[LLVMbugs] [Bug 5276] New: instcombine: missed boolean simplification opportunity

bugzilla-daemon at cs.uiuc.edu bugzilla-daemon at cs.uiuc.edu
Thu Oct 22 09:32:53 PDT 2009


           Summary: instcombine: missed boolean simplification opportunity
           Product: new-bugs
           Version: unspecified
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: new bugs
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: edwintorok at gmail.com
                CC: llvmbugs at cs.uiuc.edu

The code below is not simplified by instcombine:
target datalayout =
target triple = "x86_64-unknown-linux-gnu"

define i32 @foo(i32 %a, i32 %b, i32 %c) nounwind {
  %tobool = icmp eq i32 %a, 0                     ; <i1> [#uses=1]
  %mul.mask = and i32 %b, 2147483647              ; <i32> [#uses=1]
  %tobool2 = icmp eq i32 %mul.mask, 0             ; <i1> [#uses=1]
  %tobool5 = icmp eq i32 %c, 0                    ; <i1> [#uses=1]
  %or.cond = or i1 %tobool2, %tobool5             ; <i1> [#uses=1]
  %0 = xor i1 %or.cond, true                      ; <i1> [#uses=1]
  %1 = and i1 %tobool, %0                         ; <i1> [#uses=1]
  br i1 %1, label %UnifiedReturnBlock, label %if.then

if.then:                                          ; preds = %entry
  %call = tail call i32 (...)* @bar() nounwind    ; <i32> [#uses=0]
  ret i32 5

UnifiedReturnBlock:                               ; preds = %entry
  ret i32 4

However and (A, xor (or (B, C), 1)) can be transformed to
and(A, and (!B, !C)), and calculating !B and !C is easy, one just needs to flip
the predicate in the icmp.

So instcombine could see if xor(or (A,B), 1) can be transformed into and(!A,
by flipping predicates in ICMP.

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