[llvm-dev] trunc nsw/nuw?

[Nuno Lopes via llvm-dev]

Hi Alexandre,

LLVM currently doesn't have trunc nsw/nuw, no.
Which frontend would emit such instructions? Any application in mind?
Just asking because if no frontend could emit those, then the motivation to 
add nsw/nuw support to trunc would be very low I guess.

Thanks,
Nuno

-----Original Message----- 
From: Alexandre Isoard via llvm-dev
Sent: Monday, July 3, 2017 8:38 PM
To: llvm-dev
Subject: [llvm-dev] trunc nsw/nuw?


Hello,

>From [1], trunc does not seems to have a nsw/nuw attribute.
Is it possible to have that? Or do we have that and it is not up-to-date?

The definition would be:

If the nuw keyword is present, the result value of the trunc is a poison 
value if the truncated high order bits are non-zero. If the nsw keyword is 
present, the result value of the trunc is a poison value if the truncated 
high order bits are not all equal to the non-truncated bit of the highest 
order.

This allow to cancel out:
- sext with trunc nsw
- zext with trunc nuw

And probably to commute with add/sub/mul/lshr/ashr/shl/urem/udiv/udiv (with 
the correct flags).

[1]: http://llvm.org/docs/LangRef.html#trunc-to-instruction


-- 

Alexandre Isoard