[LLVMbugs] [Bug 1598] New: miscompilation of if(expr){ do{...}while(expr2) }

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=1598

           Summary: miscompilation of if(expr){ do{...}while(expr2) }
           Product: libraries
           Version: trunk
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: major
          Priority: P2
         Component: Loop Optimizer
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: nicholas at mxc.ca
                CC: llvmbugs at cs.uiuc.edu


Consider this C program:

  int f(int a, int b, int x, int y) {
    if (a != b) {
      do {
        ++x;
      } while (x < y);
    }
    return x;
  }

Currently, this generates:

define i32 @f(i32 %a, i32 %b, i32 %x, i32 %y) {
entry:
        %tmp3 = icmp eq i32 %a, %b
        %retval = select i1 %tmp3, i32 %x, i32 %y
        ret i32 %retval
}

which is pretty seriously funny. The bug in this case is ScalarEvolution's
HowManyLessThans search for a dominating comparison. If the comparison is for
EQ or NE then it doesn't bother to check the arguments at all.

Although the search for a dominating comparison is a good fallback, this
function should probably be changed to check whether the initial iteration of
the loop matches the condition or not using AddRec::getStart and such.


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