llvm-project/compiler-rt/lib/arm/switch16.S

44 lines
1.9 KiB
ArmAsm

//===-- switch.S - Implement switch* --------------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
#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.
//
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
ldrshcc r0, [r0, #1] // load 16-bit element if r0 is in range
add ip, lr, ip, lsl #1 // compute address of last element in table
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
// tell linker it can break up file at label boundaries
.subsections_via_symbols