[TargetLowering][X86] TeachSimplifyDemanedBits to handle cases where only the sign bit is demanded from a SETCC and can be passed through

[craig.topper]

Summary

If we're doing a compare that only tests the sign bit and only the sign bit is demanded, we can just bypass the node. This removes one of the blend dependencies in our v2i64->v2f32 uint_to_fp codegen on pre-sse4.2 targets.

[RKSimon]

Are you OK with the increase in register pressure? I think this is mainly due to SSE41 BLENDVPS being hard coded to using xmm0 for the selection mask.

[craig.topper]

Rebase after the uint_to_fp v4i64->v4f32 custom type legalization change D72368.

[RKSimon]

LGTM cheers

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

case ISD::XOR: {
     // If all of the demanded bits are known zero on one side, return the
     // other.
     if (DemandedBits.isSubsetOf(RHSKnown.Zero))
       return Op.getOperand(0);
     if (DemandedBits.isSubsetOf(LHSKnown.Zero))
       return Op.getOperand(1);
     break;
   }
+  case ISD::SETCC: {
+    SDValue Op0 = Op.getOperand(0);
+    SDValue Op1 = Op.getOperand(1);
+    ISD::CondCode CC = cast<CondCodeSDNode>(Op.getOperand(2))->get();
+    // If (1) we only need the sign-bit, (2) the setcc operands are the same
+    // width as the setcc result, and (3) the result of a setcc conforms to 0 or
+    // -1, we may be able to bypass the setcc.
+    if (DemandedBits.isSignMask() &&
+        Op0.getScalarValueSizeInBits() == DemandedBits.getBitWidth() &&
+        getBooleanContents(Op0.getValueType()) ==
+            BooleanContent::ZeroOrNegativeOneBooleanContent) {
+      // If we're testing X < 0, then this compare isn't needed - just use X!
+      // FIXME: We're limiting to integer types here, but this should also work
+      // if we don't care about FP signed-zero. The use of SETLT with FP means
+      // that we don't care about NaNs.
+      if (CC == ISD::SETLT && Op1.getValueType().isInteger() &&
+          (isNullConstant(Op1) || ISD::isBuildVectorAllZeros(Op1.getNode())))
+        return Op0;
+    }
+    break;
+  }
   case ISD::SIGN_EXTEND_INREG: {
     // If none of the extended bits are demanded, eliminate the sextinreg.
     EVT ExVT = cast<VTSDNode>(Op.getOperand(1))->getVT();
     if (DemandedBits.getActiveBits() <= ExVT.getScalarSizeInBits())
       return Op.getOperand(0);
     break;
   }
   case ISD::INSERT_VECTOR_ELT: {

llvm/test/CodeGen/X86/vec_int_to_fp.ll

 ; SSE2-NEXT:    xorps %xmm1, %xmm1
 ; SSE2-NEXT:    cvtsi2ss %rax, %xmm1
 ; SSE2-NEXT:    addss %xmm1, %xmm1
 ; SSE2-NEXT:    unpcklps {{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
 ; SSE2-NEXT:    retq
 ;
 ; SSE41-LABEL: uitofp_2i64_to_4f32:
 ; SSE41:       # %bb.0:
-; SSE41-NEXT:    movdqa %xmm0, %xmm1
-; SSE41-NEXT:    pxor %xmm0, %xmm0
-; SSE41-NEXT:    pcmpgtd %xmm1, %xmm0
-; SSE41-NEXT:    movdqa {{.*#+}} xmm2 = [1,1]
-; SSE41-NEXT:    pand %xmm1, %xmm2
-; SSE41-NEXT:    movdqa %xmm1, %xmm3
-; SSE41-NEXT:    psrlq $1, %xmm3
-; SSE41-NEXT:    por %xmm2, %xmm3
-; SSE41-NEXT:    blendvpd %xmm0, %xmm3, %xmm1
-; SSE41-NEXT:    pextrq $1, %xmm1, %rax
-; SSE41-NEXT:    xorps %xmm2, %xmm2
-; SSE41-NEXT:    cvtsi2ss %rax, %xmm2
-; SSE41-NEXT:    movq %xmm1, %rax
+; SSE41-NEXT:    movdqa {{.*#+}} xmm1 = [1,1]
+; SSE41-NEXT:    pand %xmm0, %xmm1
+; SSE41-NEXT:    movdqa %xmm0, %xmm2
+; SSE41-NEXT:    pxor %xmm3, %xmm3
+; SSE41-NEXT:    pcmpgtd %xmm0, %xmm3
+; SSE41-NEXT:    movdqa %xmm0, %xmm4
+; SSE41-NEXT:    psrlq $1, %xmm4
+; SSE41-NEXT:    por %xmm1, %xmm4
+; SSE41-NEXT:    blendvpd %xmm0, %xmm4, %xmm2
+; SSE41-NEXT:    pextrq $1, %xmm2, %rax
+; SSE41-NEXT:    xorps %xmm0, %xmm0
+; SSE41-NEXT:    cvtsi2ss %rax, %xmm0
+; SSE41-NEXT:    movq %xmm2, %rax
 ; SSE41-NEXT:    xorps %xmm1, %xmm1
 ; SSE41-NEXT:    cvtsi2ss %rax, %xmm1
-; SSE41-NEXT:    insertps {{.*#+}} xmm1 = xmm1[0],xmm2[0],zero,zero
+; SSE41-NEXT:    insertps {{.*#+}} xmm1 = xmm1[0],xmm0[0],zero,zero
 ; SSE41-NEXT:    movaps %xmm1, %xmm2
 ; SSE41-NEXT:    addps %xmm1, %xmm2
-; SSE41-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[1,3,2,3]
+; SSE41-NEXT:    pshufd {{.*#+}} xmm0 = xmm3[1,3,2,3]
 ; SSE41-NEXT:    blendvps %xmm0, %xmm2, %xmm1
 ; SSE41-NEXT:    movaps %xmm1, %xmm0
 ; SSE41-NEXT:    retq
 ;
 ; VEX-LABEL: uitofp_2i64_to_4f32:
 ; VEX:       # %bb.0:
 ; VEX-NEXT:    vpand {{.*}}(%rip), %xmm0, %xmm1
 ; VEX-NEXT:    vpsrlq $1, %xmm0, %xmm2

