[clang] [Clang] Force expressions with UO_Not to not be non-negative (PR #126846)

[Yutong Zhu via cfe-commits]

https://github.com/YutongZhuu updated https://github.com/llvm/llvm-project/pull/126846

>From 44673ebf7c3fa773ffc7c52141b889c9ea352a93 Mon Sep 17 00:00:00 2001
From: Yutong Zhu <y25zhu at uwaterloo.ca>
Date: Tue, 11 Feb 2025 22:49:40 -0500
Subject: [PATCH] Force expressions with UO_Not to not be non-negative

---
 clang/docs/ReleaseNotes.rst     |  2 ++
 clang/lib/Sema/SemaChecking.cpp | 11 +++++++++++
 clang/test/Sema/compare.c       |  4 ++++
 3 files changed, 17 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 6344c4b36e357..e8aeaa6e514e4 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -135,6 +135,8 @@ Improvements to Clang's diagnostics
 - Fixed a bug where Clang's Analysis did not correctly model the destructor behavior of ``union`` members (#GH119415).
 - A statement attribute applied to a ``case`` label no longer suppresses
   'bypassing variable initialization' diagnostics (#84072).
+- The ``-Wsign-compare`` warning now treats expressions with bitwise NOT(~) as signed integers 
+  and throws warning if they are compared with unsigned integers (##18878).
 
 Improvements to Clang's time-trace
 ----------------------------------
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 66c233de4ef30..47566d3db82d3 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -10069,6 +10069,17 @@ static std::optional<IntRange> TryGetExprRange(ASTContext &C, const Expr *E,
     case UO_AddrOf: // should be impossible
       return IntRange::forValueOfType(C, GetExprType(E));
 
+    case UO_Not: {
+      std::optional<IntRange> SubRange = TryGetExprRange(
+          C, UO->getSubExpr(), MaxWidth, InConstantContext, Approximate);
+      if (!SubRange)
+        return std::nullopt;
+
+      // The width increments by 1 if the sub-expression cannot be negative
+      // since it now can be.
+      return IntRange(SubRange->Width + (int)SubRange->NonNegative, false);
+    }
+
     default:
       return TryGetExprRange(C, UO->getSubExpr(), MaxWidth, InConstantContext,
                              Approximate);
diff --git a/clang/test/Sema/compare.c b/clang/test/Sema/compare.c
index 17cf0351ef4f5..faa17ae7546dd 100644
--- a/clang/test/Sema/compare.c
+++ b/clang/test/Sema/compare.c
@@ -419,3 +419,7 @@ void pr36008(enum PR36008EnumTest lhs) {
   if (x == y) x = y; // no warning
   if (y == x) y = x; // no warning
 }
+
+int test13(unsigned a, int *b) {
+        return a > ~(95 != *b); // expected-warning {{comparison of integers of different signs}}
+}