[PATCH] Revert 221429 and fix bug in TargetLowering::SimplifyDemandedBits

[Gil Rapaport]

This patch modifies the SIGN_EXTEND_INREG case of SimplifyDemandedBits by:

- Fixing the bug (by using the multi-use guarded) NewMask instead of DemandedMask.
- Avoiding simplifying to shl when the sext-in-reg is known to already be sign-extended, as we can expect it to be removed altogether (by DAGCombiner::visitSIGN_EXTEND_INREG()).


http://reviews.llvm.org/D6949

Files:
  lib/CodeGen/SelectionDAG/TargetLowering.cpp
  test/CodeGen/X86/vector-blend.ll
  test/CodeGen/X86/vselect-avx.ll

Index: lib/CodeGen/SelectionDAG/TargetLowering.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -793,19 +793,26 @@
 
     APInt MsbMask = APInt::getHighBitsSet(BitWidth, 1);
     // If we only care about the highest bit, don't bother shifting right.
-    if (MsbMask == DemandedMask) {
+    if (MsbMask == NewMask) {
       unsigned ShAmt = ExVT.getScalarType().getSizeInBits();
       SDValue InOp = Op.getOperand(0);
+      unsigned VTBits = Op->getValueType(0).getScalarType().getSizeInBits();
+      bool alreadySignExtended =
+        TLO.DAG.ComputeNumSignBits(InOp) >= VTBits-ShAmt+1;
+      // However if the input is already sign extended we expect the sign
+      // extension to be dropped altogether later and do not simplify.
+      if (!alreadySignExtended) {
+        // Compute the correct shift amount type, which must be getShiftAmountTy
+        // for scalar types after legalization.
+        EVT ShiftAmtTy = Op.getValueType();
+        if (TLO.LegalTypes() && !ShiftAmtTy.isVector())
+          ShiftAmtTy = getShiftAmountTy(ShiftAmtTy);
 
-      // Compute the correct shift amount type, which must be getShiftAmountTy
-      // for scalar types after legalization.
-      EVT ShiftAmtTy = Op.getValueType();
-      if (TLO.LegalTypes() && !ShiftAmtTy.isVector())
-        ShiftAmtTy = getShiftAmountTy(ShiftAmtTy);
-
-      SDValue ShiftAmt = TLO.DAG.getConstant(BitWidth - ShAmt, ShiftAmtTy);
-      return TLO.CombineTo(Op, TLO.DAG.getNode(ISD::SHL, dl,
-                                            Op.getValueType(), InOp, ShiftAmt));
+        SDValue ShiftAmt = TLO.DAG.getConstant(BitWidth - ShAmt, ShiftAmtTy);
+        return TLO.CombineTo(Op, TLO.DAG.getNode(ISD::SHL, dl,
+                                                 Op.getValueType(), InOp,
+                                                 ShiftAmt));
+      }
     }
 
     // Sign extension.  Compute the demanded bits in the result that are not
Index: test/CodeGen/X86/vector-blend.ll
===================================================================
--- test/CodeGen/X86/vector-blend.ll
+++ test/CodeGen/X86/vector-blend.ll
@@ -419,8 +419,8 @@
 ;
 ; SSE41-LABEL: vsel_i648:
 ; SSE41:       # BB#0: # %entry
-; SSE41-NEXT:    pblendw {{.*#+}} xmm0 = xmm0[0,1,2,3],xmm4[4,5,6,7]
-; SSE41-NEXT:    pblendw {{.*#+}} xmm2 = xmm2[0,1,2,3],xmm6[4,5,6,7]
+; SSE41-NEXT:    blendpd {{.*#+}} xmm0 = xmm0[0],xmm4[1]
+; SSE41-NEXT:    blendpd {{.*#+}} xmm2 = xmm2[0],xmm6[1]
 ; SSE41-NEXT:    movaps %xmm5, %xmm1
 ; SSE41-NEXT:    movaps %xmm7, %xmm3
 ; SSE41-NEXT:    retq
Index: test/CodeGen/X86/vselect-avx.ll
===================================================================
--- test/CodeGen/X86/vselect-avx.ll
+++ test/CodeGen/X86/vselect-avx.ll
@@ -59,19 +59,15 @@
 ; 
 ; <rdar://problem/18819506>
 
-; Note: For now, hard code ORIG_MASK and SHRUNK_MASK registers, because we
-; cannot express that ORIG_MASK must not be equal to ORIG_MASK. Otherwise,
-; even a faulty pattern would pass!
-;  
 ; CHECK-LABEL: test3:
-; Compute the original mask.
-;	CHECK: vpcmpeqd {{%xmm[0-9]+}}, {{%xmm[0-9]+}}, [[ORIG_MASK:%xmm0]]
-; Shrink the bit of the mask.
-; CHECK-NEXT: vpslld	$31, [[ORIG_MASK]], [[SHRUNK_MASK:%xmm3]]
-; Use the shrunk mask in the blend.
-; CHECK-NEXT:	vblendvps	[[SHRUNK_MASK]], %xmm{{[0-9]+}}, %xmm{{[0-9]+}}, %xmm{{[0-9]+}}
-; Use the original mask in the and.
-; CHECK-NEXT: vpand LCPI2_2(%rip), [[ORIG_MASK]], {{%xmm[0-9]+}} 
+; Compute the mask.
+;	CHECK: vpcmpeqd {{%xmm[0-9]+}}, {{%xmm[0-9]+}}, [[MASK:%xmm[0-9]+]]
+; Do not shrink the bit of the mask.
+; CHECK-NOT: vpslld	$31, [[MASK]], {{%xmm[0-9]+}}
+; Use the mask in the blend.
+; CHECK-NEXT:	vblendvps	[[MASK]], %xmm{{[0-9]+}}, %xmm{{[0-9]+}}, %xmm{{[0-9]+}}
+; Use the mask in the and.
+; CHECK-NEXT: vpand LCPI2_2(%rip), [[MASK]], {{%xmm[0-9]+}} 
 ; CHECK: retq
 define void @test3(<4 x i32> %induction30, <4 x i16>* %tmp16, <4 x i16>* %tmp17,  <4 x i16> %tmp3, <4 x i16> %tmp12) {
   %tmp6 = srem <4 x i32> %induction30, <i32 3, i32 3, i32 3, i32 3>

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