[Sema] Fix tautological bounds check warning with -fwrapv (#120480)

The tautological bounds check warning added in #120222 does not take into account whether signed integer overflow is well defined or not, which could result in a developer removing a bounds check that may not actually be always false because of different overflow semantics. ```c int check(const int* foo, unsigned int idx) { return foo + idx < foo; } ``` ``` $ clang -O2 -c test.c test.c:3:19: warning: pointer comparison always evaluates to false [-Wtautological-compare] 3 | return foo + idx < foo; | ^ 1 warning generated. # Bounds check is eliminated without -fwrapv, warning was correct $ llvm-objdump -dr test.o ... 0000000000000000 <check>: 0: 31 c0 xorl %eax, %eax 2: c3 retq ``` ``` $ clang -O2 -fwrapv -c test.c test.c:3:19: warning: pointer comparison always evaluates to false [-Wtautological-compare] 3 | return foo + idx < foo; | ^ 1 warning generated. # Bounds check remains, warning was wrong $ llvm-objdump -dr test.o 0000000000000000 <check>: 0: 89 f0 movl %esi, %eax 2: 48 8d 0c 87 leaq (%rdi,%rax,4), %rcx 6: 31 c0 xorl %eax, %eax 8: 48 39 f9 cmpq %rdi, %rcx b: 0f 92 c0 setb %al e: c3 retq ```

Author: nathanchance

clang/lib/Sema/SemaExpr.cpp

@@ -11789,10 +11789,11 @@ static bool checkForArray(const Expr *E) {
 /// Detect patterns ptr + size >= ptr and ptr + size < ptr, where ptr is a
 /// pointer and size is an unsigned integer. Return whether the result is
 /// always true/false.
-static std::optional<bool> isTautologicalBoundsCheck(const Expr *LHS,
+static std::optional<bool> isTautologicalBoundsCheck(Sema &S, const Expr *LHS,
  const Expr *RHS,
  BinaryOperatorKind Opc) {
- if (!LHS->getType()->isPointerType())
+ if (!LHS->getType()->isPointerType() ||
+ S.getLangOpts().isSignedOverflowDefined())
  return std::nullopt;
 
  // Canonicalize to >= or < predicate.
@@ -11940,7 +11941,7 @@ static void diagnoseTautologicalComparison(Sema &S, SourceLocation Loc,
  << 1 /*array comparison*/
  << Result);
  } else if (std::optional<bool> Res =
- isTautologicalBoundsCheck(LHS, RHS, Opc)) {
+ isTautologicalBoundsCheck(S, LHS, RHS, Opc)) {
  S.DiagRuntimeBehavior(Loc, nullptr,
  S.PDiag(diag::warn_comparison_always)
  << 2 /*pointer comparison*/

clang/test/Sema/tautological-pointer-comparison.c

@@ -1,4 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -fwrapv -verify=fwrapv %s
+
+// fwrapv-no-diagnostics
 
 int add_ptr_idx_ult_ptr(const char *ptr, unsigned index) {
  return ptr + index < ptr; // expected-warning {{pointer comparison always evaluates to false}}