[PATCH] D112138: [x86] add special-case lowering for usubsat for AVX512

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: RKSimon, pengfei, craig.topper.
Herald added subscribers: hiraditya, mcrosier.
spatel requested review of this revision.
Herald added a project: LLVM.

This is a small extension of D112095 <https://reviews.llvm.org/D112095> to avoid another regression seen with D112085 <https://reviews.llvm.org/D112085>.
In this case, we allow the same conversion from usubsat to ALU ops if the target supports vpternlog.

That pattern will get converted later in X86DAGToDAGISel::tryVPTERNLOG(). This seems better than putting a magic immediate constant directly in this code to create the exact vpternlog that we need. It's possible that there are other special-cases along these lines, so we should try to keep all of the vpternlog magic in one place.


https://reviews.llvm.org/D112138

Files:
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/X86/psubus.ll


Index: llvm/test/CodeGen/X86/psubus.ll
===================================================================
--- llvm/test/CodeGen/X86/psubus.ll
+++ llvm/test/CodeGen/X86/psubus.ll
@@ -162,9 +162,8 @@
 ;
 ; AVX512-LABEL: usubsat_custom:
 ; AVX512:       # %bb.0:
-; AVX512-NEXT:    vpbroadcastd {{.*#+}} xmm1 = [2147483648,2147483648,2147483648,2147483648]
-; AVX512-NEXT:    vpmaxud %xmm1, %xmm0, %xmm0
-; AVX512-NEXT:    vpsubd %xmm1, %xmm0, %xmm0
+; AVX512-NEXT:    vpsrad $31, %xmm0, %xmm1
+; AVX512-NEXT:    vpternlogd $72, {{\.?LCPI[0-9]+_[0-9]+}}(%rip){1to4}, %xmm1, %xmm0
 ; AVX512-NEXT:    retq
   %res = call <4 x i32> @llvm.usub.sat.v4i32(<4 x i32> %x, <4 x i32> <i32 2147483648, i32 2147483648, i32 2147483648, i32 undef>)
   ret <4 x i32> %res
Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -28139,26 +28139,32 @@
       TLI.getSetCCResultType(DAG.getDataLayout(), *DAG.getContext(), VT);
 
   unsigned BitWidth = VT.getScalarSizeInBits();
-  if (Opcode == ISD::USUBSAT && !TLI.isOperationLegal(ISD::UMAX, VT)) {
-    // Handle a special-case with a bit-hack instead of cmp+select:
-    // usubsat X, SMIN --> (X ^ SMIN) & (X s>> BW-1)
-    ConstantSDNode *C = isConstOrConstSplat(Y, true);
-    if (C && C->getAPIntValue().isSignMask()) {
-      SDValue SignMask = DAG.getConstant(C->getAPIntValue(), DL, VT);
-      SDValue ShiftAmt = DAG.getConstant(BitWidth - 1, DL, VT);
-      SDValue Xor = DAG.getNode(ISD::XOR, DL, VT, X, SignMask);
-      SDValue Sra = DAG.getNode(ISD::SRA, DL, VT, X, ShiftAmt);
-      return DAG.getNode(ISD::AND, DL, VT, Xor, Sra);
-    }
-
-    // usubsat X, Y --> (X >u Y) ? X - Y : 0
-    SDValue Sub = DAG.getNode(ISD::SUB, DL, VT, X, Y);
-    SDValue Cmp = DAG.getSetCC(DL, SetCCResultType, X, Y, ISD::SETUGT);
-    // TODO: Move this to DAGCombiner?
-    if (SetCCResultType == VT &&
-        DAG.ComputeNumSignBits(Cmp) == VT.getScalarSizeInBits())
-      return DAG.getNode(ISD::AND, DL, VT, Cmp, Sub);
-    return DAG.getSelect(DL, VT, Cmp, Sub, DAG.getConstant(0, DL, VT));
+  if (Opcode == ISD::USUBSAT) {
+    if (!TLI.isOperationLegal(ISD::UMAX, VT) || useVPTERNLOG(Subtarget, VT)) {
+      // Handle a special-case with a bit-hack instead of cmp+select:
+      // usubsat X, SMIN --> (X ^ SMIN) & (X s>> BW-1)
+      // If the target can use VPTERNLOG, DAGToDAG will match this as
+      // "vpsra + vpternlog" which is better than "vpmax + vpsub" with a
+      // "broadcast" constant load.
+      ConstantSDNode *C = isConstOrConstSplat(Y, true);
+      if (C && C->getAPIntValue().isSignMask()) {
+        SDValue SignMask = DAG.getConstant(C->getAPIntValue(), DL, VT);
+        SDValue ShiftAmt = DAG.getConstant(BitWidth - 1, DL, VT);
+        SDValue Xor = DAG.getNode(ISD::XOR, DL, VT, X, SignMask);
+        SDValue Sra = DAG.getNode(ISD::SRA, DL, VT, X, ShiftAmt);
+        return DAG.getNode(ISD::AND, DL, VT, Xor, Sra);
+      }
+    }
+    if (!TLI.isOperationLegal(ISD::UMAX, VT)) {
+      // usubsat X, Y --> (X >u Y) ? X - Y : 0
+      SDValue Sub = DAG.getNode(ISD::SUB, DL, VT, X, Y);
+      SDValue Cmp = DAG.getSetCC(DL, SetCCResultType, X, Y, ISD::SETUGT);
+      // TODO: Move this to DAGCombiner?
+      if (SetCCResultType == VT &&
+          DAG.ComputeNumSignBits(Cmp) == VT.getScalarSizeInBits())
+        return DAG.getNode(ISD::AND, DL, VT, Cmp, Sub);
+      return DAG.getSelect(DL, VT, Cmp, Sub, DAG.getConstant(0, DL, VT));
+    }
   }
 
   if ((Opcode == ISD::SADDSAT || Opcode == ISD::SSUBSAT) &&


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