[LLVMdev] Do we abuse the "nsw" flag

[Shuxin Yang]

It works like a charm! Thanks a lot!

Shuxin

On 5/6/13 4:59 PM, Cameron Zwarich wrote:
> This has come up before, and we just added -fwrapv to work around the problem:
>
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20110131/115969.html
>
> Are you compiling without -fwrapv?
>
> Cameron
>
> On May 6, 2013, at 4:55 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>
>> Hi, There:
>>
>>    Clang fails to compile 254.gap @ CPU2000int suite.  The symptom is that executable fail to run
>> with reference input.
>>
>>    The root cause is that the compiler mistakenly optimizes expr "x * y / y" into x where the x*y is blindly
>> flagged with nsw without any analysis.
>>
>>     The preproceeded code is excerpted bellow:
>>
>> cat -n integer.i
>> ---------------------------------
>> 2361 TypHandle ProdInt ( hdL, hdR )
>> 2362     TypHandle hdL, hdR;
>> 2363 {
>> ....
>> 2373     if ( (int)hdL & (int)hdR & 1 ) {
>> 2374
>> 2375
>> 2376         i = ((int)hdL - 1) * ((int)hdR >> 1);  /* !!! No barbaric NSW, please !!! */
>> 2377         if ( ((int)hdR >> 1) == 0 || i / ((int)hdR >> 1) == ((int)hdL-1) ) {
>> ---------------------------------
>>
>>    I catch the case where the multiplication overflow: the 1st and 2nd operand hold the
>> value 0x4000 and 0x20000, respectively.
>>
>>    Dose gap source code has some "undefined" behavior according to some spec
>> on this planet? Or it is compiler's fault for blindly mark any mul NSW?
>>
>> Thanks
>> Shuxin
>>
>>
>>
>>
>>
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