[llvm] [GISel] Legalize bitreverse with types smaller than 8 bits (PR #92998)

[Matt Arsenault via llvm-commits]

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@@ -7975,27 +7975,50 @@ LegalizerHelper::LegalizeResult
 LegalizerHelper::lowerBitreverse(MachineInstr &MI) {
   auto [Dst, Src] = MI.getFirst2Regs();
   const LLT Ty = MRI.getType(Src);
-  unsigned Size = Ty.getSizeInBits();
+  unsigned Size = Ty.getScalarSizeInBits();
+
+  if (Size >= 8) {
+    MachineInstrBuilder BSWAP =
+        MIRBuilder.buildInstr(TargetOpcode::G_BSWAP, {Ty}, {Src});
+
+    // swap high and low 4 bits in 8 bit blocks 7654|3210 -> 3210|7654
+    //    [(val & 0xF0F0F0F0) >> 4] | [(val & 0x0F0F0F0F) << 4]
+    // -> [(val & 0xF0F0F0F0) >> 4] | [(val << 4) & 0xF0F0F0F0]
+    MachineInstrBuilder Swap4 =
+        SwapN(4, Ty, MIRBuilder, BSWAP, APInt::getSplat(Size, APInt(8, 0xF0)));
+
+    // swap high and low 2 bits in 4 bit blocks 32|10 76|54 -> 10|32 54|76
+    //    [(val & 0xCCCCCCCC) >> 2] & [(val & 0x33333333) << 2]
+    // -> [(val & 0xCCCCCCCC) >> 2] & [(val << 2) & 0xCCCCCCCC]
+    MachineInstrBuilder Swap2 =
+        SwapN(2, Ty, MIRBuilder, Swap4, APInt::getSplat(Size, APInt(8, 0xCC)));
+
+    // swap high and low 1 bit in 2 bit blocks 1|0 3|2 5|4 7|6 -> 0|1 2|3 4|5
+    // 6|7
+    //    [(val & 0xAAAAAAAA) >> 1] & [(val & 0x55555555) << 1]
+    // -> [(val & 0xAAAAAAAA) >> 1] & [(val << 1) & 0xAAAAAAAA]
+    SwapN(1, Dst, MIRBuilder, Swap2, APInt::getSplat(Size, APInt(8, 0xAA)));
+  } else {
+    // Expand bitreverse for types smaller than 8 bits.
+    MachineInstrBuilder Tmp;
+    for (unsigned I = 0, J = Size - 1; I < Size; ++I, --J) {
+      MachineInstrBuilder Tmp2;
+      if (I < J)
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arsenm wrote:

Braces, and split buildConstant to temp variable? The line break is pretty ugly 

https://github.com/llvm/llvm-project/pull/92998