switch8.S revision 2d1fdb26e458c4ddc04155c1d421bced3ba90cd0 
1//===-- switch.S - Implement switch* --------------------------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is dual licensed under the MIT and the University of Illinois Open
6// Source Licenses. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9
10#include "../assembly.h"
11
12//
13// When compiling switch statements in thumb mode, the compiler
14// can use these __switch* helper functions  The compiler emits a blx to
15// the __switch* function followed by a table of displacements for each
16// case statement.  On entry, R0 is the index into the table. The __switch*
17// function uses the return address in lr to find the start of the table.
18// The first entry in the table is the count of the entries in the table.
19// It then uses R0 to index into the table and get the displacement of the
20// address to jump to.  If R0 is greater than the size of the table, it jumps
21// to the last entry in the table. Each displacement in the table is actually
22// the distance from lr to the label, thus making the tables PIC.
23
24
25	.text
26	.syntax unified
27
28//
29// The table contains signed byte sized elements which are 1/2 the distance
30// from lr to the target label.
31//
32	.p2align 2
33DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch8)
34	ldrb    ip, [lr, #-1]           // get first byte in table
35	cmp     r0, ip                  // signed compare with index
36	ite lo
37	ldrsblo r0, [lr, r0]            // get indexed byte out of table
38	ldrsbhs r0, [lr, ip]            // if out of range, use last entry in table
39	add     ip, lr, r0, lsl #1      // compute label = lr + element*2
40	bx      ip                      // jump to computed label
41END_COMPILERRT_FUNCTION(__switch8)
42
43