Re: [PATCH] PR17544, a C99 and C++ Wunreachable-code problem

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Steven Bosscher <stevenb@suse.de> writes:

| Hi,
| 
| This patch is marked "middle-end", but really it's a problem in the
| middle-end *and* the front ends.  There was no way to mark a complete
| statement as compiler generated in a way that would survive through
| gimplification, and the gimplifier would notice that some statements
| do not have a source location.
| 
| In this particular case, the "return 0;" statement required in C99
| mode and by C++ would get annotated with a locus by the gimplifier.
| 
| But often the compiler generated "return 0;" is not reachable.  The
| -Wunreachable-code warnings only happen for statements with a source
| location, and since the compiler generated statement got a locus in
| the gimplifier, we warn if they're unreachable.
| 
| My proposed fix is to put a locus on the compiler generated statement
| in the front end: just put it in the current file, on line 0.  Then
| teach the places where we issue unreachable code warnings to issue
| the warning only if the unreachable code is on a line number >0.
| 
| The attached patch was bootstrapped and tested (all except Ada) on
| x86_64-suse-linux-gnu, and it fixes PR17544, a regression from all
| earlier GCC releases.  OK to apply?

Here is an alternative way to that.  Look for one of the unused flag
in tree_common, for statements.  Choices are

   private_flag
   unsigned_flag
   nothrow_flag
   deprecated_flag

Alternatively, you can set TREE_NO_WARNING (which I would think much
cleaner). 

-- Gaby