[clang] f0dcf32 - [Sema] Fix tautological bounds check warning with -fwrapv (#120480)

[via cfe-commits]

Author: Nathan Chancellor
Date: 2024-12-18T20:23:50-07:00
New Revision: f0dcf3240dffe3767c7f3a2e2da5b92ae9fd1bef

URL: https://github.com/llvm/llvm-project/commit/f0dcf3240dffe3767c7f3a2e2da5b92ae9fd1bef
DIFF: https://github.com/llvm/llvm-project/commit/f0dcf3240dffe3767c7f3a2e2da5b92ae9fd1bef.diff

LOG: [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
```

Added: 
    

Modified: 
    clang/lib/Sema/SemaExpr.cpp
    clang/test/Sema/tautological-pointer-comparison.c

Removed: 
    


################################################################################
diff  --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index e06a092177ef02..24f7d27c691154 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/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*/

diff  --git a/clang/test/Sema/tautological-pointer-comparison.c b/clang/test/Sema/tautological-pointer-comparison.c
index 19cd20e5f7d21c..1c5973b01a30d0 100644
--- a/clang/test/Sema/tautological-pointer-comparison.c
+++ b/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}}