[Bug target/35658] [4.3/4.4 regression] Bad interaction on ia64 between -funroll-loops -fno-automatic -O2 and common block variable

["sje at cup dot hp dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #7 from sje at cup dot hp dot com  2008-06-05 23:02 -------
I now think this is a register scheduling bug.  If I use -fno-schedule-insns2
then the bug doesn't happen even with "-O2 fno-automatic -frename-registers".

The problem seems to be scheduling the assignment to TEMP2 and assigning TEMP2
to F(K).

In the good code, in C201S I see the following instructions, in this order:

         addl r17 = @gprel(temp2.717#), gp - r17 is addr of temp2
         stfd [r17] = f0                   - zero out temp2
         (p11) stfd [r17] = f10            - (maybe) put value (2.0) in temp2
         ldfd f9 = [r17]                   - load temp2
         add r16 = r34, r37                - r16 is addr of F(1)
         stfd [r16] = f9                   - store temp2 in F(1)

In the bad code (with instruction scheduling turned on) I see:


         addl r17 = @gprel(temp2.717#), gp - r17 is addr of temp2
         add r16 = r34, r37                - r16 is addr of F(1)
         ldfd f9 = [r17]                   - load temp2
         stfd [r17] = f0                   - zero out temp2
         stfd [r16] = f9                   - store (old) temp2 in F(1)
         (p11) stfd [r17] = f10            - (maybe) put value (2.0) in temp2

The store into F(1) is done before we have put the correct value int temp2.
I don't understand how instruction scheduling could be this broken.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35658