llvm dev - Jul 2017 - 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*
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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

Hi,
> 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.
I think the clang frontend could use that to allow better static analysis of
integer overflows
on the LLVM IR.

It might be interesting for static analysis tools that want to know if you
truncate something
that is too large for the target range but you need the sign to determine, e.g.
the C/C++ frontend
could use that flag for trunc of signed/unsigned integers, then you could e.g.
check if

(uint8_t)x

changes the values if x has stuff out of the 0..255 range.

e.g. x as int with -100 => trunc nuw to 8 => bad

(int8_t)x

  => trunc nsw to 8 => ok

Greetings
Christoph
> 
> 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
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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