[LLVMbugs] [Bug 10054] New: LLVM should transform bit test branch into select for better backend codegen

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=10054

           Summary: LLVM should transform bit test branch into select for
                    better backend codegen
           Product: new-bugs
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: david.majnemer at gmail.com
                CC: llvmbugs at cs.uiuc.edu


take the following for example:
unsigned sb_4(unsigned x, unsigned y) {
        if ((x & y) == 0) {
                x &= ~y;
        }
        return x;
}

this ends up turning into:
define i32 @sb_4(i32 %x, i32 %y) nounwind uwtable readnone ssp {
entry:
  %and = and i32 %y, %x
  %cmp = icmp eq i32 %and, 0
  br i1 %cmp, label %if.then, label %if.end

if.then:                                          ; preds = %entry
  %neg = xor i32 %y, -1
  %and4 = and i32 %neg, %x
  br label %if.end

if.end:                                           ; preds = %if.then, %entry
  %x.addr.0 = phi i32 [ %and4, %if.then ], [ %x, %entry ]
  ret i32 %x.addr.0
}

however, I think it should turn into:
define i32 @sb_5(i32 %x, i32 %y) nounwind uwtable readnone ssp {
entry:
  %and = and i32 %x, %y
  %neg = xor i32 %y, -1
  %and2 = and i32 %x, %neg
  %cmp = icmp eq i32 %and, 0
  %and2.x = select i1 %cmp, i32 %and2, i32 %x
  ret i32 %and2.x
}

on x86-64 this results in nicer codegen because there is no branch, just a cmov

-- 
Configure bugmail: http://llvm.org/bugs/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.