[PATCH] D65192: [Sema] Disable some enabled-by-default -Wparentheses diagnostics

Thu Aug 1 06:11:44 PDT 2019

aaron.ballman added inline comments.

================
Comment at: include/clang/Basic/DiagnosticSemaKinds.td:5654-5656
 def warn_addition_in_bitshift : Warning<
   "operator '%0' has lower precedence than '%1'; "
+  "'%1' will be evaluated first">, InGroup<ShiftOpParentheses>, DefaultIgnore;
----------------
MaskRay wrote:
> rsmith wrote:
> > Do you have evidence that this warning has a significant false-positive rate? In my experience it's very common for people to think of `<<` as being a multiplication-like operator and be surprised when it turns out to have lower precedence than addition.
> warn_addition_in_bitshift has many false positives. Some results when searching for `[-Wshift-op-parentheses]` and the most common diagnostic `operator '<<' has lower precedence than '+'; '+' will be evaluated first`: 
> 
> https://gitship.com/srutscher/pdp-6-emulator/blob/f41b119eee2f409ea519b15f3a76cfecb70c03d8/emu/Makefile
> https://www.openwall.com/lists/musl/2014/04/04/12
> https://salmonella-freebsd-x86-64.call-cc.org/chicken-4/clang/freebsd/x86-64/2018/06/21/salmonella-report/install/bvsp-spline.html
> https://software.intel.com/en-us/forums/intel-threading-building-blocks/topic/431422
> https://github.com/rdkit/rdkit/issues/145
> ffmpeg https://bugs.freebsd.org/bugzilla/show_bug.cgi?format=multiple&id=191743
> https://clang.debian.net/logs/2015-03-25/fairymax_4.8v-1_unstable_clang.log
> 
Some of those look like true positives. For instance, the fix to https://github.com/rdkit/rdkit/issues/145 was https://github.com/rdkit/rdkit/commit/38ca41c8abc3f4429ae8df2ad79d3ff8b3fea0b6 which looks like the warning behaved as intended. https://software.intel.com/en-us/forums/intel-threading-building-blocks/topic/431422 doesn't really have anything to do with the diagnostic.

FWIW, in most of the cases, I feel like parens would clarify the code (heck, they're already using parens in many of these cases). e.g.,
```
fairymax.c:776:46: warning: operator '>>' has lower precedence than '+'; '+' will be evaluated first [-Wshift-op-parentheses]
                        MovesLeft = -(GamePtr+(Side==WHITE)>>1);
                                      ~~~~~~~^~~~~~~~~~~~~~~~

dbvspis.c:606:20: warning: operator '<<' has lower precedence than '+'; '+' will be evaluated first [-Wshift-op-parentheses]
    i3 = i2 + (*np + 1 << 1);
               ~~~~^~~ ~~
```
FWIW, I'm fine leaving this default on.

Repository:
  rC Clang

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D65192/new/

https://reviews.llvm.org/D65192