[LLVMdev] "icmp sgt" when it should be "ugt" ?

[Chris Lattner]

Icmp sgt is correct.  Note that "ugt x, 0" is the same as "x != 0" which is not what you want.

-Chris

On Aug 1, 2011, at 9:11 AM, Jonas Gefele <llvm.org at schrieb.de> wrote:

> Hello,
> 
> while writing a new LLVM backend I have observed that in some cases the
> optimizer produces an "icmp sgt i32 %a, 0" where I would have expected an
> "icmp ugt i32 %a, 0".
> 
> For example when I feed "opt -O3 -S ..."  (LLVM 2.9, Windows) with
> 
> 
> ------------------------------------------------------------------------
> target datalayout = "E-p:32:32:32"
> 
> define void @foo(i8* %buf, i32 %bufLen, i32* %ret) nounwind {
> entry:
>  %add.ptr = getelementptr inbounds i8* %buf, i32 %bufLen
>  %cmp = icmp ult i8* %buf, %add.ptr
>  br i1 %cmp, label %if.then, label %if.end
> 
> if.then:                                          ; preds = %entry
>  store i32 0, i32* %ret
>  br label %if.end
> 
> if.end:                                           ; preds = %if.then, %entry
>  ret void
> }
> ------------------------------------------------------------------------
> 
> 
> I get as output
> 
> 
> ------------------------------------------------------------------------
> ...
> entry:
>  %cmp = icmp sgt i32 %bufLen, 0
>  br i1 %cmp, label %if.then, label %if.end
> ...
> ------------------------------------------------------------------------
> 
> 
> This is no problem if one thinks of "icmp sgt %a, 0" as "compute %a - 0
> and set flags accordingly" because then signedness makes no difference.
> But if I understand things correctly, it is a problem according to the
> LLVM IR reference which states "sgt: interprets the operands as signed
> values and yields true if op1 is greater than op2". Under this definition
> and for %a=0xFFFFFFFF "ugt" would return true while "sgt" would return
> false, wouldn't it?
> 
> Is this a bug in my thinking, a bug in the reference manual, or is this a
> bug in some pass which introduces the "sgt"?
> 
> 
> Cheers,
> Jonas
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