[llvm-commits] SETCC InstructionCombining.cpp [#2/3]

[Chris Lattner]

On Dec 22, 2006, at 3:08 PM, Reid Spencer wrote:

> On Fri, 2006-12-22 at 12:08 -0800, Chris Lattner wrote:
>
>>
>> +/// Four bits are used to represent the condition, as follows:
>> +///   0  A > B
>> +///   1  A == B
>> +///   2  A < B
>> +///   3  A and B are signed
>> +///
>> +/// S<=>   Definition              S<=>   Definition
>> +/// 0000   Always false            1000   Alwasy false
>> +/// 0001   A u> B                  1001   A s> B
>> +/// 0010   A == B                  1010   A == B
>> +/// 0011   A u>= B                 1011   A s>= B
>> +/// 0100   A u< B                  1100   A s< B
>> +/// 0101   A != B                  1101   A != B
>> +/// 0110   A u<= B                 1110   A s<= B
>> +/// 0111   Always true             1111   Always true
>> +///
>> +static unsigned getICmpCode(const ICmpInst *ICI) {
>>
>>
>> This table (and it's use) is not correct.  It will miscompile:
>>
>>
>> (a <u b) | (a <s b)
>>
>>
>> You really want separate bits for s<, s>, u<, u>.  In this case,  
>> you'd
>> detect that the merged comparison can't be done with a single  
>> compare,
>> so you give up.
>
> I don't understand. There are separate bits for each of those.
>
> s< = 1100
> s> = 1001
> u< = 0100
> u> = 0001

If you or together s< and u< you get 1100, which is thought to be  
s<.  This is not valid.

You want
== -> 00001
s< -> 00010
s> -> 00100
u< -> 01000
u> -> 10000

or something

-Chris