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

[Gabriel Dos Reis]

Steven Bosscher <stevenb@suse.de> writes:

| On Friday 31 December 2004 14:21, Gabriel Dos Reis wrote:
| > 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).
| 
| Maybe you didn't read the first paragraph of my first mail,

I did.

| so I'll  repeat:

That is a waste.

| 
| "There was no way to mark a complete statement as compiler generated
| in a way that would survive through gimplification"
| 
| If you set TREE_NO_WARNING, after gimplification, you *still* have a
| locus and your flag setting is gone.

My point is that the gimplification stage should preserve the flag,
instead of clearing it.

-- Gaby