LambdaCaptureKind CK = Capture.getCaptureKind();
if (CK != LCK_ByRef)
continue;

VarDecl *VD = dyn_cast<VarDecl>(Capture.getCapturedVar());
if (VD && Decls.count(VD))
FoundDecl = true;
}
}
}

void VisitPseudoObjectExpr(PseudoObjectExpr *POE) {
Expand All @@ -2019,7 +2034,7 @@ namespace {

bool FoundDeclInUse() { return FoundDecl; }

}; // end class DeclMatcher
}; // end class DeclMatcher

void CheckForLoopConditionalStatement(Sema &S, Expr *Second,
Expr *Third, Stmt *Body) {
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42 changes: 42 additions & 0 deletions clang/test/SemaCXX/warn-loop-analysis.cpp
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Expand Up @@ -299,3 +299,45 @@ void test10() {
for (auto[i, j, k] = arr; i < a; ++i) { }
for (auto[i, j, k] = arr; i < a; ++arr[0]) { }
};

namespace GH132038 {
extern void foo(int);
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We normally wrap tests from bug reports in namespace GHXXXX where XXXX is the bug report number.

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Can we also get tests that show diagnostics for lambda cases as well.

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@shafik, thanks for the review. I've updated tests

void test1() {
int a = 0;
auto incr_a = [&a]() { ++a; };

for (int b = 10; a <= b; incr_a())
foo(a);

for (int b = 10; a <= b;)
incr_a();

for (int b = 10; a <= b; [&a]() { ++a; }()) { }
for (int b = 10; a <= b; [&a]() { }()) { }
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do you know why we don't complain about this case?

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@zyn0217, that's a good question. In this case, I followed a similar example with functions where the body isn't taken into account

llvm-project/clang/test/SemaCXX/warn-loop-analysis.cpp

Line 23 in db0f754

for (int i; i < 1; ) { by_ref(i); }

llvm-project/clang/test/SemaCXX/warn-loop-analysis.cpp

Line 8 in db0f754

void by_ref(int &value) { }

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@zyn0217 zyn0217 Apr 18, 2025 •
edited
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Can you test this case?

void test() {
  int a = 0;
  int *b = &a;
  auto increase = [b]() { ++*b; };
  for (a = 10; a <= 20; increase()) {}
}

(Sorry, I mean for (a = 10; ...))

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Or if that is something we can't handle at the moment, can you please add a FIXME?

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@zyn0217 I've added a test case with a FIXME. The original test already briefly mentions cases involving dereferencing...

llvm-project/clang/test/SemaCXX/warn-loop-analysis.cpp

Lines 40 to 41 in db0f754

// Dereferencing pointers is ignored for now.
for (int *i; *i; ) {}

rewritten using dataflow analysis

I suppose it’s better to rely on dataflow analysis when handling more complex cases...

}

void test2() {
int a = 0;
int *c = &a;

auto incr_a = [a]() { };
auto incr_b = [](int b) { };
auto incr_c = [c]() { ++*c; };

for (int b = 10; a <= b; incr_a()) // expected-warning {{variables 'a' and 'b' used in loop condition not modified in loop body}}
foo(a);

for (int b = 10; a <= b;) // expected-warning {{variables 'a' and 'b' used in loop condition not modified in loop body}}
incr_a();

for (int b = 10; a <= b; incr_b(b)) // expected-warning {{variables 'a' and 'b' used in loop condition not modified in loop body}}
foo(a);

for (int b = 10; a <= b;) // expected-warning {{variables 'a' and 'b' used in loop condition not modified in loop body}}
incr_b(b);

// FIXME: handle modification of loop control variable inside lambda body
for (a = 10; a <= 20; incr_c()) // expected-warning {{variable 'a' used in loop condition not modified in loop body}}
foo(a);
}
}
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