[llvm] r249390 - [InstCombine] Teach SimplifyDemandedVectorElts how to handle ConstantVector select masks with ConstantExpr elements (PR24922)

[Andrea Di Biagio via llvm-commits]

Author: adibiagio
Date: Tue Oct  6 05:34:53 2015
New Revision: 249390

URL: http://llvm.org/viewvc/llvm-project?rev=249390&view=rev
Log:
[InstCombine] Teach SimplifyDemandedVectorElts how to handle ConstantVector select masks with ConstantExpr elements (PR24922)

If the mask of a select instruction is a ConstantVector, method
SimplifyDemandedVectorElts iterates over the mask elements to identify which
values are selected from the select inputs.

Before this patch, method SimplifyDemandedVectorElts always used method
Constant::isNullValue() to check if a value in the mask was zero. Unfortunately
that method always returns false when called on a ConstantExpr.

This patch fixes the problem in SimplifyDemandedVectorElts by adding an explicit
check for ConstantExpr values. Now, if a value in the mask is a ConstantExpr, we
avoid calling isNullValue() on it.

Fixes PR24922.

Differential Revision: http://reviews.llvm.org/D13219

Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
    llvm/trunk/test/Transforms/InstCombine/vec_demanded_elts.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp?rev=249390&r1=249389&r2=249390&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp Tue Oct  6 05:34:53 2015
@@ -1057,7 +1057,13 @@ Value *InstCombiner::SimplifyDemandedVec
     APInt LeftDemanded(DemandedElts), RightDemanded(DemandedElts);
     if (ConstantVector* CV = dyn_cast<ConstantVector>(I->getOperand(0))) {
       for (unsigned i = 0; i < VWidth; i++) {
-        if (CV->getAggregateElement(i)->isNullValue())
+        Constant *CElt = CV->getAggregateElement(i);
+        // Method isNullValue always returns false when called on a
+        // ConstantExpr. If CElt is a ConstantExpr then skip it in order to
+        // to avoid propagating incorrect information.
+        if (isa<ConstantExpr>(CElt))
+          continue;
+        if (CElt->isNullValue())
           LeftDemanded.clearBit(i);
         else
           RightDemanded.clearBit(i);

Modified: llvm/trunk/test/Transforms/InstCombine/vec_demanded_elts.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/vec_demanded_elts.ll?rev=249390&r1=249389&r2=249390&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/vec_demanded_elts.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/vec_demanded_elts.ll Tue Oct  6 05:34:53 2015
@@ -253,3 +253,16 @@ define <4 x double> @test_vpermilvar_pd_
   %a = tail call <4 x double> @llvm.x86.avx.vpermilvar.pd.256(<4 x double> %v, <4 x i64> zeroinitializer)
   ret <4 x double> %a
 }
+
+define <2 x i64> @PR24922(<2 x i64> %v) {
+; CHECK-LABEL: @PR24922
+; CHECK: select <2 x i1> 
+;
+; Check that instcombine doesn't wrongly fold the select statement into a
+; ret <2 x i64> %v
+;
+; FIXME: We should be able to simplify the ConstantExpr in the select mask.
+entry:
+  %result = select <2 x i1> <i1 icmp eq (i64 extractelement (<2 x i64> bitcast (<4 x i32> <i32 15, i32 15, i32 15, i32 15> to <2 x i64>), i64 0), i64 0), i1 true>, <2 x i64> %v, <2 x i64> zeroinitializer
+  ret <2 x i64> %result
+}