[LLVMbugs] [Bug 1941] New: instcombine doesn't merge mixed sign-unsign comparisons

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Mon Jan 21 23:01:40 PST 2008


           Summary: instcombine doesn't merge mixed sign-unsign comparisons
           Product: libraries
           Version: trunk
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nicholas at mxc.ca
                CC: llvmbugs at cs.uiuc.edu

Instcombine will merge comparisons like (x >= 10) && (x < 20) by producing (x -
10) u< 10, but only when the comparisons have matching sign.

This program:

define i1 @test(i8 %x) {
  %A = icmp uge i8 %x, 5
  %B = icmp slt i8 %x, 20
  %C = and i1 %A, %B
  ret i1 %C

could be converted using the same technique. There's a lot of corner cases here
that would need to be thought through thoroughly.

(This was found by inspection, I doubt if any llvm-gcc generated input would
trigger this.)

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