[PATCH] D127518: [Diagnostics] Fix inconsistent shift-overflow warnings in C++20

[Shivam Gupta via Phabricator via cfe-commits]

xgupta created this revision.
xgupta added a reviewer: aaron.ballman.
Herald added a project: All.
xgupta requested review of this revision.
Herald added a project: clang.
Herald added a subscriber: cfe-commits.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D127518

Files:
  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


Index: clang/test/SemaCXX/shift.cpp
===================================================================
--- clang/test/SemaCXX/shift.cpp
+++ clang/test/SemaCXX/shift.cpp
@@ -41,7 +41,7 @@
 
   int i;
   i = 1 << (WORD_BIT - 2);
-  i = 2 << (WORD_BIT - 1); // expected-warning {{bits to represent, but 'int' only has}}
+  i = 2 << (WORD_BIT - 1); // cxx17-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); // cxx17-warning {{shifting a negative signed value is undefined}}
   i = -1 << 0; // cxx17-warning {{shifting a negative signed value is undefined}}
@@ -53,7 +53,7 @@
   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);
 }
 
Index: clang/test/SemaCXX/constant-expression-cxx2a.cpp
===================================================================
--- clang/test/SemaCXX/constant-expression-cxx2a.cpp
+++ clang/test/SemaCXX/constant-expression-cxx2a.cpp
@@ -530,8 +530,8 @@
   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);
Index: clang/test/CXX/expr/expr.const/p2-0x.cpp
===================================================================
--- clang/test/CXX/expr/expr.const/p2-0x.cpp
+++ clang/test/CXX/expr/expr.const/p2-0x.cpp
@@ -162,9 +162,9 @@
   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}}
Index: clang/lib/Sema/SemaExpr.cpp
===================================================================
--- clang/lib/Sema/SemaExpr.cpp
+++ clang/lib/Sema/SemaExpr.cpp
@@ -11453,6 +11453,9 @@
     return;
   }
 
+  if (S.getLangOpts().CPlusPlus20)
+    return;
+
   S.Diag(Loc, diag::warn_shift_result_gt_typewidth)
     << HexResult.str() << Result.getMinSignedBits() << LHSType
     << Left.getBitWidth() << LHS.get()->getSourceRange()


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