[X86] Use packusdw+vpmovuswb to implement v16i32->V16i8 that clamps signed inputs to be between 0 and 255 when zmm registers are disabled on SKX.

If we've disable zmm registers, the v16i32 will need to be split. This split will propagate through min/max the truncate. This creates two sequences that need to be concatenated back to v16i8. We can instead use packusdw to do part of the clamping, truncating, and concatenating all at once. Then we can use a vpmovuswb to finish off the clamp.

Differential Revision: https://reviews.llvm.org/D68763

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374431 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86ISelLowering.cpp

@@ -39841,6 +39841,21 @@ static SDValue combineTruncateWithSat(SDValue In, EVT VT, const SDLoc &DL,
     if (auto USatVal = detectUSatPattern(In, VT, DAG, DL))
       return DAG.getNode(X86ISD::VTRUNCUS, DL, VT, USatVal);
   }
+
+  // If we're clamping a signed 32-bit vector to 0-255 and the 32-bit vector is
+  // split across two registers. We can use a packusdw+perm to clamp to 0-65535
+  // and concatenate at the same time. Then we can use a final vpmovuswb to
+  // clip to 0-255.
+  if (Subtarget.hasBWI() && !Subtarget.useAVX512Regs() &&
+      InVT == MVT::v16i32 && VT == MVT::v16i8) {
+    if (auto USatVal = detectSSatPattern(In, VT, true)) {
+      // Emit a VPACKUSDW+VPERMQ followed by a VPMOVUSWB.
+      SDValue Mid = truncateVectorWithPACK(X86ISD::PACKUS, MVT::v16i16, USatVal,
+                                           DL, DAG, Subtarget);
+      return DAG.getNode(X86ISD::VTRUNCUS, DL, VT, Mid);
+    }
+  }
+
   if (VT.isVector() && isPowerOf2_32(VT.getVectorNumElements()) &&
       !(Subtarget.hasAVX512() && InSVT == MVT::i32) &&
       !(Subtarget.hasBWI() && InSVT == MVT::i16) &&

test/CodeGen/X86/min-legal-vector-width.ll

@@ -1083,12 +1083,10 @@ define void @vselect_split_v16i16_setcc(<16 x i16> %s, <16 x i16> %t, <16 x i32>
 define <16 x i8> @trunc_packus_v16i32_v16i8(<16 x i32>* %p, <16 x i8>* %q) "min-legal-vector-width"="256" {
 ; CHECK-LABEL: trunc_packus_v16i32_v16i8:
 ; CHECK:       # %bb.0:
-; CHECK-NEXT:    vpxor %xmm0, %xmm0, %xmm0
-; CHECK-NEXT:    vpmaxsd 32(%rdi), %ymm0, %ymm1
-; CHECK-NEXT:    vpmovusdb %ymm1, %xmm1
-; CHECK-NEXT:    vpmaxsd (%rdi), %ymm0, %ymm0
-; CHECK-NEXT:    vpmovusdb %ymm0, %xmm0
-; CHECK-NEXT:    vpunpcklqdq {{.*#+}} xmm0 = xmm0[0],xmm1[0]
+; CHECK-NEXT:    vmovdqa (%rdi), %ymm0
+; CHECK-NEXT:    vpackusdw 32(%rdi), %ymm0, %ymm0
+; CHECK-NEXT:    vpermq {{.*#+}} ymm0 = ymm0[0,2,1,3]
+; CHECK-NEXT:    vpmovuswb %ymm0, %xmm0
 ; CHECK-NEXT:    vzeroupper
 ; CHECK-NEXT:    retq
   %a = load <16 x i32>, <16 x i32>* %p