Created attachment 74639 [details]
Test case illustrating bsfl validity bit failure

This is with current SVN (revision 13057).

To reproduce:

1) Compile the attached program with "gcc -O3 -Wall -o test test.c"
2) Run "valgrind test"

I expect this to produce no warnings, because even though the integer starts as an undefined value, the code explicitly clears its low bit and sets its second bit before calling __builtin_ffsl (aka. "bsfl").

Since all of the bits up to and including the first "1" are explicitly set, this program's behavior is perfectly deterministic; it is guaranteed to return "2" from main.

Similar code shows up in certain optimized word-at-a-time string routines, so this is not just hypothetical.  (And it will become even less hypothetical if we ever fix bug 294285.)