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 4-byte sized elements which are the distance 29// from lr to the target label. 30// 31 .p2align 2 32DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch32) 33 ldr ip, [lr, #-1] // get first 32-bit word in table 34 cmp r0, ip // compare with index 35 add r0, lr, r0, lsl #2 // compute address of element in table 36 add ip, lr, ip, lsl #2 // compute address of last element in table 37 ite lo 38 ldrlo r0, [r0, #3] // load 32-bit element if r0 is in range 39 ldrhs r0, [ip, #3] // load 32-bit element if r0 out of range 40 add ip, lr, r0 // compute label = lr + element 41 bx ip // jump to computed label 42END_COMPILERRT_FUNCTION(__switch32) 43 44NO_EXEC_STACK_DIRECTIVE 45 46