[PATCH] D112095: [x86] add special-case lowering for usubsat for pre-SSE4

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: RKSimon, pengfei, craig.topper.
Herald added subscribers: hiraditya, mcrosier.
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usubsat X, SMIN --> (X ^ SMIN) & (X s>> BW-1)

This would be a regression with D112085 <https://reviews.llvm.org/D112085> where we combine to usubsat more aggressively, so avoid that by matching the special-case where we are subtracting SMIN (signmask):
https://alive2.llvm.org/ce/z/4_3gBD


https://reviews.llvm.org/D112095

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
@@ -130,11 +130,9 @@
 define <4 x i32> @usubsat_custom(<4 x i32> %x) nounwind {
 ; SSE2OR3-LABEL: usubsat_custom:
 ; SSE2OR3:       # %bb.0:
-; SSE2OR3-NEXT:    movdqa {{.*#+}} xmm1 = [2147483648,2147483648,2147483648,2147483648]
-; SSE2OR3-NEXT:    pxor %xmm0, %xmm1
-; SSE2OR3-NEXT:    pxor %xmm2, %xmm2
-; SSE2OR3-NEXT:    pcmpgtd %xmm2, %xmm1
-; SSE2OR3-NEXT:    psubd {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm0
+; SSE2OR3-NEXT:    movdqa %xmm0, %xmm1
+; SSE2OR3-NEXT:    psrad $31, %xmm1
+; SSE2OR3-NEXT:    pxor {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm0
 ; SSE2OR3-NEXT:    pand %xmm1, %xmm0
 ; SSE2OR3-NEXT:    retq
 ;
Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -28135,7 +28135,19 @@
   EVT SetCCResultType =
       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);
@@ -28148,7 +28160,6 @@
 
   if ((Opcode == ISD::SADDSAT || Opcode == ISD::SSUBSAT) &&
       (!VT.isVector() || VT == MVT::v2i64)) {
-    unsigned BitWidth = VT.getScalarSizeInBits();
     APInt MinVal = APInt::getSignedMinValue(BitWidth);
     APInt MaxVal = APInt::getSignedMaxValue(BitWidth);
     SDValue Zero = DAG.getConstant(0, DL, VT);


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