x64 backend: fix cmpxchg (don't return RealReg as result). (#4243)

The current lowering helper for `cmpxchg` returns the literal RealReg
`rax` as its result. However, this breaks a number of invariants, and
eventually causes a regalloc panic if used as a blockparam arg (pinned
vregs cannot be used in this way).

In general we have to return regular vregs, not a RealReg, as results of
instructions during lowering. However #4223 added a helper for
`x64_cmpxchg` that returns a literal `rax`.

Fortunately we can do the right thing here by just giving a fresh vreg
to the instruction; the regalloc constraints mean that this vreg is
constrained to `rax` at the instruction (at its def/late point), so the
generator of the instruction need not worry about `rax` here.

cranelift/codegen/src/isa/x64/inst.isle

@@ -1067,11 +1067,6 @@
 (decl encode_fcmp_imm (FcmpImm) u8)
 (extern constructor encode_fcmp_imm encode_fcmp_imm)
 
-;;;; Registers ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
-
-(decl rax () WritableGpr)
-(extern constructor rax rax)
-
 ;;;; Newtypes for Different Register Classes ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (type Gpr (primitive Gpr))
@@ -2906,8 +2901,9 @@
 
 (decl x64_cmpxchg (Type Gpr Gpr SyntheticAmode) Gpr)
 (rule (x64_cmpxchg ty expected replacement addr)
-      (let ((_ Unit (emit (MInst.LockCmpxchg ty replacement expected addr (rax)))))
-        (rax)))
+      (let ((dst WritableGpr (temp_writable_gpr))
+            (_ Unit (emit (MInst.LockCmpxchg ty replacement expected addr dst))))
+        dst))
 
 ;;;; Automatic conversions ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 

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

@@ -547,12 +547,6 @@ where
     fn zero_offset(&mut self) -> Offset32 {
         Offset32::new(0)
     }
-
-    #[inline]
-    fn rax(&mut self) -> WritableGpr {
-        let gpr = Gpr::new(regs::rax()).unwrap();
-        WritableGpr::from_reg(gpr)
-    }
 }
 
 // Since x64 doesn't have 8x16 shifts and we must use a 16x8 shift instead, we