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 unsigned byte sized elements which are 1/2 the distance 29// from lr to the target label. 30// 31 .p2align 2 32DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switchu8) 33 ldrb ip, [lr, #-1] // get first byte in table 34 cmp r0, ip // compare with index 35 ite lo 36 ldrblo r0, [lr, r0] // get indexed byte out of table 37 ldrbhs r0, [lr, ip] // if out of range, use last entry in table 38 add ip, lr, r0, lsl #1 // compute label = lr + element*2 39 bx ip // jump to computed label 40END_COMPILERRT_FUNCTION(__switchu8) 41 42NO_EXEC_STACK_DIRECTIVE 43 44