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 unsigned byte sized elements which are 1/2 the distance 30// from lr to the target label. 31// 32 .p2align 2 33DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switchu8) 34 ldrb ip, [lr, #-1] // get first byte in table 35 cmp r0, ip // compare with index 36 ite lo 37 ldrblo r0, [lr, r0] // get indexed byte out of table 38 ldrbhs 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(__switchu8) 42 43NO_EXEC_STACK_DIRECTIVE 44 45