 ; SSE2-NEXT:    addss %xmm0, %xmm0
 ; SSE2-NEXT:  .LBB42_6:
 ; SSE2-NEXT:    unpcklps {{.*#+}} xmm0 = xmm0[0],xmm1[0],xmm0[1],xmm1[1]
 ; SSE2-NEXT:    movq {{.*#+}} xmm0 = xmm0[0],zero
 ; SSE2-NEXT:    retq
 ;
 ; SSE41-LABEL: uitofp_2i64_to_2f32:
 ; SSE41:       # %bb.0:
-; SSE41-NEXT:    movdqa %xmm0, %xmm1
-; SSE41-NEXT:    pxor %xmm0, %xmm0
-; SSE41-NEXT:    pcmpgtd %xmm1, %xmm0
-; SSE41-NEXT:    movdqa {{.*#+}} xmm2 = [1,1]
-; SSE41-NEXT:    pand %xmm1, %xmm2
-; SSE41-NEXT:    movdqa %xmm1, %xmm3
-; SSE41-NEXT:    psrlq $1, %xmm3
-; SSE41-NEXT:    por %xmm2, %xmm3
-; SSE41-NEXT:    blendvpd %xmm0, %xmm3, %xmm1
-; SSE41-NEXT:    pextrq $1, %xmm1, %rax
-; SSE41-NEXT:    xorps %xmm2, %xmm2
-; SSE41-NEXT:    cvtsi2ss %rax, %xmm2
-; SSE41-NEXT:    movq %xmm1, %rax
+; SSE41-NEXT:    movdqa {{.*#+}} xmm1 = [1,1]
+; SSE41-NEXT:    pand %xmm0, %xmm1
+; SSE41-NEXT:    movdqa %xmm0, %xmm2
+; SSE41-NEXT:    pxor %xmm3, %xmm3
+; SSE41-NEXT:    pcmpgtd %xmm0, %xmm3
+; SSE41-NEXT:    movdqa %xmm0, %xmm4
+; SSE41-NEXT:    psrlq $1, %xmm4
+; SSE41-NEXT:    por %xmm1, %xmm4
+; SSE41-NEXT:    blendvpd %xmm0, %xmm4, %xmm2
+; SSE41-NEXT:    pextrq $1, %xmm2, %rax
+; SSE41-NEXT:    xorps %xmm0, %xmm0
+; SSE41-NEXT:    cvtsi2ss %rax, %xmm0
+; SSE41-NEXT:    movq %xmm2, %rax
 ; SSE41-NEXT:    xorps %xmm1, %xmm1
 ; SSE41-NEXT:    cvtsi2ss %rax, %xmm1
-; SSE41-NEXT:    insertps {{.*#+}} xmm1 = xmm1[0],xmm2[0],zero,zero
+; SSE41-NEXT:    insertps {{.*#+}} xmm1 = xmm1[0],xmm0[0],zero,zero
 ; SSE41-NEXT:    movaps %xmm1, %xmm2
 ; SSE41-NEXT:    addps %xmm1, %xmm2
-; SSE41-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[1,3,2,3]
+; SSE41-NEXT:    pshufd {{.*#+}} xmm0 = xmm3[1,3,2,3]
 ; SSE41-NEXT:    blendvps %xmm0, %xmm2, %xmm1
 ; SSE41-NEXT:    movq {{.*#+}} xmm0 = xmm1[0],zero
 ; SSE41-NEXT:    retq
 ;
 ; VEX-LABEL: uitofp_2i64_to_2f32:
 ; VEX:       # %bb.0:
 ; VEX-NEXT:    vpand {{.*}}(%rip), %xmm0, %xmm1
 ; VEX-NEXT:    vpsrlq $1, %xmm0, %xmm2

