Terminate blocks with a single branch in riscv64 (#5374)

Ensure that we're terminating blocks with a single branch instruction, when testing I128 values against zero.

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

@@ -1921,6 +1921,13 @@
   (lower_branch (brz v @ (value_type ty) _ _) targets)
   (lower_brz_or_nz (IntCC.Equal) (normalize_cmp_value ty v) targets ty))
 
+;; Special case for SI128 to reify the comparison value and branch on it.
+(rule 2
+  (lower_branch (brz v @ (value_type $I128) _ _) targets)
+  (let ((zero ValueRegs (value_regs (zero_reg) (zero_reg)))
+        (cmp Reg (gen_icmp (IntCC.Equal) v zero $I128)))
+    (lower_brz_or_nz (IntCC.NotEqual) cmp targets $I64)))
+
 (rule 1
   (lower_branch (brz (icmp cc a @ (value_type ty) b) _ _) targets)
   (lower_br_icmp (intcc_inverse cc) a b targets ty))
@@ -1934,6 +1941,13 @@
   (lower_branch (brnz v @ (value_type ty) _ _) targets)
   (lower_brz_or_nz (IntCC.NotEqual) (normalize_cmp_value ty v) targets ty))
 
+;; Special case for SI128 to reify the comparison value and branch on it.
+(rule 2
+  (lower_branch (brnz v @ (value_type $I128) _ _) targets)
+  (let ((zero ValueRegs (value_regs (zero_reg) (zero_reg)))
+        (cmp Reg (gen_icmp (IntCC.NotEqual) v zero $I128)))
+    (lower_brz_or_nz (IntCC.NotEqual) cmp targets $I64)))
+
 (rule 1
   (lower_branch (brnz (icmp cc a @ (value_type ty) b) _ _) targets)
   (lower_br_icmp cc a b targets ty))

cranelift/filetests/filetests/isa/riscv64/condbr.clif

@@ -170,8 +170,8 @@ block1:
 }
 
 ; block0:
-;   bne a1,zero,taken(label2),not_taken(0)
-;   beq a0,zero,taken(label1),not_taken(label2)
+;   eq a0,[a0,a1],[zerozero]##ty=i128
+;   bne a0,zero,taken(label1),not_taken(label2)
 ; block1:
 ;   j label3
 ; block2:
@@ -190,7 +190,7 @@ block1:
 }
 
 ; block0:
-;   bne a1,zero,taken(label1),not_taken(0)
+;   ne a0,[a0,a1],[zerozero]##ty=i128
 ;   bne a0,zero,taken(label1),not_taken(label2)
 ; block1:
 ;   j label3