Hi Tianyou,

I am not enough of a physicist to know what “the qualification of the ground 
state energy in a quantum system” means, but this does not sound like a Clang 
bug to me. You did not say whether `a` in your example is a float or a double, 
but the minimum legal values for these types are the constants FLT_MIN and 
DBL_MIN. There is no need to try to compute them. Exceeding the range of values 
representable in a type — [FLT_MIN, FLT_MAX] or [DBL_MIN, DBL_MAX] — is 
undefined behavior, so the compiler is free to emit anything.

Thanks,
Matt

> On May 26, 2020, at 20:31, Tianyou via cfe-users <cfe-users@lists.llvm.org> 
> wrote:
> 
> Hello everyone,
> I found some questionable behaviors in the clang compiler.
> I tried to calculate the smallest number in my machine with clang. I did it 
> by looping as: 
> a = a / 1.73
> In g++, after 1358 loops, I found that a = 4.94066E-324, and the value of a 
> never changes from loop 1358 till the end 1500 loop.
> It is questionable for me that a non zero real number here a = 4.94066E-324 
> divided by a non zero and non one real number here 1.73 will keep the same 
> value. The phenomenon seems to the qualification of the ground state energy 
> in a quantum system.
> Is this bug for the clang compiler?
> I tested my code with, g++ (Apple LLVM version 10.0.0, clang-1000.10.44.4) on 
> Macintosh. I also tried a = a / 1.7, a = a / 1.77. 
> All have similar results. A ground state qualification and unchanged value of 
> a. 
> I guess that it is related to the binary system used by the computer system. 
> Here with my codes. 
> 
> Best wishes
> Tianyou Yi
> Postdoc researcher in National Tsing Hua University
> 
> Best wishes
> Tianyou Yi 
> Please send me to 
> yitianyo...@qq.com
> yitianyou9...@gzemail.cn
> 
> 
>  
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