 ; SSE2-NEXT:    addss %xmm0, %xmm0
 ; SSE2-NEXT:  .LBB43_6:
 ; SSE2-NEXT:    unpcklps {{.*#+}} xmm1 = xmm1[0],xmm0[0],xmm1[1],xmm0[1]
 ; SSE2-NEXT:    movq {{.*#+}} xmm0 = xmm1[0],zero
 ; SSE2-NEXT:    retq
 ;
 ; SSE41-LABEL: uitofp_4i64_to_4f32_undef:
 ; SSE41:       # %bb.0:
-; SSE41-NEXT:    movdqa %xmm0, %xmm1
-; SSE41-NEXT:    pxor %xmm0, %xmm0
-; SSE41-NEXT:    pcmpgtd %xmm1, %xmm0
-; SSE41-NEXT:    movdqa {{.*#+}} xmm2 = [1,1]
-; SSE41-NEXT:    pand %xmm1, %xmm2
-; SSE41-NEXT:    movdqa %xmm1, %xmm3
-; SSE41-NEXT:    psrlq $1, %xmm3
-; SSE41-NEXT:    por %xmm2, %xmm3
-; SSE41-NEXT:    blendvpd %xmm0, %xmm3, %xmm1
-; SSE41-NEXT:    pextrq $1, %xmm1, %rax
+; SSE41-NEXT:    movdqa {{.*#+}} xmm1 = [1,1]
+; SSE41-NEXT:    pand %xmm0, %xmm1
+; SSE41-NEXT:    movdqa %xmm0, %xmm2
+; SSE41-NEXT:    psrlq $1, %xmm2
+; SSE41-NEXT:    por %xmm1, %xmm2
+; SSE41-NEXT:    pshufd {{.*#+}} xmm1 = xmm0[1,3,2,3]
+; SSE41-NEXT:    blendvpd %xmm0, %xmm2, %xmm0
+; SSE41-NEXT:    pextrq $1, %xmm0, %rax
+; SSE41-NEXT:    cvtsi2ss %rax, %xmm3
+; SSE41-NEXT:    movq %xmm0, %rax
 ; SSE41-NEXT:    xorps %xmm2, %xmm2
 ; SSE41-NEXT:    cvtsi2ss %rax, %xmm2
-; SSE41-NEXT:    movq %xmm1, %rax
-; SSE41-NEXT:    xorps %xmm1, %xmm1
-; SSE41-NEXT:    cvtsi2ss %rax, %xmm1
-; SSE41-NEXT:    insertps {{.*#+}} xmm1 = xmm1[0],xmm2[0],zero,zero
-; SSE41-NEXT:    movaps %xmm1, %xmm2
-; SSE41-NEXT:    addps %xmm1, %xmm2
-; SSE41-NEXT:    pshufd {{.*#+}} xmm0 = xmm0[1,3,2,3]
-; SSE41-NEXT:    blendvps %xmm0, %xmm2, %xmm1
-; SSE41-NEXT:    movaps %xmm1, %xmm0
+; SSE41-NEXT:    insertps {{.*#+}} xmm2 = xmm2[0],xmm3[0],zero,zero
+; SSE41-NEXT:    movaps %xmm2, %xmm3
+; SSE41-NEXT:    addps %xmm2, %xmm3
+; SSE41-NEXT:    movdqa %xmm1, %xmm0
+; SSE41-NEXT:    blendvps %xmm0, %xmm3, %xmm2
+; SSE41-NEXT:    movaps %xmm2, %xmm0
 ; SSE41-NEXT:    retq
 ;
 ; AVX1-LABEL: uitofp_4i64_to_4f32_undef:
 ; AVX1:       # %bb.0:
 ; AVX1-NEXT:    # kill: def $xmm0 killed $xmm0 def $ymm0
 ; AVX1-NEXT:    vandps {{.*}}(%rip), %ymm0, %ymm1
 ; AVX1-NEXT:    vpsrlq $1, %xmm0, %xmm2
 ; AVX1-NEXT:    vorps %ymm1, %ymm2, %ymm1