switch16.S
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//===-- switch.S - Implement switch* --------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
#include "../assembly.h"
//
// When compiling switch statements in thumb mode, the compiler
// can use these __switch* helper functions The compiler emits a blx to
// the __switch* function followed by a table of displacements for each
// case statement. On entry, R0 is the index into the table. The __switch*
// function uses the return address in lr to find the start of the table.
// The first entry in the table is the count of the entries in the table.
// It then uses R0 to index into the table and get the displacement of the
// address to jump to. If R0 is greater than the size of the table, it jumps
// to the last entry in the table. Each displacement in the table is actually
// the distance from lr to the label, thus making the tables PIC.
.text
.syntax unified
//
// The table contains signed 2-byte sized elements which are 1/2 the distance
// from lr to the target label.
//
.p2align 2
DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch16)
ldrh ip, [lr, #-1] // get first 16-bit word in table
cmp r0, ip // compare with index
add r0, lr, r0, lsl #1 // compute address of element in table
add ip, lr, ip, lsl #1 // compute address of last element in table
ite lo
ldrshlo r0, [r0, #1] // load 16-bit element if r0 is in range
ldrshhs r0, [ip, #1] // load 16-bit element if r0 out of range
add ip, lr, r0, lsl #1 // compute label = lr + element*2
bx ip // jump to computed label
END_COMPILERRT_FUNCTION(__switch16)
NO_EXEC_STACK_DIRECTIVE