machinst x64: implement one way conditional jmp

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

@@ -1021,6 +1021,18 @@ pub(crate) fn emit(
             sink.put4(nt_disp);
         }
 
+        Inst::OneWayJmpCond { cc, dst } => {
+            let cond_start = sink.cur_offset();
+            let cond_disp_off = cond_start + 2;
+            if let Some(l) = dst.as_label() {
+                sink.use_label_at_offset(cond_disp_off, l, LabelUse::JmpRel32);
+            }
+            let dst_disp = dst.as_offset32_or_zero() as u32;
+            sink.put1(0x0F);
+            sink.put1(0x80 + cc.get_enc());
+            sink.put4(dst_disp);
+        }
+
         Inst::JmpUnknown { target } => {
             match target {
                 RegMem::Reg { reg } => {

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

@@ -200,6 +200,10 @@ pub enum Inst {
         not_taken: BranchTarget,
     },
 
+    /// A one-way conditional branch, invisible to the CFG processing; used *only* as part of
+    /// straight-line sequences in code to be emitted.
+    OneWayJmpCond { cc: CC, dst: BranchTarget },
+
     /// Indirect jump: jmpq (reg mem).
     JmpUnknown { target: RegMem },
 
@@ -634,7 +638,11 @@ impl ShowWithRRU for Inst {
                 taken.show_rru(mb_rru),
                 not_taken.show_rru(mb_rru)
             ),
-            //
+            Inst::OneWayJmpCond { cc, dst } => format!(
+                "{} {}",
+                ljustify2("j".to_string(), cc.to_string()),
+                dst.show_rru(mb_rru),
+            ),
             Inst::JmpUnknown { target } => format!(
                 "{} *{}",
                 ljustify("jmp".to_string()),
@@ -757,6 +765,7 @@ fn x64_get_regs(inst: &Inst, collector: &mut RegUsageCollector) {
         | Inst::EpiloguePlaceholder
         | Inst::JmpKnown { .. }
         | Inst::JmpCond { .. }
+        | Inst::OneWayJmpCond { .. }
         | Inst::Nop { .. }
         | Inst::JmpUnknown { .. }
         | Inst::VirtualSPOffsetAdj { .. }
@@ -970,6 +979,7 @@ fn x64_map_regs<RUM: RegUsageMapper>(inst: &mut Inst, mapper: &RUM) {
         | Inst::EpiloguePlaceholder
         | Inst::JmpKnown { .. }
         | Inst::JmpCond { .. }
+        | Inst::OneWayJmpCond { .. }
         | Inst::Nop { .. }
         | Inst::JmpUnknown { .. }
         | Inst::VirtualSPOffsetAdj { .. }