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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-exception6//7//===----------------------------------------------------------------------===//8 9#include "../assembly.h"10 11//12// When compiling switch statements in thumb mode, the compiler13// can use these __switch* helper functions The compiler emits a blx to14// the __switch* function followed by a table of displacements for each15// 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 the19// address to jump to. If R0 is greater than the size of the table, it jumps20// to the last entry in the table. Each displacement in the table is actually21// the distance from lr to the label, thus making the tables PIC.22 23 24 .text25 .syntax unified26 27//28// The table contains signed byte sized elements which are 1/2 the distance29// from lr to the target label.30//31 .p2align 232DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch8)33 ldrb ip, [lr, #-1] // get first byte in table34 cmp r0, ip // signed compare with index35 ite lo36 ldrsblo r0, [lr, r0] // get indexed byte out of table37 ldrsbhs r0, [lr, ip] // if out of range, use last entry in table38 add ip, lr, r0, lsl #1 // compute label = lr + element*239 bx ip // jump to computed label40END_COMPILERRT_FUNCTION(__switch8)41 42NO_EXEC_STACK_DIRECTIVE43 44