From 681829bec2957f9c23bbba87170836e4d360a237 Mon Sep 17 00:00:00 2001 From: Davide Italiano Date: Mon, 6 Jul 2015 18:02:09 +0000 Subject: [PATCH] [Sema] Warn when shifting a negative value. Example: % ./clang -Wshift-negative-value emit.c emit.c:3:14: warning: shifting a negative signed value is undefined [-Wshift-negative-value] int a = -1 << 3; ~~ ^ 1 warning generated. PR: 24026 Differential Revision: http://reviews.llvm.org/D10938 Reviewed by: rsmith git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@241478 91177308-0d34-0410-b5e6-96231b3b80d8 --- include/clang/Basic/DiagnosticSemaKinds.td | 2 ++ lib/Sema/SemaExpr.cpp | 14 ++++++++++++-- test/CXX/expr/expr.const/p2-0x.cpp | 2 +- test/Sema/shift.c | 5 +++-- 4 files changed, 18 insertions(+), 5 deletions(-) diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td index 750219483c..39b602c551 100644 --- a/include/clang/Basic/DiagnosticSemaKinds.td +++ b/include/clang/Basic/DiagnosticSemaKinds.td @@ -4740,6 +4740,8 @@ def warn_division_by_zero : Warning<"division by zero is undefined">, InGroup; def warn_remainder_by_zero : Warning<"remainder by zero is undefined">, InGroup; +def warn_shift_lhs_negative : Warning<"shifting a negative signed value is undefined">, + InGroup>; def warn_shift_negative : Warning<"shift count is negative">, InGroup>; def warn_shift_gt_typewidth : Warning<"shift count >= width of type">, diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp index c023c8523a..01a4d48805 100644 --- a/lib/Sema/SemaExpr.cpp +++ b/lib/Sema/SemaExpr.cpp @@ -7937,9 +7937,19 @@ static void DiagnoseBadShiftValues(Sema& S, ExprResult &LHS, ExprResult &RHS, // representable in the result type, so never warn for those. llvm::APSInt Left; if (LHS.get()->isValueDependent() || - !LHS.get()->isIntegerConstantExpr(Left, S.Context) || - LHSType->hasUnsignedIntegerRepresentation()) + LHSType->hasUnsignedIntegerRepresentation() || + !LHS.get()->EvaluateAsInt(Left, S.Context)) return; + + // If LHS does not have a signed type and non-negative value + // then, the behavior is undefined. Warn about it. + if (Left.isNegative()) { + S.DiagRuntimeBehavior(Loc, LHS.get(), + S.PDiag(diag::warn_shift_lhs_negative) + << LHS.get()->getSourceRange()); + return; + } + llvm::APInt ResultBits = static_cast(Right) + Left.getMinSignedBits(); if (LeftBits.uge(ResultBits)) diff --git a/test/CXX/expr/expr.const/p2-0x.cpp b/test/CXX/expr/expr.const/p2-0x.cpp index d027c7a388..2adefd928a 100644 --- a/test/CXX/expr/expr.const/p2-0x.cpp +++ b/test/CXX/expr/expr.const/p2-0x.cpp @@ -157,7 +157,7 @@ namespace UndefinedBehavior { constexpr int shl_unsigned_negative = unsigned(-3) << 1; // ok constexpr int shl_unsigned_into_sign = 1u << 31; // ok constexpr int shl_unsigned_overflow = 1024u << 31; // ok - constexpr int shl_signed_negative = (-3) << 1; // expected-error {{constant expression}} expected-note {{left shift of negative value -3}} + constexpr int shl_signed_negative = (-3) << 1; // expected-warning {{shifting a negative signed value is undefined}} // expected-error {{constant expression}} expected-note {{left shift of negative value -3}} constexpr int shl_signed_ok = 1 << 30; // ok constexpr int shl_signed_into_sign = 1 << 31; // ok (DR1457) constexpr int shl_signed_into_sign_2 = 0x7fffffff << 1; // ok (DR1457) diff --git a/test/Sema/shift.c b/test/Sema/shift.c index d355544885..07c5fe5280 100644 --- a/test/Sema/shift.c +++ b/test/Sema/shift.c @@ -39,7 +39,8 @@ void test() { i = 1 << (WORD_BIT - 2); i = 2 << (WORD_BIT - 1); // expected-warning {{bits to represent, but 'int' only has}} i = 1 << (WORD_BIT - 1); // expected-warning {{sets the sign bit of the shift expression}} - i = -1 << (WORD_BIT - 1); + i = -1 << (WORD_BIT - 1); // expected-warning {{shifting a negative signed value is undefined}} + i = -1 << 0; // expected-warning {{shifting a negative signed value is undefined}} i = 0 << (WORD_BIT - 1); i = (char)1 << (WORD_BIT - 2); @@ -48,7 +49,7 @@ void test() { u = 5U << (WORD_BIT - 1); long long int lli; - lli = INT_MIN << 2; // expected-warning {{bits to represent, but 'int' only has}} + lli = INT_MIN << 2; // expected-warning {{shifting a negative signed value is undefined}} lli = 1LL << (sizeof(long long) * CHAR_BIT - 2); } -- 2.40.0