-Wsign-compare shouldn't warn when the signed operand is a conditional operator
whose operands are non-negative integer constant expressions. This comes up
in LLVM in a few places.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@92525 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index a8be333..121ed6a 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -5110,6 +5110,22 @@
return LHSTy;
}
+/// Returns true if we can prove that the result of the given
+/// integral expression will not have its sign bit set.
+static bool IsSignBitProvablyZero(ASTContext &Context, Expr *E) {
+ E = E->IgnoreParens();
+
+ llvm::APSInt value;
+ if (E->isIntegerConstantExpr(value, Context))
+ return value.isNonNegative();
+
+ if (ConditionalOperator *CO = dyn_cast<ConditionalOperator>(E))
+ return IsSignBitProvablyZero(Context, CO->getLHS()) &&
+ IsSignBitProvablyZero(Context, CO->getRHS());
+
+ return false;
+}
+
/// \brief Implements -Wsign-compare.
///
/// \param lex the left-hand expression
@@ -5157,27 +5173,15 @@
// If the value is a non-negative integer constant, then the
// signed->unsigned conversion won't change it.
- llvm::APSInt value;
- if (signedOperand->isIntegerConstantExpr(value, Context)) {
- assert(value.isSigned() && "result of signed expression not signed");
+ if (IsSignBitProvablyZero(Context, signedOperand))
+ return;
- if (value.isNonNegative())
- return;
- }
-
- if (Equality) {
- // For (in)equality comparisons, if the unsigned operand is a
- // constant which cannot collide with a overflowed signed operand,
- // then reinterpreting the signed operand as unsigned will not
- // change the result of the comparison.
- if (unsignedOperand->isIntegerConstantExpr(value, Context)) {
- assert(!value.isSigned() && "result of unsigned expression is signed");
-
- // 2's complement: test the top bit.
- if (value.isNonNegative())
- return;
- }
- }
+ // For (in)equality comparisons, if the unsigned operand is a
+ // constant which cannot collide with a overflowed signed operand,
+ // then reinterpreting the signed operand as unsigned will not
+ // change the result of the comparison.
+ if (Equality && IsSignBitProvablyZero(Context, unsignedOperand))
+ return;
Diag(OpLoc, PD)
<< lex->getType() << rex->getType()
diff --git a/test/Sema/compare.c b/test/Sema/compare.c
index 75a3cf1..bacd47e 100644
--- a/test/Sema/compare.c
+++ b/test/Sema/compare.c
@@ -194,6 +194,9 @@
((short) a < (unsigned short) 0x80000) + // expected-warning {{comparison of integers of different signs}}
((signed char) a < (unsigned char) 0x80000) + // expected-warning {{comparison of integers of different signs}}
+ // We should be able to avoid warning about this.
+ (b != (a < 4 ? 1 : 2)) +
+
10
;
}