[LLVMbugs] [Bug 4657] New: register scavenger asserts with subreg lowering

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Thu Jul 30 17:51:29 PDT 2009


           Summary: register scavenger asserts with subreg lowering
           Product: libraries
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Register Allocator
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: bob.wilson at apple.com
                CC: llvmbugs at cs.uiuc.edu

Created an attachment (id=3262)
 --> (http://llvm.org/bugs/attachment.cgi?id=3262)

The attached file causes the scavenger to assert when compiled with "llc
-march=arm -mattr=+neon".  Here is what I think is happening.  (I've actually
been looking at a slightly different case that doesn't occur without some of my
local changes, but I think it is the same problem.)  A Neon quad register is
defined by a pair of insert_subregs.  The subreg accesses are lowered into
copies that assign the underlying Neon double registers.  Now you've got a quad
register that is used but never (directly) defined.  Or, at least that seems to
be what the scavenger is thinking.

For example, in the following Q1 is defined:

        %D0<def> = FMDRR %R2<kill>, %R3<kill>, 14, %reg0
        %D1<def> = FMDRR %R0<kill>, %R1<kill>, 14, %reg0
        %Q1<def> = INSERT_SUBREG %Q1<undef>, %D1<kill>, 5
        %Q1<def> = INSERT_SUBREG %Q1, %D0<kill>, 6
        %R0<def> = ADDri <fi#0>, 0, 14, %reg0, %reg0
        VSTRQ %Q1<kill>, %R0, Mem:ST(16,16) [v_addr + 0]

but then the INSERT_SUBREGs are lowered:

********** LOWERING SUBREG INSTRS **********
********** Function: scale
subreg: CONVERTING: %Q1<def> = INSERT_SUBREG %Q1<undef>, %D1<kill>, 5
subreg: %D2<def> = FCPYD %D1, 14, %reg0
subreg: CONVERTING: %Q1<def> = INSERT_SUBREG %Q1, %D0<kill>, 6subreg: %D3<def>
= FCPYD %D0, 14, %reg0

and the scavenger asserts:

Assertion failed: (isUsed(Reg) && "Using an undefined register!"), function
forward, file
/Users/bwilson/local/llvm/llvm/lib/CodeGen/RegisterScavenging.cpp, line 221.

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