[LLVMdev] Linearscan allocator bug?

[Vladimir Prus]

Chris Lattner wrote:
> On Tue, 22 Jun 2004, Vladimir Prus wrote:
> > First, I attach two files -- LLVM asm and the asm for my target. The
> > problem with assembler is: on line 171 it uses register gr2, which is
> > copied from gr6 above, on line 161. The only predecessor of this basic
> > block is jump on line 90. The problem is that gr6 is not initialized in
> > the interval from the function entry till the jump.
>
> Okay, I see the problem.  You need to tell the compiler that the
> conditional branches are terminators.  You're getting code that looks like
> this:
>
> <LBB7> // // shortcirc_next.0.selectcont.selectcont
>         gr1 = gr1;
>         gr1 = gr1;
>         gr5 = 0;
>         gr2 - gr5;
>         if <>0 goto LBB11;
> *       gr2 = gr4;
> *       gr5 = gr1;
> *       gr6 = gr4;
> *       gr1 = gr1;
>         goto LBB8;
>
> I'm guessing that those copies are inserted by the register allocator, and
> in particular, that is probably where gr6 is supposed to get it's value.
> If you set the isTerminator flag on your 'if <>0 goto LBB11;' things will
> probably magically start working for you, as the copies will be inserted
> BEFORE the branch instead of after it.

Hmm.. this is what I have in td file already:

let isTerminator = 1 in
    def GOTO : Unknown<"goto">;
    def IFEQ : Unknown<"if =0 goto">;
    def IFNEQ : Unknown<"if <>0 goto">;
    .....

Should this work?

> Also, if you haven't already, you might want to teach TargetInstrInfo
> that '=' is a move instruction (implement isMoveInstr), so instructinos
> like 'gr1 = gr1' will go away and you'll get coallescing.  :)

BTW, is it possible to set some instruction flag, instead of overriding a 
function? Something like:

let isMove = 1 in
      def MOVE :.....

?

- Volodya