[GISel] Legalize bitreverse with types smaller than 8 bits (#92998)
This patch adds support for lowering `bitreverse` with types smaller
than 8 bits. It also fixes an existing assertion failure in
`llvm::APInt::getSplat`: https://godbolt.org/z/7crs8xrcG
The lowering logic is copied from SDAG:
https://github.com/llvm/llvm-project/blob/2034f2fc8729bd4645ef7caa3c5c6efa284d2d3f/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp#L9384-L9398
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index d8b0f52..9208b09 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -7977,27 +7977,51 @@
LegalizerHelper::lowerBitreverse(MachineInstr &MI) {
auto [Dst, Src] = MI.getFirst2Regs();
const LLT Ty = MRI.getType(Src);
- unsigned Size = Ty.getSizeInBits();
+ unsigned Size = Ty.getScalarSizeInBits();
- MachineInstrBuilder BSWAP =
- MIRBuilder.buildInstr(TargetOpcode::G_BSWAP, {Ty}, {Src});
+ 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 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 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)));
+ // 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) {
+ auto ShAmt = MIRBuilder.buildConstant(Ty, J - I);
+ Tmp2 = MIRBuilder.buildShl(Ty, Src, ShAmt);
+ } else {
+ auto ShAmt = MIRBuilder.buildConstant(Ty, I - J);
+ Tmp2 = MIRBuilder.buildLShr(Ty, Src, ShAmt);
+ }
+
+ auto Mask = MIRBuilder.buildConstant(Ty, 1U << J);
+ Tmp2 = MIRBuilder.buildAnd(Ty, Tmp2, Mask);
+ if (I == 0)
+ Tmp = Tmp2;
+ else
+ Tmp = MIRBuilder.buildOr(Ty, Tmp, Tmp2);
+ }
+ MIRBuilder.buildCopy(Dst, Tmp);
+ }
MI.eraseFromParent();
return Legalized;