[SelectionDAG] ComputeNumSignBits add getValidMaximumShiftAmountConstant() for ISD::SHL support

Allows us to handle non-uniform SHL shifts to determine the minimum number of sign bits remaining (based off the maximum shift amount value)

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2449,6 +2449,32 @@ getValidMinimumShiftAmountConstant(SDValue V, const APInt &DemandedElts) {
   return MinShAmt;
 }
 
+/// If a SHL/SRA/SRL node has constant vector shift amounts that are all less
+/// than the element bit-width of the shift node, return the maximum value.
+static const APInt *
+getValidMaximumShiftAmountConstant(SDValue V, const APInt &DemandedElts) {
+  unsigned BitWidth = V.getScalarValueSizeInBits();
+  auto *BV = dyn_cast<BuildVectorSDNode>(V.getOperand(1));
+  if (!BV)
+    return nullptr;
+  const APInt *MaxShAmt = nullptr;
+  for (unsigned i = 0, e = BV->getNumOperands(); i != e; ++i) {
+    if (!DemandedElts[i])
+      continue;
+    auto *SA = dyn_cast<ConstantSDNode>(BV->getOperand(i));
+    if (!SA)
+      return nullptr;
+    // Shifting more than the bitwidth is not valid.
+    const APInt &ShAmt = SA->getAPIntValue();
+    if (ShAmt.uge(BitWidth))
+      return nullptr;
+    if (MaxShAmt && MaxShAmt->uge(ShAmt))
+      continue;
+    MaxShAmt = &ShAmt;
+  }
+  return MaxShAmt;
+}
+
 /// Determine which bits of Op are known to be either zero or one and return
 /// them in Known. For vectors, the known bits are those that are shared by
 /// every vector element.
@@ -3621,6 +3647,11 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
       Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1);
       if (ShAmt->ult(Tmp))
         return Tmp - ShAmt->getZExtValue();
+    } else if (const APInt *ShAmt =
+                   getValidMaximumShiftAmountConstant(Op, DemandedElts)) {
+      Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1);
+      if (ShAmt->ult(Tmp))
+        return Tmp - ShAmt->getZExtValue();
     }
     break;
   case ISD::AND:

llvm/test/CodeGen/X86/known-signbits-vector.ll

@@ -257,45 +257,30 @@ define <4 x double> @signbits_sext_shuffle_sitofp(<4 x i32> %a0, <4 x i64> %a1)
 define <2 x double> @signbits_sext_shl_sitofp(<2 x i16> %a0) nounwind {
 ; X86-LABEL: signbits_sext_shl_sitofp:
 ; X86:       # %bb.0:
-; X86-NEXT:    pushl %ebp
-; X86-NEXT:    movl %esp, %ebp
-; X86-NEXT:    andl $-8, %esp
-; X86-NEXT:    subl $32, %esp
 ; X86-NEXT:    vpmovsxwq %xmm0, %xmm0
 ; X86-NEXT:    vpsllq $5, %xmm0, %xmm1
 ; X86-NEXT:    vpsllq $11, %xmm0, %xmm0
-; X86-NEXT:    vmovq %xmm0, {{[0-9]+}}(%esp)
-; X86-NEXT:    vpshufd {{.*#+}} xmm0 = xmm1[2,3,0,1]
-; X86-NEXT:    vmovq %xmm0, {{[0-9]+}}(%esp)
-; X86-NEXT:    fildll {{[0-9]+}}(%esp)
-; X86-NEXT:    fstpl {{[0-9]+}}(%esp)
-; X86-NEXT:    fildll {{[0-9]+}}(%esp)
-; X86-NEXT:    fstpl (%esp)
-; X86-NEXT:    vmovsd {{.*#+}} xmm0 = mem[0],zero
-; X86-NEXT:    vmovhps {{.*#+}} xmm0 = xmm0[0,1],mem[0,1]
-; X86-NEXT:    movl %ebp, %esp
-; X86-NEXT:    popl %ebp
+; X86-NEXT:    vpblendw {{.*#+}} xmm0 = xmm0[0,1,2,3],xmm1[4,5,6,7]
+; X86-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,2,2,3]
+; X86-NEXT:    vcvtdq2pd %xmm0, %xmm0
 ; X86-NEXT:    retl
 ;
 ; X64-AVX1-LABEL: signbits_sext_shl_sitofp:
 ; X64-AVX1:       # %bb.0:
 ; X64-AVX1-NEXT:    vpmovsxwq %xmm0, %xmm0
-; X64-AVX1-NEXT:    vpsllq $11, %xmm0, %xmm1
-; X64-AVX1-NEXT:    vpsllq $5, %xmm0, %xmm0
-; X64-AVX1-NEXT:    vpextrq $1, %xmm0, %rax
-; X64-AVX1-NEXT:    vcvtsi2sd %eax, %xmm2, %xmm0
-; X64-AVX1-NEXT:    vcvtdq2pd %xmm1, %xmm1
-; X64-AVX1-NEXT:    vunpcklpd {{.*#+}} xmm0 = xmm1[0],xmm0[0]
+; X64-AVX1-NEXT:    vpsllq $5, %xmm0, %xmm1
+; X64-AVX1-NEXT:    vpsllq $11, %xmm0, %xmm0
+; X64-AVX1-NEXT:    vpblendw {{.*#+}} xmm0 = xmm0[0,1,2,3],xmm1[4,5,6,7]
+; X64-AVX1-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,2,2,3]
+; X64-AVX1-NEXT:    vcvtdq2pd %xmm0, %xmm0
 ; X64-AVX1-NEXT:    retq
 ;
 ; X64-AVX2-LABEL: signbits_sext_shl_sitofp:
 ; X64-AVX2:       # %bb.0:
 ; X64-AVX2-NEXT:    vpmovsxwq %xmm0, %xmm0
 ; X64-AVX2-NEXT:    vpsllvq {{.*}}(%rip), %xmm0, %xmm0
-; X64-AVX2-NEXT:    vpextrq $1, %xmm0, %rax
-; X64-AVX2-NEXT:    vcvtsi2sd %eax, %xmm1, %xmm1
+; X64-AVX2-NEXT:    vpshufd {{.*#+}} xmm0 = xmm0[0,2,2,3]
 ; X64-AVX2-NEXT:    vcvtdq2pd %xmm0, %xmm0
-; X64-AVX2-NEXT:    vunpcklpd {{.*#+}} xmm0 = xmm0[0],xmm1[0]
 ; X64-AVX2-NEXT:    retq
   %1 = sext <2 x i16> %a0 to <2 x i64>
   %2 = shl <2 x i64> %1, <i64 11, i64 5>