cranelift x64: use the LZCNT instruction for Clz when it's available;

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

@@ -402,6 +402,8 @@ pub enum UnaryRmROpcode {
     Bsr,
     /// Bit-scan forward.
     Bsf,
+    /// Counts leading zeroes (Leading Zero CouNT).
+    Lzcnt,
 }
 
 impl fmt::Debug for UnaryRmROpcode {
@@ -409,6 +411,7 @@ impl fmt::Debug for UnaryRmROpcode {
         match self {
             UnaryRmROpcode::Bsr => write!(fmt, "bsr"),
             UnaryRmROpcode::Bsf => write!(fmt, "bsf"),
+            UnaryRmROpcode::Lzcnt => write!(fmt, "lzcnt"),
         }
     }
 }

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

@@ -129,18 +129,20 @@ impl RexFlags {
 /// We may need to include one or more legacy prefix bytes before the REX prefix.  This enum
 /// covers only the small set of possibilities that we actually need.
 enum LegacyPrefixes {
-    /// No prefix bytes
+    /// No prefix bytes.
     None,
-    /// Operand Size Override -- here, denoting "16-bit operation"
+    /// Operand Size Override -- here, denoting "16-bit operation".
     _66,
-    /// The Lock prefix
+    /// The Lock prefix.
     _F0,
-    /// Operand size override and Lock
+    /// Operand size override and Lock.
     _66F0,
-    /// REPNE, but no specific meaning here -- is just an opcode extension
+    /// REPNE, but no specific meaning here -- is just an opcode extension.
     _F2,
-    /// REP/REPE, but no specific meaning here -- is just an opcode extension
+    /// REP/REPE, but no specific meaning here -- is just an opcode extension.
     _F3,
+    /// Operand size override and same effect as F3.
+    _66F3,
 }
 
 impl LegacyPrefixes {
@@ -157,6 +159,10 @@ impl LegacyPrefixes {
             }
             LegacyPrefixes::_F2 => sink.put1(0xF2),
             LegacyPrefixes::_F3 => sink.put1(0xF3),
+            LegacyPrefixes::_66F3 => {
+                sink.put1(0x66);
+                sink.put1(0xF3);
+            }
             LegacyPrefixes::None => (),
         }
     }
@@ -665,16 +671,28 @@ pub(crate) fn emit(
         }
 
         Inst::UnaryRmR { size, op, src, dst } => {
-            let (prefix, rex_flags) = match size {
+            let rex_flags = match size {
-                2 => (LegacyPrefixes::_66, RexFlags::clear_w()),
+                2 | 4 => RexFlags::clear_w(),
-                4 => (LegacyPrefixes::None, RexFlags::clear_w()),
+                8 => RexFlags::set_w(),
-                8 => (LegacyPrefixes::None, RexFlags::set_w()),
+                _ => unreachable!(),
+            };
+
+            let prefix = match size {
+                2 => match op {
+                    UnaryRmROpcode::Bsr | UnaryRmROpcode::Bsf => LegacyPrefixes::_66,
+                    UnaryRmROpcode::Lzcnt => LegacyPrefixes::_66F3,
+                },
+                4 | 8 => match op {
+                    UnaryRmROpcode::Bsr | UnaryRmROpcode::Bsf => LegacyPrefixes::None,
+                    UnaryRmROpcode::Lzcnt => LegacyPrefixes::_F3,
+                },
                 _ => unreachable!(),
             };
 
             let (opcode, num_opcodes) = match op {
                 UnaryRmROpcode::Bsr => (0x0fbd, 2),
                 UnaryRmROpcode::Bsf => (0x0fbc, 2),
+                UnaryRmROpcode::Lzcnt => (0x0fbd, 2),
             };
 
             match src {

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

@@ -2276,7 +2276,22 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
         }
 
         Opcode::Clz => {
-            // TODO when the x86 flags have use_lzcnt, we can use LZCNT.
+            let orig_ty = ty.unwrap();
+
+            if isa_flags.use_lzcnt() && (orig_ty == types::I32 || orig_ty == types::I64) {
+                // We can use a plain lzcnt instruction here. Note no special handling is required
+                // for zero inputs, because the machine instruction does what the CLIF expects for
+                // zero, i.e. it returns zero.
+                let src = input_to_reg_mem(ctx, inputs[0]);
+                let dst = get_output_reg(ctx, outputs[0]).only_reg().unwrap();
+                ctx.emit(Inst::unary_rm_r(
+                    orig_ty.bytes() as u8,
+                    UnaryRmROpcode::Lzcnt,
+                    src,
+                    dst,
+                ));
+                return Ok(());
+            }
 
             // General formula using bit-scan reverse (BSR):
             // mov -1, %dst
@@ -2285,7 +2300,6 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             // mov $(size_bits - 1), %dst
             // sub %tmp, %dst
 
-            let orig_ty = ty.unwrap();
             if orig_ty == types::I128 {
                 // clz upper, tmp1
                 // clz lower, dst
@@ -4427,6 +4441,7 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
                 }
             }
         }
+
         Opcode::Store
         | Opcode::Istore8
         | Opcode::Istore16

cranelift/filetests/filetests/isa/x64/clz-lzcnt.clif

@@ -0,0 +1,31 @@
+test compile
+target x86_64 has_lzcnt
+feature "experimental_x64"
+
+function %clz(i64) -> i64 {
+block0(v0: i64):
+    v1 = clz v0
+    return v1
+}
+
+; check: pushq   %rbp
+; check: movq    %rsp, %rbp
+; check: lzcntq  %rdi, %rsi
+; check: movq    %rsi, %rax
+; check: movq    %rbp, %rsp
+; check: popq    %rbp
+; check: ret
+
+function %clz(i32) -> i32 {
+block0(v0: i32):
+    v1 = clz v0
+    return v1
+}
+
+; check: pushq   %rbp
+; check: movq    %rsp, %rbp
+; check: lzcntl  %edi, %esi
+; check: movq    %rsi, %rax
+; check: movq    %rbp, %rsp
+; check: popq    %rbp
+; check: ret