[LLVMbugs] [Bug 8994] New: Constant prop fails on vector code

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=8994

           Summary: Constant prop fails on vector code
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: boaz.ouriel at intel.com
                CC: llvmbugs at cs.uiuc.edu


The instruction combine pass carshes when trying to fold a bitcast from a
vector type to a scalar type.  Reproduce with:

define i48 @test(<3 x i1> %icmp1, <3 x i1> %icmp2, i16 addrspace(1)* %dst)
nounwind {
entry:
  %select1 = select <3 x i1> %icmp1, <3 x i16> zeroinitializer, <3 x i16> <i16
-1, i16 -1, i16 -1> 
  %select2 = select <3 x i1> %icmp2, <3 x i16> zeroinitializer, <3 x i16>
%select1
  %tmp2 = bitcast <3 x i16> %select2 to i48
  ret i48 %tmp2
}

here's a fix:

Index: InstructionCombining.cpp
===================================================================
--- InstructionCombining.cpp    (revision xxx)
+++ InstructionCombining.cpp    (revision xxx)
@@ -221,6 +221,12 @@
 Instruction *InstCombiner::FoldOpIntoSelect(Instruction &Op, SelectInst *SI) {
   // Don't modify shared select instructions
   if (!SI->hasOneUse()) return 0;
+  
+  // we cannot fold bitcasts which transform a vector to scalars
+  if ( BitCastInst* BI = dyn_cast<BitCastInst>(&Op) )  {
+      if ( BI->getSrcTy()->isVectorTy() ) return 0;
+  }
+
   Value *TV = SI->getOperand(1);
   Value *FV = SI->getOperand(2);

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