[LLVMdev] Re: Preferring cast over seteq with 0

[Chris Lattner]

On Thu, 19 May 2005, Eric Zimmerman wrote:
>>> In my situation, this bool value feeds a select instruction.  Because
>>> casting inverts the condition, the select would have to switch the
>>> operands, but I think it would be beneficial.
>>
>> I think that the instcombine pass should take care of this situation.  Do
>> you have a simple testcase that shows the problem?

> Hi Chris,
>  I am targeting the alpha platform.  I pasted a portion of the
> problem code below.  I starred a select in the join block that is fed
> by the seteq (%tmp.37) in the entry block.  The resultant alpha code
> places an unnecessary cmpeq in block 'entry'.  When I move the
> definition of %tmp.37 to block 'join', the Alpha backend correctly
> eliminates the cmpeq.

I don't think that this is due to a cast vs setcc issue.  In theory they 
should both generate identical code.

> I think rather than changing the cast/seteq behavior, I can get the
> same results for alpha by moving the seteq to the block where it is
> used.

Yup, I think this is the real issue.  It should be straight-forward to 
make the inst combine pass sink setcc to the select instruction.  If 
you're interested in tackling this, you want to add something like this to 
visitSelectInst:

   if (SetCondInst *SCI = dyn_cast<SetCondInst>(CondVal))
     if (SCI->hasOneUse() && SCI->getParent() != SI.getParent()) {
       SI.getParent().getInstList().splice(&SI,
                       SCI->getParent()->getInstList(), SCI);
       return &SI;
     }

I'm sorry I can't take care of this myself, the computers here have 
decided to not cooperate.

-Chri

> -Eric
>
> implementation   ; Functions:
>
> int %FirstOne(ulong %arg1) {
> entry:
>        %x = alloca ulong               ; <ulong*> [#uses=2]
>        store ulong %arg1, ulong* %x
>        %tmp.3 = cast ulong* %x to [4 x ushort]*                ; <[4 x ushort]*> [#uses=2]
>        %tmp.35 = getelementptr [4 x ushort]* %tmp.3, int 0, int 1              ; <ushort*> [#uses=1]
>        %tmp.36 = load ushort* %tmp.35          ; <ushort> [#uses=1]
> ***     %tmp.37 = seteq ushort %tmp.36, 0               ; <bool> [#uses=1]
>        %tmp.51 = getelementptr [4 x ushort]* %tmp.3, int 0, int 0              ; <ushort*> [#uses=1]
>        %tmp.52 = load ushort* %tmp.51          ; <ushort> [#uses=2]
>        %tmp.53 = seteq ushort %tmp.52, 0               ; <bool> [#uses=1]
>        br bool %tmp.53, label %join, label %then.3
>
> then.3:         ; preds = %entry
>        %tmp.59 = cast ushort %tmp.52 to int            ; <int> [#uses=1]
>        %tmp.64 = add int %tmp.59, 48           ; <int> [#uses=1]
>        br label %join
>
> join:           ; preds = %then.3, %entry
>        %joinPHI = phi int [ %tmp.64, %then.3 ], [ 64, %entry ]         ; <int> [#uses=1]
> ***     %joinPHI.tmp.48 = select bool %tmp.37, int %joinPHI, int 42             ; <int> [#uses=1]
>        ret int %joinPHI.tmp.48
> }
>
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-Chris

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