[LLVMdev] How to check for "SPARC code generation" in MachineBasicBlock.cpp?

[Nathan Keynes]

On 11/12/2009, at 10:43 AM, Anton Korobeynikov wrote:

> Hi, Chris
> 
>> That is target independent code, so you should not put sparc specific changes there.  It sounds like one of the sparc-specific target hooks is wrong.
> Since sparc does not provide any hooks for operation of branches (e.g.
> AnalyzeBranch and friends) it might be possible that generic codegen
> code is broken in absence of these hooks.

Hi Anton,

   I've had a little time to look into this now, and I think there's actually two separate problems here. By way of example, what's happening is that the following pair of MBBs: 

BB#315: derived from LLVM BB %bb
    Predecessors according to CFG: BB#314
        %reg1731<def> = SETHIi 1856
        %reg1732<def> = ORri %G0, 1
        %reg1733<def> = SLLrr %reg1732, %reg1729
        %reg1734<def> = ORri %reg1731, 1
        %reg1735<def> = ANDrr %reg1733, %reg1734
        %reg1736<def> = SUBCCri %reg1735, 0, %ICC<imp-def>
        BCOND <BB#3>, 9, %ICC<imp-use>
        BA <BB#53>
    Successors according to CFG: BB#3 BB#53

BB#3: derived from LLVM BB %bb1
    Predecessors according to CFG: BB#315
        %reg1740<def> = ANDri %reg1067, 255
        %reg1741<def> = SUBCCri %reg1740, 1, %ICC<imp-def>; dbg:expr.c:757
        BCOND <BB#5>, 1, %ICC<imp-use>; dbg:expr.c:757
    Successors according to CFG: BB#5 BB#4


is compiled down to 

! BB#7:                                                     ! %bb
                                                            !   in Loop: Header=BB1_2 Depth=2
        sethi 1856, %l5
        or %g0, 1, %l6
        sll %l6, %l3, %l3
        or %l5, 1, %l5
        and %l3, %l5, %l3
        subcc %l3, 0, %l3
        bne .LBB1_8
        nop
        ba .LBB1_68
        nop
! BB#8:                                                     ! %bb1
                                                            !   in Loop: Header=BB1_2 Depth=2
        ld [%fp+-212], %l3
        and %l3, 255, %l3
.Llabel18:
        subcc %l3, 1, %l3
                                                            ! expr.c:757
        be .LBB1_10
                                                            ! expr.c:757
        nop

(Note the missing .LBB1_8:, which is the immediate problem). 

Firstly, the BNE/BA pair should be reduced to a BE (I assume this is the responsibility of AnalyzeBranch and friends that you mention). However I still wouldn't have expected that to result in the label being omitted (assuming multiple branches are legal in an MBB). It appears that the branch delay-slots are specifically to blame here - the above BB#315 immediately prior to output is

BB#7: derived from LLVM BB %bb
    Live Ins: %L1 %L0 %L3 %L2 %L4
    Predecessors according to CFG: BB#6
        %L5<def> = SETHIi 1856
        %L6<def> = ORri %G0, 1
        %L3<def> = SLLrr %L6<kill>, %L3<kill>
        %L5<def> = ORri %L5<kill>, 1
        %L3<def> = ANDrr %L3<kill>, %L5<kill>
        %L3<def,dead> = SUBCCri %L3<kill>, 0, %ICC<imp-def>
        BCOND <BB#8>, 9, %ICC<imp-use,kill>
        NOP
        BA <BB#68>
        NOP

which leads MachineBasicBlock::isOnlyReachableByFallthrough() to return TRUE for BB#8, since the final NOP is not a 'barrier instruction', and so the label is skipped. Assuming I've got this right so far, I'd expect this to be affecting MIPS as well as SPARC. 

So I guess the question I have is, are MBBs like BB#7 above legal, and if so would something like

BB#7:
    ...
    BCOND cond1, BB#8
    BCOND cond2, BB#57
BB#8:
    ...
also be legal - ie BB7 contains both a branch and a fallthrough to the same successor block? 

If so, it seems that isOnlyReachableByFallthrough() would need to check all label references in the predecessor to ensure none of them correspond to the target block rather than just the final instruction (or alternatively one could add additional tracking information to the MBB for this purpose?).

Cheers,
Nathan