[llvm-bugs] [Bug 42618] New: CVP: adding nuw flags not correct in the presence of undef

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42618

            Bug ID: 42618
           Summary: CVP: adding nuw flags not correct in the presence of
                    undef
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: nunoplopes at sapo.pt
                CC: jdoerfert at anl.gov, llvm-bugs at lists.llvm.org,
                    me+llvm at luqman.ca, nikita.ppv at gmail.com,
                    regehr at cs.utah.edu

The following unit test shows incorrect behavior regarding undef inputs:
$ opt -correlated-propagation -cvp-dont-add-nowrap-flags=false
Transforms/CorrelatedValuePropagation/sub.ll

The issue is the branch on undef, which if we assume is a non-deterministic
jump, leads to branch %then, which then does a 'sub nuw' on an undef (which is
poison).

Possible solutions: same as in #42617.


define i32 @test11(* %p, i32 %i) {
  %limit = load i32, * %p, align 4
  %range_l#0%limit = icmp sge i32 %limit, 0
  %range_h#0%limit = icmp slt i32 %limit, 2147483647
  %range#0%limit = and i1 %range_l#0%limit, %range_h#0%limit
  assume_non_poison i1 %range#0%limit
  %within.1 = icmp slt i32 %limit, %i
  %i.minus.7 = add i32 %i, 4294967289
  %within.2 = icmp slt i32 %limit, %i.minus.7
  %within = and i1 %within.1, %within.2
  br i1 %within, label %then, label %else

%then:
  %i.minus.6 = sub i32 %i, 6
  ret i32 %i.minus.6

%else:
  ret i32 0
}
=>
define i32 @test11(* %p, i32 %i) {
  %limit = load i32, * %p, align 4
  %range_l#0%limit = icmp sge i32 %limit, 0
  %range_h#0%limit = icmp slt i32 %limit, 2147483647
  %range#0%limit = and i1 %range_l#0%limit, %range_h#0%limit
  assume_non_poison i1 %range#0%limit
  %within.1 = icmp slt i32 %limit, %i
  %i.minus.7 = add i32 %i, 4294967289
  %within.2 = icmp slt i32 %limit, %i.minus.7
  %within = and i1 %within.1, %within.2
  br i1 %within, label %then, label %else

%then:
  %i.minus.6 = sub nsw nuw i32 %i, 6
  ret i32 %i.minus.6

%else:
  ret i32 0
}
Transformation doesn't verify!
ERROR: Target is more poisonous than source

Example:
* %p = #x2dc920000000
i32 %i = undef

Source:
i32 %limit = #x00000000 (0)
i1 %range_l#0%limit = #x1 (1)
i1 %range_h#0%limit = #x1 (1)
i1 %range#0%limit = #x1 (1)
i1 %within.1 = undef
i32 %i.minus.7 = undef
i1 %within.2 = undef
i1 %within = undef
i32 %i.minus.6 = undef

Target:
i32 %limit = #x00000000 (0)
i1 %range_l#0%limit = #x1 (1)
i1 %range_h#0%limit = #x1 (1)
i1 %range#0%limit = #x1 (1)
i1 %within.1 = undef
i32 %i.minus.7 = undef
i1 %within.2 = undef
i1 %within = undef
i32 %i.minus.6 = poison
Source value: undef
Target value: poison

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