1//===-- switch.S - Implement switch* --------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "../assembly.h"
10
11//
12// When compiling switch statements in thumb mode, the compiler
13// can use these __switch* helper functions  The compiler emits a blx to
14// the __switch* function followed by a table of displacements for each
15// case statement.  On entry, R0 is the index into the table. The __switch*
16// function uses the return address in lr to find the start of the table.
17// The first entry in the table is the count of the entries in the table.
18// It then uses R0 to index into the table and get the displacement of the
19// address to jump to.  If R0 is greater than the size of the table, it jumps
20// to the last entry in the table. Each displacement in the table is actually
21// the distance from lr to the label, thus making the tables PIC.
22
23
24	.text
25	.syntax unified
26
27//
28// The table contains signed 2-byte sized elements which are 1/2 the distance
29// from lr to the target label.
30//
31	.p2align 2
32DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch16)
33	ldrh    ip, [lr, #-1]           // get first 16-bit word in table
34	cmp     r0, ip                  // compare with index
35	add     r0, lr, r0, lsl #1      // compute address of element in table
36	add     ip, lr, ip, lsl #1      // compute address of last element in table
37	ite lo
38	ldrshlo r0, [r0, #1]            // load 16-bit element if r0 is in range
39	ldrshhs r0, [ip, #1]            // load 16-bit element if r0 out of range
40	add     ip, lr, r0, lsl #1      // compute label = lr + element*2
41	bx      ip                      // jump to computed label
42END_COMPILERRT_FUNCTION(__switch16)
43
44NO_EXEC_STACK_DIRECTIVE
45
46