lib/arm/switch.S

//===-- 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 unsigned byte sized elements which are 1/2 the distance
// from lr to the target label.
//
DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switchu8)
	ldrb    ip, [lr, #-1]           // get first byte in table
	cmp     r0, ip                  // compare with index
	ldrbcc  r0, [lr, r0]            // get indexed byte out of table
	ldrbhs  r0, [lr, ip]            // if out of range, use last entry in table
	add     ip, lr, r0, lsl #1      // compute label = lr + element*2
	bx      ip                      // jump to computed label



//
// The table contains signed byte sized elements which are 1/2 the distance
// from lr to the target label.  
//
DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch8)
	ldrb    ip, [lr, #-1]           // get first byte in table
	cmp     r0, ip                  // signed compare with index
	ldrsbcc r0, [lr, r0]            // get indexed byte out of table
	ldrsbhs r0, [lr, ip]            // if out of range, use last entry in table
	add     ip, lr, r0, lsl #1      // compute label = lr + element*2
	bx      ip                      // jump to computed label


//
// 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


//
// The table contains signed 4-byte sized elements which are the distance
// from lr to the target label.    
//
DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch32)
	ldr     ip, [lr, #-1]            // get first 32-bit word in table
	cmp     r0, ip                   // compare with index
	add     r0, lr, r0, lsl #2       // compute address of element in table
	ldrcc   r0, [r0, #3]             // load 32-bit element if r0 is in range
	add     ip, lr, ip, lsl #2       // compute address of last element in table
	ldrcs   r0, [ip, #3]             // load 32-bit element if r0 out of range
	add     ip, lr, r0               // compute label = lr + element
	bx      ip                       // jump to computed label


	// tell linker it can break up file at label boundaries
	.subsections_via_symbols