[llvm-dev] Potential problem with -Wunreachable-code

[David Blaikie via llvm-dev]

Not a bug - intended behavior & not the only diagnostic with this property
(that the behavior is different between preprocessed and unpreprocessed
source). Clang uses macro spelling as a means to reduce false positives in
diagnostics - without them, there are more false positives.

Clang does have the -frewrite-includes flag that should preserve more of
the macro content while still making a standalone file that could be
compiled without searching for includes.

On Thu, Nov 21, 2019 at 7:29 PM Ian Collins via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello, hopefully this is the correct place to ask this...
>
>
> We use distcc with clang++ and I have recently added -Wunreachable-code
> to our set of warnings.  The problem I am seeing is the compile fails
> with (valid) unreachable code warnings on the slave, but passes (no
> warning) locally.  All machines have the same compiler version
>
> clang version 8.0.1-svn369350-1~exp1~20190820121219.79
> (branches/release_80)
>
> Distcc takes the compile line, removes -o <output> and replaces it with
> -E and sends the preprocessed result to the slave where it is compiled
> without any -I, -isystem or -D options. Generating the preprocessed
> output and compiling it locally finds the warnings.  I'm concerned that
> we are seeing a different result for the output of -E and the standard
> compile.
>
>
> The compiler options are:
> -g
> -Wall
> -Werror
> -Wno-strict-aliasing
> -Wno-missing-braces
> -m64
> -Wno-switch
> -fno-diagnostics-fixit-info
> -fshow-overloads=best
> -Wno-unknown-warning-option
> -Wno-unknown-pragmas
> -Wno-logical-op-parentheses
> -Wno-tautological-undefined-compare
> -Wno-tautological-unsigned-zero-compare
> -Wno-tautological-unsigned-enum-zero-compare
> -Wno-expansion-to-defined
> -Wno-format-security
> -Wnarrowing
> -Warray-bounds-pointer-arithmetic
> -Wduplicate-method-match
> -Wmove
> -Wunreachable-code
> -Wno-return-std-move
> -ftemplate-depth=512
> -Wpessimizing-move
> -gline-tables-only
> -fprofile-instr-generate
> -fcoverage-mapping
> -Og
> -std=c++14
> -Wno-reorder
> -Wno-literal-suffix
> -Wcomma
> -fPIC
>
> Thanks,
>
> Ian.
>
>
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