[llvm-dev] unsigned operations with negative numbers

[Bruce Hoult via llvm-dev]

I'm surprised llvm (or clang) is emitting that -2 without a cast. But I'm
sure it is treating it as MAXUINT-1 i.e. 0xfffffffe = 4294967294 on a 32
bit machine.


On Mon, Jul 31, 2017 at 11:56 AM, Anastasiya Ruzhanskaya via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
> I want to know, if I can always assume that when I do unsigned operations
> like
>
> udiv, urem
>
> I will get the both operands converted to unsigned values? with under
> optimized version of code I sometimes receive these lines:
>         unsigned a = 123;
>         int b = -2;
>         int c = a / b;
> -> %1 = udiv i32 123, -2
>
> and get the result 0. Will it always be zero? or is it undefined?
> Somewhere I have read that it may produce garbage.
> will it be zero in this case : %1 = udiv i32 -123, 2?
>
> However, when I set only the result as unsigned, then :
>         int a = 123;
>         int b = -2;
>         unsigned int c = a / b;
> -> %1 = sdiv i32 123, -2
> the result seems to be correct. So one operand (not the result variable)
> needs to be unsigned in order the result was unsigned too?
>
> What should I expect from sign of the operand if I get the line:
> %1 = udiv i32 %a.o, 2. Can it in this case be negative. Or this situation
> is only the result of under optimization and zero value is ok?
>
> These questions may be related mostly to c, but still, perhaps llvm has
> some additional rules that I didn't find. The general thing I wanted to
> know what to expect from sign of value when I get urem/udiv ops in final
> code?
>
> Thank you in advance for the answer.
>
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