machinst x64: lower Ctz using the Bsf x86 instruction

cranelift/codegen/src/isa/x64/inst/args.rs

@@ -293,12 +293,15 @@ impl ToString for AluRmiROpcode {
 pub enum ReadOnlyGprRmROpcode {
     /// Bit-scan reverse.
     Bsr,
+    /// Bit-scan forward.
+    Bsf,
 }
 
 impl fmt::Debug for ReadOnlyGprRmROpcode {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         match self {
             ReadOnlyGprRmROpcode::Bsr => write!(fmt, "bsr"),
+            ReadOnlyGprRmROpcode::Bsf => write!(fmt, "bsf"),
         }
     }
 }

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -566,6 +566,7 @@ pub(crate) fn emit(
 
             let (opcode, num_opcodes) = match op {
                 ReadOnlyGprRmROpcode::Bsr => (0x0fbd, 2),
+                ReadOnlyGprRmROpcode::Bsf => (0x0fbc, 2),
             };
 
             match src {

cranelift/codegen/src/isa/x64/lower.rs

@@ -358,6 +358,38 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             ));
         }
 
+        Opcode::Ctz => {
+            // TODO when the x86 flags have use_bmi1, we can use TZCNT.
+
+            // General formula using bit-scan forward (BSF):
+            // bsf %src, %dst
+            // mov $(size_bits), %tmp
+            // cmovz %tmp, %dst
+            let ty = ctx.input_ty(insn, 0);
+            let ty = if ty.bits() < 32 { I32 } else { ty };
+            debug_assert!(ty == I32 || ty == I64);
+
+            let src = input_to_reg_mem(ctx, inputs[0]);
+            let dst = output_to_reg(ctx, outputs[0]);
+
+            let tmp = ctx.alloc_tmp(RegClass::I64, ty);
+            ctx.emit(Inst::imm_r(false /* 64 bits */, ty.bits() as u64, tmp));
+
+            ctx.emit(Inst::read_only_gpr_rm_r(
+                ty.bytes() as u8,
+                ReadOnlyGprRmROpcode::Bsf,
+                src,
+                dst,
+            ));
+
+            ctx.emit(Inst::cmove(
+                ty.bytes() as u8,
+                CC::Z,
+                RegMem::reg(tmp.to_reg()),
+                dst,
+            ));
+        }
+
         Opcode::Uextend
         | Opcode::Sextend
         | Opcode::Bint