[LLVMdev] Question about nsw and nuw flags

[Douglas do Couto Teixeira]

Hi everybody,

I saw that there are some instructions in the programs in which the flag nsw or nuw is placed. I wonder what rules does LLVM follows to put these flags in some instructions.


Best,

Douglas

[Chris Lattner]

On Jan 12, 2011, at 11:31 AM, Douglas do Couto Teixeira wrote:

> Hi everybody,
>
> I saw that there are some instructions in the programs in which the flag nsw or nuw is placed. I wonder what rules does LLVM follows to put these flags in some instructions.

Hi Douglas,

The canonical reference for these types of questions is the IR description, here: http://llvm.org/docs/LangRef.html

-Chris
_______________________________________________
LLVM Developers mailing list
LLV...@cs.uiuc.edu http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

[Brian West]

Douglas do Couto Teixeira <douglasdocouto <at> gmail.com> writes:
> Hi everybody,I saw that there are some instructions in the programs in which

the flag nsw or nuw is placed. I wonder what rules does LLVM follows to put
these flags in some instructions.Best,Douglas

Language requirements appears to drive their placement. In C, "signed int"
overflow produces a trap value and undefined behavior. Thus, C "signed int"
operations do not wrap on overflow and get tagged with "nsw" in the LLVM IR.

Brian West