[GlobalISel] Add computeNumSignBits for G_BUILD_VECTOR. (llvm#139506)

The code is similar to SelectionDAG::ComputeNumSignBits, but does not
deal with truncating buildvectors.

Author: davemgreen

llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp

@@ -836,6 +836,24 @@ unsigned GISelValueTracking::computeNumSignBits(Register R,
       return TyBits - 1; // Every always-zero bit is a sign bit.
     break;
   }
+  case TargetOpcode::G_BUILD_VECTOR: {
+    // Collect the known bits that are shared by every demanded vector element.
+    FirstAnswer = TyBits;
+    APInt SingleDemandedElt(1, 1);
+    for (unsigned i = 0, e = MI.getNumOperands() - 1; i < e; ++i) {
+      if (!DemandedElts[i])
+        continue;
+
+      unsigned Tmp2 = computeNumSignBits(MI.getOperand(i + 1).getReg(),
+                                         SingleDemandedElt, Depth + 1);
+      FirstAnswer = std::min(FirstAnswer, Tmp2);
+
+      // If we don't know any bits, early out.
+      if (FirstAnswer == 1)
+        break;
+    }
+    break;
+  }
   case TargetOpcode::G_INTRINSIC:
   case TargetOpcode::G_INTRINSIC_W_SIDE_EFFECTS:
   case TargetOpcode::G_INTRINSIC_CONVERGENT:

llvm/test/CodeGen/AArch64/aarch64-dup-ext.ll

@@ -312,15 +312,14 @@ define <4 x i32> @nonsplat_shuffleinsert2(<4 x i16> %b, i16 %b0, i16 %b1, i16 %b
 ; CHECK-GI-LABEL: nonsplat_shuffleinsert2:
 ; CHECK-GI:       // %bb.0: // %entry
 ; CHECK-GI-NEXT:    sxth w8, w0
-; CHECK-GI-NEXT:    sshll v0.4s, v0.4h, #0
-; CHECK-GI-NEXT:    mov v1.s[0], w8
-; CHECK-GI-NEXT:    sxth w8, w1
-; CHECK-GI-NEXT:    mov v1.s[1], w8
+; CHECK-GI-NEXT:    sxth w9, w1
+; CHECK-GI-NEXT:    fmov s1, w8
 ; CHECK-GI-NEXT:    sxth w8, w2
-; CHECK-GI-NEXT:    mov v1.s[2], w8
+; CHECK-GI-NEXT:    mov v1.h[1], w9
+; CHECK-GI-NEXT:    mov v1.h[2], w8
 ; CHECK-GI-NEXT:    sxth w8, w3
-; CHECK-GI-NEXT:    mov v1.s[3], w8
-; CHECK-GI-NEXT:    mul v0.4s, v1.4s, v0.4s
+; CHECK-GI-NEXT:    mov v1.h[3], w8
+; CHECK-GI-NEXT:    smull v0.4s, v1.4h, v0.4h
 ; CHECK-GI-NEXT:    ret
 entry:
   %s0 = sext i16 %b0 to i32

llvm/unittests/CodeGen/GlobalISel/KnownBitsVectorTest.cpp

@@ -692,7 +692,7 @@ TEST_F(AArch64GISelMITest, TestVectorNumSignBitsConstant) {
   EXPECT_EQ(2u, Info.computeNumSignBits(CopyReg32));
   EXPECT_EQ(3u, Info.computeNumSignBits(CopyRegNeg32));
   EXPECT_EQ(3u, Info.computeNumSignBits(NonSplatSameSign));
-  EXPECT_EQ(1u, Info.computeNumSignBits(NonSplatDifferentSign));
+  EXPECT_EQ(2u, Info.computeNumSignBits(NonSplatDifferentSign));
 }
 
 TEST_F(AArch64GISelMITest, TestVectorNumSignBitsSext) {