[llvm-bugs] [Bug 33165] New: NewGVN miscompiles select undef

via llvm-bugs llvm-bugs at lists.llvm.org
Thu May 25 02:13:43 PDT 2017


            Bug ID: 33165
           Summary: NewGVN miscompiles select undef
           Product: libraries
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Keywords: miscompilation
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: nunoplopes at sapo.pt
                CC: davide at freebsd.org, dberlin at dberlin.org,
                    llvm-bugs at lists.llvm.org, regehr at cs.utah.edu

$ cat a.ll
define i2 @f(i2, i1) {
  %a = xor i2 %0, -1
  %b = select i1 %1, i2 %a, i2 undef
  %c = and i2 %a, %b
  ret i2 %c

$ opt -S a.ll -newgvn
define i2 @f(i2, i1) {
  %a = xor i2 %0, -1
  %b = select i1 %1, i2 %a, i2 undef
  ret i2 %b

This is incorrect. Alive report:

ERROR: Mismatch in values of i2 ret_

%0 i2 = 0x2 (2, -2)
%1 i1 = 0x0 (0)
%a i2 = 0x1 (1)
%b i2 = 0x0 (0)
%c i2 = 0x0 (0)
Source value: 0x0 (0)
Target value: 0x2 (2, -2)


GVN gets this right (http://rise4fun.com/Alive/n6D):
$ opt -S a.ll -gvn
define i2 @f(i2, i1) {
  %a = xor i2 %0, -1
  ret i2 %a

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