Method build_jump_table

cranelift/frontend/src/switch.rs:201–258

(
        bx: &mut FunctionBuilder,
        val: Value,
        otherwise: Block,
        first_index: EntryIndex,
        blocks: &[Block],
    )

    }

    fn build_jump_table(
        bx: &mut FunctionBuilder,
        val: Value,
        otherwise: Block,
        first_index: EntryIndex,
        blocks: &[Block],
    ) {
        // There are currently no 128bit systems supported by rustc, but once we do ensure that
        // we don't silently ignore a part of the jump table for 128bit integers on 128bit systems.
        assert!(
            u32::try_from(blocks.len()).is_ok(),
            "Jump tables bigger than 2^32-1 are not yet supported"
        );

        let jt_data = JumpTableData::new(
            bx.func.dfg.block_call(otherwise, &[]),
            &blocks
                .iter()
                .map(|block| bx.func.dfg.block_call(*block, &[]))
                .collect::<Vec<_>>(),
        );
        let jump_table = bx.create_jump_table(jt_data);

        let discr = if first_index == 0 {
            val
        } else {
            if let Ok(first_index) = u64::try_from(first_index) {
                bx.ins().iadd_imm(val, (first_index as i64).wrapping_neg())
            } else {
                let (lsb, msb) = (first_index as u64, (first_index >> 64) as u64);
                let lsb = bx.ins().iconst(types::I64, lsb as i64);
                let msb = bx.ins().iconst(types::I64, msb as i64);
                let index = bx.ins().iconcat(lsb, msb);
                bx.ins().isub(val, index)
            }
        };

        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()
                    .brif(bigger_than_u32, otherwise, &[], new_block, &[]);
                bx.seal_block(new_block);
                bx.switch_to_block(new_block);

                // 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, jump_table);
    }