[X86] Fix a bug in the lowering of the mask of VSELECT.

X86 code to lower VSELECT messed a bit with the bits set in the mask of VSELECT
when it knows it can be lowered into BLEND. Indeed, only the high bits need to be
set for those and it optimizes those accordingly.
However, when the mask is a compile time constant, the lowering will be handled
by the generic optimizer and those modifications will generate bad code in the
generic optimizer.

This patch fixes that by preventing the optimization if the VSELECT will be
handled by the generic optimizer.

<rdar://problem/18675020>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220242 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 33 insertions, 1 deletions

diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index a8afe816d902..543a2fdc99f0 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -22598,7 +22598,12 @@ static SDValue PerformSELECTCombine(SDNode *N, SelectionDAG &DAG,
     TargetLowering::TargetLoweringOpt TLO(DAG, DCI.isBeforeLegalize(),
                                           DCI.isBeforeLegalizeOps());
     if (TLO.ShrinkDemandedConstant(Cond, DemandedMask) ||
-        TLI.SimplifyDemandedBits(Cond, DemandedMask, KnownZero, KnownOne, TLO))
+        (TLI.SimplifyDemandedBits(Cond, DemandedMask, KnownZero, KnownOne,
+                                  TLO) &&
+         // Don't optimize vector of constants. Those are handled by
+         // the generic code and all the bits must be properly set for
+         // the generic optimizer.
+         !ISD::isBuildVectorOfConstantSDNodes(TLO.New.getNode())))
       DCI.CommitTargetLoweringOpt(TLO);
   }
 
diff --git a/test/CodeGen/X86/vselect-avx.ll b/test/CodeGen/X86/vselect-avx.ll
new file mode 100644
index 000000000000..2d7ccf39d387
--- /dev/null
+++ b/test/CodeGen/X86/vselect-avx.ll
@@ -0,0 +1,27 @@
+; RUN: llc %s -o - -mattr=+avx | FileCheck %s
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx"
+
+; For this test we used to optimize the <i1 true, i1 false, i1 false, i1 true>
+; mask into <i32 2147483648, i32 0, i32 0, i32 2147483648> because we thought
+; we would lower that into a blend where only the high bit is relevant.
+; However, since the whole mask is constant, this is simplified incorrectly
+; by the generic code, because it was expecting -1 in place of 2147483648.
+; 
+; The problem does not occur without AVX, because vselect of v4i32 is not legal
+; nor custom.
+;
+; <rdar://problem/18675020>
+
+; CHECK-LABEL: test:
+; CHECK: vmovdqa {{.*#+}} xmm1 = [65533,124,125,14807]
+; CHECK: vmovdqa {{.*#+}} xmm1 = [65535,0,0,65535]
+; CHECK: ret
+define void @test(<4 x i16>* %a, <4 x i16>* %b) {
+body:
+  %predphi = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>, <4 x i16> <i16 -3, i16 545, i16 4385, i16 14807>, <4 x i16> <i16 123, i16 124, i16 125, i16 127>
+  %predphi42 = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>, <4 x i16> <i16 -1, i16 -1, i16 -1, i16 -1>, <4 x i16> zeroinitializer
+  store <4 x i16> %predphi, <4 x i16>* %a, align 8
+  store <4 x i16> %predphi42, <4 x i16>* %b, align 8
+  ret void
+}