[all-commits] [llvm/llvm-project] f0dcf3: [Sema] Fix tautological bounds check warning with ...

[Nathan Chancellor via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: f0dcf3240dffe3767c7f3a2e2da5b92ae9fd1bef
      https://github.com/llvm/llvm-project/commit/f0dcf3240dffe3767c7f3a2e2da5b92ae9fd1bef
  Author: Nathan Chancellor <nathan at kernel.org>
  Date:   2024-12-18 (Wed, 18 Dec 2024)

  Changed paths:
    M clang/lib/Sema/SemaExpr.cpp
    M clang/test/Sema/tautological-pointer-comparison.c

  Log Message:
  -----------
  [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
```



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