[llvm-bugs] [Bug 45160] New: [InstCombine] Missed @llvm.usub.sat opportunity

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45160

            Bug ID: 45160
           Summary: [InstCombine] Missed @llvm.usub.sat opportunity
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: lebedev.ri at gmail.com
                CC: llvm-bugs at lists.llvm.org

Given https://godbolt.org/z/gGcybp

using size_type = unsigned;

size_type t0(size_type pos, size_type size) {
        size_type bytesRemaining = (pos < size) ? size - pos : 0;
    return ((bytesRemaining > 4)
                       ? (size - pos - 4)
                       : 0);
}

we currently produce 

define dso_local i32 @_Z2t0jj(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = tail call i32 @llvm.usub.sat.i32(i32 %1, i32 %0)
  %4 = icmp ugt i32 %3, 4
  %5 = sub i32 -4, %0
  %6 = add i32 %5, %1
  %7 = select i1 %4, i32 %6, i32 0
  ret i32 %7
}

declare i32 @llvm.usub.sat.i32(i32, i32) #1

but we should instead produce

define dso_local i32 @_Z6squarejj(i32 %0, i32 %1) local_unnamed_addr #0 {
  %3 = tail call i32 @llvm.usub.sat.i32(i32 %1, i32 %0)
  %4 = tail call i32 @llvm.usub.sat.i32(i32 %3, i32 4)
  ret i32 %4
}
declare i32 @llvm.usub.sat.i32(i32, i32) #1



----------------------------------------
define i8 @_Z6squarejj(i8 %0, i8 %1) {
%2:
  %3 = usub_sat i8 %1, %0
  %4 = icmp ugt i8 %3, 4
  %5 = sub i8 252, %0
  %6 = add i8 %5, %1
  %7 = select i1 %4, i8 %6, i8 0
  ret i8 %7
}
=>
define i8 @_Z6squarejj(i8 %0, i8 %1) {
%2:
  %3 = usub_sat i8 %1, %0
  %4 = usub_sat i8 %3, 4
  ret i8 %4
}
Transformation seems to be correct!

Summary:
  1 correct transformations
  0 incorrect transformations
  0 Alive2 errors

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