[InstSimplify] fold icmp based on range of abs/nabs (2nd try)

This is retrying the fold from rL345717 
(reverted at rL347780)
...with a fix for the miscompile
demonstrated by PR39510:
https://bugs.llvm.org/show_bug.cgi?id=39510

Original commit message:

This is a fix for PR39475:
https://bugs.llvm.org/show_bug.cgi?id=39475

We managed to get some of these patterns using computeKnownBits in https://reviews.llvm.org/D47041, but that
can't be used for nabs(). Instead, put in some range-based logic, so we can fold
both abs/nabs with icmp with a constant value.

Alive proofs:
https://rise4fun.com/Alive/21r

Name: abs_nsw_is_positive

  %cmp = icmp slt i32 %x, 0
  %negx = sub nsw i32 0, %x
  %abs = select i1 %cmp, i32 %negx, i32 %x
  %r = icmp sgt i32 %abs, -1
    =>
  %r = i1 true


Name: abs_nsw_is_not_negative

  %cmp = icmp slt i32 %x, 0
  %negx = sub nsw i32 0, %x
  %abs = select i1 %cmp, i32 %negx, i32 %x
  %r = icmp slt i32 %abs, 0
    =>
  %r = i1 false


Name: nabs_is_negative_or_0

  %cmp = icmp slt i32 %x, 0
  %negx = sub i32 0, %x
  %nabs = select i1 %cmp, i32 %x, i32 %negx
  %r = icmp slt i32 %nabs, 1
    =>
  %r = i1 true

Name: nabs_is_not_over_0

  %cmp = icmp slt i32 %x, 0
  %negx = sub i32 0, %x
  %nabs = select i1 %cmp, i32 %x, i32 %negx
  %r = icmp sgt i32 %nabs, 0
    =>
  %r = i1 false

Differential Revision: https://reviews.llvm.org/D53844

llvm-svn: 345832

0 files changed, 0 insertions, 0 deletions