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

[Duncan Sands]

Hi Chris,

> Icmp sgt is correct.

while ugt would be wrong, I think sgt is too!

For example, suppose %buf is 0 and %bufLen is ~0U.  Then %add.ptr is ~0U, and
%cmp is true, so control branches to %if.then.  However in the optimized version
%cmp is false and control branches to %if.end.

The GEP does have an inbounds attribute, I'm not sure if that is relevant here.

Ciao, Duncan.

   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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