riscv64: Remove support for fixed offset jumps from Jump instructions (…

…#6988)

* riscv64: Use `MachLabel` for Jal

* riscv64: Use `Inst::gen_jump` for jumps

This is mostly a personal preference. It emits the exact same code.

* riscv64: Use VecMachLabel on BrTable

* riscv64: Remove `BranchTarget::Offset` arm

Replaces it with `Fallthrough` which works the same way with
a fixed offset of 0.

* riscv64: Rename `BranchTarget`

It is now only used for `CondBr`

* riscv64: Panic on fallthrough taken target in condbr

cranelift/codegen/src/isa/riscv64/inst.isle

@@ -132,11 +132,11 @@
 
     (Jal
       ;; (rd WritableReg) don't use
-      (dest BranchTarget))
+      (label MachLabel))
 
     (CondBr
-      (taken BranchTarget)
-      (not_taken BranchTarget)
+      (taken CondBrTarget)
+      (not_taken CondBrTarget)
       (kind IntegerCompare))
 
     ;; Load an inline symbol reference.
@@ -227,7 +227,7 @@
       (index Reg)
       (tmp1 WritableReg)
       (tmp2 WritableReg)
-      (targets VecBranchTarget))
+      (targets VecMachLabel))
 
     ;; atomic compare and set operation
     (AtomicCas
@@ -763,7 +763,6 @@
 ;;;; lowest four bit are used.
 (type FenceReq (primitive u8))
 
-(type VecBranchTarget (primitive VecBranchTarget))
 (type BoxCallInfo (primitive BoxCallInfo))
 (type BoxCallIndInfo (primitive BoxCallIndInfo))
 (type BoxReturnCallInfo (primitive BoxReturnCallInfo))
@@ -777,7 +776,7 @@
 (type Imm5 (primitive Imm5))
 (type Imm20 (primitive Imm20))
 (type Imm3 (primitive Imm3))
-(type BranchTarget (primitive BranchTarget))
+(type CondBrTarget (primitive CondBrTarget))
 (type OptionFloatRoundingMode (primitive OptionFloatRoundingMode))
 (type VecU8 (primitive VecU8))
 (type AMO (primitive AMO))
@@ -2660,21 +2659,6 @@
         (test XReg (rv_srli sum (imm12_const (ty_bits ty)))))
     (value_regs sum test)))
 
-(decl label_to_br_target (MachLabel) BranchTarget)
-(extern constructor label_to_br_target label_to_br_target)
-
-(decl gen_jump (MachLabel) MInst)
-(rule
-  (gen_jump v)
-  (MInst.Jal (label_to_br_target v)))
-
-(decl vec_label_get (VecMachLabel u8) MachLabel )
-(extern constructor vec_label_get vec_label_get)
-
-(decl partial lower_branch (Inst VecMachLabel) Unit)
-(rule (lower_branch (jump _) targets )
-      (emit_side_effect (SideEffectNoResult.Inst (gen_jump (vec_label_get targets 0)))))
-
 ;;; cc a b targets Type
 (decl lower_br_icmp (IntCC ValueRegs ValueRegs VecMachLabel Type) Unit)
 (extern constructor lower_br_icmp lower_br_icmp)
@@ -2718,6 +2702,17 @@
             (hi XReg (value_regs_get regs 1)))
         (rv_or lo hi)))
 
+
+(decl label_to_br_target (MachLabel) CondBrTarget)
+(extern constructor label_to_br_target label_to_br_target)
+
+(decl vec_label_get (VecMachLabel u8) MachLabel)
+(extern constructor vec_label_get vec_label_get)
+
+(decl partial lower_branch (Inst VecMachLabel) Unit)
+(rule (lower_branch (jump _) targets )
+      (emit_side_effect (SideEffectNoResult.Inst (MInst.Jal (vec_label_get targets 0)))))
+
 ;; Default behavior for branching based on an input value.
 (rule
   (lower_branch (brif v @ (value_type ty) _ _) targets)
@@ -2739,23 +2734,22 @@
 (rule 1
   (lower_branch (brif (maybe_uextend (fcmp cc a @ (value_type ty) b)) _ _) targets)
   (if-let $true (floatcc_unordered cc))
-  (let ((then BranchTarget (label_to_br_target (vec_label_get targets 0)))
-        (else BranchTarget (label_to_br_target (vec_label_get targets 1))))
+  (let ((then CondBrTarget (label_to_br_target (vec_label_get targets 0)))
+        (else CondBrTarget (label_to_br_target (vec_label_get targets 1))))
     (emit_side_effect (cond_br (emit_fcmp (floatcc_complement cc) ty a b) else then))))
 
 (rule 1
   (lower_branch (brif (maybe_uextend (fcmp cc a @ (value_type ty) b)) _ _) targets)
   (if-let $false (floatcc_unordered cc))
-  (let ((then BranchTarget (label_to_br_target (vec_label_get targets 0)))
-        (else BranchTarget (label_to_br_target (vec_label_get targets 1))))
+  (let ((then CondBrTarget (label_to_br_target (vec_label_get targets 0)))
+        (else CondBrTarget (label_to_br_target (vec_label_get targets 1))))
     (emit_side_effect (cond_br (emit_fcmp cc ty a b) then else))))
 
-;;;
+
 (decl lower_br_table (Reg VecMachLabel) Unit)
 (extern constructor lower_br_table lower_br_table)
 
-(rule
-  (lower_branch (br_table index _) targets)
+(rule (lower_branch (br_table index _) targets)
   (lower_br_table index targets))
 
 (decl load_ra () Reg)
@@ -2983,7 +2977,7 @@
 (rule (cmp_result_invert result) (CmpResult.Result result $true))
 
 ;; Consume a CmpResult, producing a branch on its result.
-(decl cond_br (CmpResult BranchTarget BranchTarget) SideEffectNoResult)
+(decl cond_br (CmpResult CondBrTarget CondBrTarget) SideEffectNoResult)
 (rule (cond_br cmp then else)
       (SideEffectNoResult.Inst
         (MInst.CondBr then else (cmp_integer_compare cmp))))