llvm dev - Jun 2020 - Why doesn't this `and` get eliminated

define dso_local i32 @f(i32 %0) {
  %2 = and i32 %0, 7
  %3 = icmp eq i32 %2, 7
  %4 = zext i1 %3 to i32
  ret i32 %4
}

I thought instcombine would remove it. It doesn't and nothing else does
either. LLVM Version is 10.0.0.

/Riyaz
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200612/43c0ad0a/attachment-0001.html>

I don't think its legal to remove it. That code is equivalent to asking (%0
mod 8) == 7. Which is true for 7 and 15 and 23, etc. You can't just remove
the mod part of that or it would just return true for %0 == 7.

~Craig


On Thu, Jun 11, 2020 at 6:31 PM Riyaz Puthiyapurayil via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> define dso_local i32 @f(i32 %0) {
>
>   %2 = and i32 %0, 7
>
>   %3 = icmp eq i32 %2, 7
>
>   %4 = zext i1 %3 to i32
>
>   ret i32 %4
>
> }
>
>
>
> I thought instcombine would remove it. It doesn’t and nothing else does
> either. LLVM Version is 10.0.0.
>
>
>
> /Riyaz
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200611/22199736/attachment.html>

alive2 agrees with Craig:

----------------------------------------
define i32 @src(i32 %0) {
%1:
   %2 = and i32 %0, 7
   %3 = icmp eq i32 %2, 7
   %4 = zext i1 %3 to i32
   ret i32 %4
}
=>
define i32 @tgt(i32 %0) {
%1:
   %x3 = icmp eq i32 %0, 7
   %x4 = zext i1 %x3 to i32
   ret i32 %x4
}
Transformation doesn't verify!
ERROR: Value mismatch

Example:
i32 %0 = #x0000000f (15)

Source:
i32 %2 = #x00000007 (7)
i1 %3 = #x1 (1)
i32 %4 = #x00000001 (1)

Target:
i1 %x3 = #x0 (0)
i32 %x4 = #x00000000 (0)
Source value: #x00000001 (1)
Target value: #x00000000 (0)

Oops! My bad! The code I intended to write was:
define dso_local i32 @f(i3 %0) {
    %2 = and i3 %0, 7
  %3 = icmp eq i3 %2, 7
  %4 = zext i1 %3 to i32
  ret i32 %4
}

The `and` does get removed in this case as expected. Thanks for answering the
dumb question. 😊

/Riyaz

From: Craig Topper <craig.topper at gmail.com>
Sent: Thursday, June 11, 2020 6:57 PM
To: Riyaz Puthiyapurayil <riyaz at synopsys.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Why doesn't this `and` get eliminated

I don't think its legal to remove it. That code is equivalent to asking (%0
mod 8) == 7. Which is true for 7 and 15 and 23, etc. You can't just remove
the mod part of that or it would just return true for %0 == 7.

~Craig


On Thu, Jun 11, 2020 at 6:31 PM Riyaz Puthiyapurayil via llvm-dev <llvm-dev
at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
define dso_local i32 @f(i32 %0) {
  %2 = and i32 %0, 7
  %3 = icmp eq i32 %2, 7
  %4 = zext i1 %3 to i32
  ret i32 %4
}

I thought instcombine would remove it. It doesn’t and nothing else does either.
LLVM Version is 10.0.0.

/Riyaz
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev__;!!A4F2R9G_pg!JPHHxK08PLHdcVuEWI6SHbEK3lkgLp9Y9oRN7k0Y23Pa82isuL-auNioBqQUN_JsRYx5NrpuPTlv$>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20200617/db0fcd21/attachment.html>