> Hmm, I think instead of special-casing UNKNOWN_LOCATION
> what gimple_set_location should probably do is either not copy
> warnings at all or union them.  Btw, gimple_set_location also
> removes a previously set BLOCK (but gimple_set_block preserves
> the location locus and diagnostic override).
> 
> So I'd be tempted to axe the copy_warning () completely here. 

The first thing I tried, but it regressed the original testcase IIRC.

Even my minimal patch manages to break bootstrap on ARM:

buildslave/workspace/tcwg_gnu_1/abe/snapshots/gcc.git~master/libcpp/lex.cc:
1523:9: error: pointer used after ‘void operator delete(void*, std::size_t)’ 
[-Werror=use-after-free]
# 00:31:04 make[3]: *** [Makefile:227: lex.o] Error 1
# 00:31:04 make[2]: *** [Makefile:9527: all-stage3-libcpp] Error 2
# 00:31:35 make[1]: *** [Makefile:25887: stage3-bubble] Error 2
# 00:31:35 make: *** [Makefile:1072: all] Error 2

      /* Don't warn for cases like when a cdtor returns 'this' on ARM.  */
      else if (warning_suppressed_p (var, OPT_Wuse_after_free))
	return;

because warning-control.cc:copy_warning also clobbers the warning data of the 
destination.  We have in cp/decl.cc:maybe_return_this the lines:

      /* Return the address of the object.  */
      tree val = DECL_ARGUMENTS (current_function_decl);
      suppress_warning (val, OPT_Wuse_after_free);

-Wuse-after-free is suppressed for the location of VAL and the TREE_NO_WARNING 
bit set on it.  But other expressions may have the same location as VAL and 
the TREE_NO_WARNING bit _not_ set, so when you call copy_warning (expr, expr) 
(we do that a lot after failed folding) for them, copy_warning erases the 
warning data of the location.

I have installed the obvious fixlet after testing on x86-64/Linux, but the 
decoupling between TREE_NO_WARNING bit and location looks a bit problematic.


	* warning-control.cc (copy_warning) [generic version]: Do not erase
	the warning data of the destination location when the no-warning
	bit is not set on the source.
	(copy_warning) [tree version]: Return early if TO is equal to FROM.
	(copy_warning) [gimple version]: Likewise.
testsuite/
	* g++.dg/warn/Wuse-after-free5.C: New test.

-- 
Eric Botcazou