[Bug middle-end/37674] New: Bootstrap failure due to miscompilation of genattrtab

[krebbel at gcc dot gnu dot org]

With the patch for PR37535 applied to mainline GCC the bootstrap still fails
due to a miscompilation.  The problem is that r52 is assigned to r6 what
collides with an INSN loading r6 with a parameter for a CALL.

I think the problem is created in ira_flattening.  Allocno a3 is a parent of
a87. For a87 the conflict with r6 is properly recorded. Afterwards ira_flatting
merges a87 into a3 without propagating the conflict_hard_regs set. So the
conflict with r6 is lost and ira_reassign_pseudos later on assigns r52 to hard
reg r6. 

There is already code in ira_flattening which propagates the conflict sets. But
the code is only enabled if propagate_p is true. This in turn seems only to get
set if the loop once merged allocnos with different regnos?! I unfortunately
don't understand this enough to come up with a patch. So I better leave that to
you :)  Please contact me if you need more information.

I wasn't able to reduce the testcase (genattrtab) since it is quite difficult
to see from the code if the miscompile occurred or not.

;; a3(r52,l0) conflicts: a1(r87,l0) ......
;;     total conflict hard regs: 0-6 14
;;     conflict hard regs: 0-5 14

;; a87(r52,l2) conflicts: a86(r48,l2) .....
;;     total conflict hard regs: 0-6 14
;;     conflict hard regs: 0-6 14

...
Moving ranges of a87r52 to a3r52:  [245..248] [234..243] [229..232] [218..227]
[173..189] [126..169]

...
 Try assign 52(a3), cost=268: reassign to 6


-- 
           Summary: Bootstrap failure due to miscompilation of genattrtab
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: krebbel at gcc dot gnu dot org
 GCC build triplet: s390x-ibm-linux
  GCC host triplet: s390x-ibm-linux
GCC target triplet: s390x-ibm-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37674