cranelift: Restrict br_table to i32 indices (#4510)

* cranelift: Restrict `br_table` to `i32` indices

In #4498 it was proposed that we should only accept `i32` indices
to `br_table`. The rationale for this is that larger types lead the
users to a false sense of flexibility (since we don't support jump
tables larger than u32's), and narrower types are not well tested
paths that would be safer if we removed them.

* cranelift: Reduce directly from i128 to i32 in Switch

cranelift/frontend/src/switch.rs

@@ -264,21 +264,23 @@ impl Switch {
                 }
             };
 
-            let discr = if bx.func.dfg.value_type(discr).bits() > 64 {
-                // Check for overflow of cast to u32. This is the max supported jump table entries.
-                let new_block = bx.create_block();
-                let bigger_than_u32 =
-                    bx.ins()
-                        .icmp_imm(IntCC::UnsignedGreaterThan, discr, u32::MAX as i64);
-                bx.ins().brnz(bigger_than_u32, otherwise, &[]);
-                bx.ins().jump(new_block, &[]);
-                bx.seal_block(new_block);
-                bx.switch_to_block(new_block);
+            let discr = match bx.func.dfg.value_type(discr).bits() {
+                bits if bits > 32 => {
+                    // Check for overflow of cast to u32. This is the max supported jump table entries.
+                    let new_block = bx.create_block();
+                    let bigger_than_u32 =
+                        bx.ins()
+                            .icmp_imm(IntCC::UnsignedGreaterThan, discr, u32::MAX as i64);
+                    bx.ins().brnz(bigger_than_u32, otherwise, &[]);
+                    bx.ins().jump(new_block, &[]);
+                    bx.seal_block(new_block);
+                    bx.switch_to_block(new_block);
 
-                // Cast to u64, as br_table is not implemented for i128
-                bx.ins().isplit(discr).0
-            } else {
-                discr
+                    // Cast to i32, as br_table is not implemented for i64/i128
+                    bx.ins().ireduce(types::I32, discr)
+                }
+                bits if bits < 32 => bx.ins().uextend(types::I32, discr),
+                _ => discr,
             };
 
             bx.ins().br_table(discr, otherwise, jump_table);
@@ -419,7 +421,8 @@ block0:
     jump block3
 
 block3:
-    br_table.i8 v0, block0, jt0"
+    v1 = uextend.i32 v0
+    br_table v1, block0, jt0"
         );
     }
 
@@ -470,11 +473,13 @@ block8:
     jump block12
 
 block12:
-    br_table.i8 v0, block0, jt0
+    v5 = uextend.i32 v0
+    br_table v5, block0, jt0
 
 block10:
-    v5 = iadd_imm.i8 v0, -10
-    br_table v5, block0, jt1"
+    v6 = iadd_imm.i8 v0, -10
+    v7 = uextend.i32 v6
+    br_table v7, block0, jt1"
         );
     }
 
@@ -528,7 +533,8 @@ block0:
     jump block4
 
 block4:
-    br_table.i8 v0, block0, jt0"
+    v2 = uextend.i32 v0
+    br_table v2, block0, jt0"
         );
     }
 
@@ -620,7 +626,13 @@ block0:
     jump block4
 
 block4:
-    br_table.i64 v0, block3, jt0"
+    v1 = icmp_imm.i64 ugt v0, 0xffff_ffff
+    brnz v1, block3
+    jump block5
+
+block5:
+    v2 = ireduce.i32 v0
+    br_table v2, block3, jt0"
         );
     }
 
@@ -660,7 +672,7 @@ block4:
     jump block5
 
 block5:
-    v2, v3 = isplit.i128 v0
+    v2 = ireduce.i32 v0
     br_table v2, block3, jt0"
         );
     }