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Roman Lebedev c5417aafec [Sema] -Wtautological-constant-compare is too good. Cripple it.
Summary:
The diagnostic was mostly introduced in D38101 by me, as a reaction to wasting a lot of time, see [[ https://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20171009/206427.html | mail ]].
However, the diagnostic is pretty dumb. While it works with no false-positives,
there are some questionable cases that are diagnosed when one would argue that they should not be.

The common complaint is that it diagnoses the comparisons between an `int` and
`long` when compiling for a 32-bit target as tautological, but not when
compiling for 64-bit targets. The underlying problem is obvious: data model.
In most cases, 64-bit target is `LP64` (`int` is 32-bit, `long` and pointer are
64-bit), and the 32-bit target is `ILP32` (`int`, `long`, and pointer are 32-bit).

I.e. the common pattern is: (pseudocode)
```
#include <limits>
#include <cstdint>
int main() {
  using T1 = long;
  using T2 = int;

  T1 r;
  if (r < std::numeric_limits<T2>::min()) {}
  if (r > std::numeric_limits<T2>::max()) {}
}
```
As an example, D39149 was trying to fix this diagnostic in libc++, and it was not well-received.

This *could* be "fixed", by changing the diagnostics logic to something like
`if the types of the values being compared are different, but are of the same size, then do diagnose`,
and i even attempted to do so in D39462, but as @rjmccall rightfully commented,
that implementation is incomplete to say the least.

So to stop causing trouble, and avoid contaminating upcoming release, lets do this workaround:
* move these three diags (`warn_unsigned_always_true_comparison`, `warn_unsigned_enum_always_true_comparison`, `warn_tautological_constant_compare`) into it's own `-Wtautological-constant-in-range-compare`
* Disable them by default
* Make them part of `-Wextra`
* Additionally, give `warn_tautological_constant_compare` it's own flag `-Wtautological-type-limit-compare`.
  I'm not happy about that name, but i can't come up with anything better.

This way all three of them can be enabled/disabled either altogether, or one-by-one.

Reviewers: aaron.ballman, rsmith, smeenai, rjmccall, rnk, mclow.lists, dim

Reviewed By: aaron.ballman, rsmith, dim

Subscribers: thakis, compnerd, mehdi_amini, dim, hans, cfe-commits, rjmccall

Tags: #clang

Differential Revision: https://reviews.llvm.org/D41512

llvm-svn: 321691
2018-01-03 08:45:19 +00:00
clang [Sema] -Wtautological-constant-compare is too good. Cripple it. 2018-01-03 08:45:19 +00:00
clang-tools-extra [clangd] Properly set filterText for index-based completion items 2017-12-29 14:59:22 +00:00
compiler-rt [asan] Restore asan_device_setup compatibility with older libraries. 2018-01-02 21:35:23 +00:00
debuginfo-tests [debuginfo-tests] Support moving debuginfo-tests to llvm/projects 2017-12-12 16:54:20 +00:00
libclc configure.py: Add gfx900 (Vega, Raven) 2017-11-27 11:14:06 +00:00
libcxx Mark LWG2824 as complete. We already did it, but I added a test to be sure 2018-01-03 04:37:30 +00:00
libcxxabi Try again, this time with the correct address 2017-12-29 19:26:28 +00:00
libunwind Reland [PPC64] Port to ppc64le - initial version 2018-01-02 22:11:30 +00:00
lld Don't assume that size relocations are always constant. 2018-01-03 03:58:58 +00:00
lldb [Core/Debugger] Remove some code that doesn't compile anymore. 2018-01-02 16:27:01 +00:00
llgo irgen: Create functions instead of global variables for builtin hash and equal algorithms. 2017-06-04 22:11:28 +00:00
llvm [GlobalISel][Legalizer] Fix legalization of llvm.smul.with.overflow 2018-01-03 04:56:56 +00:00
openmp [OMPT] Build runtime with OMPT support by default 2018-01-02 21:09:00 +00:00
parallel-libs [Axccel] Remove -Wno-missing-braces in build 2016-12-19 21:34:07 +00:00
polly [CodeGen] Fix noalias annotations for memcpy/memmove. 2017-12-22 17:44:53 +00:00
README.md Add an svn project to contain the files that appear at the root of the 2017-10-19 21:09:49 +00:00

README.md

Low Level Virtual Machine (LLVM)

This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments.