[LLVMbugs] [Bug 8866] New: Scev does not get max if hidden by sext. Canonicalization problem?

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http://llvm.org/bugs/show_bug.cgi?id=8866

           Summary: Scev does not get max if hidden by sext.
                    Canonicalization problem?
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: grosser at fim.uni-passau.de
                CC: llvmbugs at cs.uiuc.edu


Hi,

I just tried to "opt -scalar-evolution -analyze" on this:

define i64 @make_positive(i64 %a) nounwind readnone {
entry:
  %is_a_nonnegative = icmp slt i64 %a, 0
  %max = select i1 %is_a_nonnegative, i64 0, i64 %a
  ret i64 %max
}

  %max = select i1 %is_a_nonnegative, i64 0, i64 %a
  -->  (0 smax %a)

This one works fine and gives the right expression. As soon as a sext
obfuscates
this expression, scalar evolution does not work any more:

define i64 @make_positive_sextend(i32 %a) nounwind readnone {
entry:
  %a_ext = sext i32 %a to i64
  %is_a_nonnegative = icmp sgt i32 %a, -1
  %max = select i1 %is_a_nonnegative, i64 %a_ext, i64 0
  ret i64 %max
}

  %a_ext = sext i32 %a to i64
  -->  (sext i32 %a to i64)
  %max = select i1 %is_a_nonnegative, i64 %a_ext, i64 0
  -->  %max

Any change this can be solved in scev - or maybe even better - canonicalized
such that svec works unchanged. I see those two options:

  %a_ext = sext i32 %a to i64
  %is_a_nonnegative = icmp sgt i64 %a_ext, -1
  %max = select i1 %is_a_nonnegative, i64 %a_ext, i64 0
  ret i64 %max

  %is_a_nonnegative = icmp sgt i32 %a, -1
  %max = select i1 %is_a_nonnegative, i32 %a, i32 0
  %max_sext = sext i32 %max to i64
  ret i64 %max_sext

This also blocks some further optimizations, as the non obfuscated code is 
optimized to a comparison to 0:

  %is_a_nonnegative = icmp slt i64 %a, 0
  %max = select i1 %is_a_nonnegative, i64 0, i64 %a
  ret i64 %max

Running -O3 on the test file does not change the obfuscated code at all.

Cheers
Tobi

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