Author: Shivam Gupta Date: 2022-06-14T20:19:46+05:30 New Revision: 48e1829874df15fd57e731d4824604f28e177585
URL: https://github.com/llvm/llvm-project/commit/48e1829874df15fd57e731d4824604f28e177585 DIFF: https://github.com/llvm/llvm-project/commit/48e1829874df15fd57e731d4824604f28e177585.diff LOG: [Diagnostics] Fix inconsistent shift-overflow warnings in C++20 This fixes https://github.com/llvm/llvm-project/issues/52873. Don't warn in C++2A mode (and newer), as signed left shifts always wrap and never overflow. Ref. - https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1236r1.html. Added: Modified: clang/docs/ReleaseNotes.rst clang/lib/Sema/SemaExpr.cpp clang/test/CXX/expr/expr.const/p2-0x.cpp clang/test/SemaCXX/constant-expression-cxx2a.cpp clang/test/SemaCXX/shift.cpp Removed: ################################################################################ diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst index e68640074cea5..8766adb27a09a 100644 --- a/clang/docs/ReleaseNotes.rst +++ b/clang/docs/ReleaseNotes.rst @@ -256,6 +256,10 @@ Improvements to Clang's diagnostics - The ``-Wdeprecated`` diagnostic will now warn on out-of-line ``constexpr`` declarations downgraded to definitions in C++1z, in addition to the existing warning on out-of-line ``const`` declarations. +- ``-Wshift-overflow`` will not warn for signed left shifts in C++20 mode + (and newer), as it will always wrap and never overflow. This fixes + `Issue 52873 <https://github.com/llvm/llvm-project/issues/52873>`_. + Non-comprehensive list of changes in this release ------------------------------------------------- diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp index 7e6618bd2d11d..6edf5b355d029 100644 --- a/clang/lib/Sema/SemaExpr.cpp +++ b/clang/lib/Sema/SemaExpr.cpp @@ -11416,10 +11416,16 @@ static void DiagnoseBadShiftValues(Sema& S, ExprResult &LHS, ExprResult &RHS, return; llvm::APSInt Left = LHSResult.Val.getInt(); - // If LHS does not have a signed type and non-negative value - // then, the behavior is undefined before C++2a. Warn about it. - if (Left.isNegative() && !S.getLangOpts().isSignedOverflowDefined() && - !S.getLangOpts().CPlusPlus20) { + // Don't warn if signed overflow is defined, then all the rest of the + // diagnostics will not be triggered because the behavior is defined. + // Also don't warn in C++20 mode (and newer), as signed left shifts + // always wrap and never overflow. + if (S.getLangOpts().isSignedOverflowDefined() || S.getLangOpts().CPlusPlus20) + return; + + // If LHS does not have a non-negative value then, the + // behavior is undefined before C++2a. Warn about it. + if (Left.isNegative()) { S.DiagRuntimeBehavior(Loc, LHS.get(), S.PDiag(diag::warn_shift_lhs_negative) << LHS.get()->getSourceRange()); diff --git a/clang/test/CXX/expr/expr.const/p2-0x.cpp b/clang/test/CXX/expr/expr.const/p2-0x.cpp index 81efbb0a47271..6d2bc3f25179b 100644 --- a/clang/test/CXX/expr/expr.const/p2-0x.cpp +++ b/clang/test/CXX/expr/expr.const/p2-0x.cpp @@ -162,9 +162,9 @@ namespace UndefinedBehavior { 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) - constexpr int shl_signed_off_end = 2 << 31; // cxx11-error {{constant expression}} cxx11-note {{signed left shift discards bits}} expected-warning {{signed shift result (0x100000000) requires 34 bits to represent, but 'int' only has 32 bits}} - constexpr int shl_signed_off_end_2 = 0x7fffffff << 2; // cxx11-error {{constant expression}} cxx11-note {{signed left shift discards bits}} expected-warning {{signed shift result (0x1FFFFFFFC) requires 34 bits to represent, but 'int' only has 32 bits}} - constexpr int shl_signed_overflow = 1024 << 31; // cxx11-error {{constant expression}} cxx11-note {{signed left shift discards bits}} expected-warning {{requires 43 bits to represent}} + constexpr int shl_signed_off_end = 2 << 31; // cxx11-error {{constant expression}} cxx11-note {{signed left shift discards bits}} cxx11-warning {{signed shift result (0x100000000) requires 34 bits to represent, but 'int' only has 32 bits}} + constexpr int shl_signed_off_end_2 = 0x7fffffff << 2; // cxx11-error {{constant expression}} cxx11-note {{signed left shift discards bits}} cxx11-warning {{signed shift result (0x1FFFFFFFC) requires 34 bits to represent, but 'int' only has 32 bits}} + constexpr int shl_signed_overflow = 1024 << 31; // cxx11-error {{constant expression}} cxx11-note {{signed left shift discards bits}} cxx11-warning {{requires 43 bits to represent}} constexpr int shl_signed_ok2 = 1024 << 20; // ok constexpr int shr_m1 = 0 >> -1; // expected-error {{constant expression}} expected-note {{negative shift count -1}} diff --git a/clang/test/SemaCXX/constant-expression-cxx2a.cpp b/clang/test/SemaCXX/constant-expression-cxx2a.cpp index 4aaf6328aaf63..760821eb88373 100644 --- a/clang/test/SemaCXX/constant-expression-cxx2a.cpp +++ b/clang/test/SemaCXX/constant-expression-cxx2a.cpp @@ -530,8 +530,8 @@ namespace TwosComplementShifts { using int32 = __INT32_TYPE__; static_assert(uint32(int32(0x1234) << 16) == 0x12340000); static_assert(uint32(int32(0x1234) << 19) == 0x91a00000); - static_assert(uint32(int32(0x1234) << 20) == 0x23400000); // expected-warning {{requires 34 bits}} - static_assert(uint32(int32(0x1234) << 24) == 0x34000000); // expected-warning {{requires 38 bits}} + static_assert(uint32(int32(0x1234) << 20) == 0x23400000); + static_assert(uint32(int32(0x1234) << 24) == 0x34000000); static_assert(uint32(int32(-1) << 31) == 0x80000000); static_assert(-1 >> 1 == -1); diff --git a/clang/test/SemaCXX/shift.cpp b/clang/test/SemaCXX/shift.cpp index 144c18849050c..89a98791d3eba 100644 --- a/clang/test/SemaCXX/shift.cpp +++ b/clang/test/SemaCXX/shift.cpp @@ -41,8 +41,8 @@ void test() { int i; 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 = 2 << (WORD_BIT - 1); // cxx17-warning {{bits to represent, but 'int' only has}} + i = 1 << (WORD_BIT - 1); // cxx17-warning {{sets the sign bit of the shift expression}} i = -1 << (WORD_BIT - 1); // cxx17-warning {{shifting a negative signed value is undefined}} i = -1 << 0; // cxx17-warning {{shifting a negative signed value is undefined}} i = 0 << (WORD_BIT - 1); @@ -53,7 +53,7 @@ void test() { u = 5U << (WORD_BIT - 1); long long int lli; - lli = INT_MIN << 2; // cxx17-warning {{shifting a negative signed value is undefined}} cxx2a-warning {{requires 34 bits to represent}} + lli = INT_MIN << 2; // cxx17-warning {{shifting a negative signed value is undefined}} lli = 1LL << (sizeof(long long) * CHAR_BIT - 2); } _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
