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      1 //===-- switch.S - Implement switch* --------------------------------------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is dual licensed under the MIT and the University of Illinois Open
      6 // Source Licenses. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 
     10 #include "../assembly.h"
     11 
     12 //
     13 // When compiling switch statements in thumb mode, the compiler
     14 // can use these __switch* helper functions  The compiler emits a blx to
     15 // the __switch* function followed by a table of displacements for each
     16 // case statement.  On entry, R0 is the index into the table. The __switch*
     17 // function uses the return address in lr to find the start of the table.
     18 // The first entry in the table is the count of the entries in the table.
     19 // It then uses R0 to index into the table and get the displacement of the
     20 // address to jump to.  If R0 is greater than the size of the table, it jumps
     21 // to the last entry in the table. Each displacement in the table is actually
     22 // the distance from lr to the label, thus making the tables PIC.
     23 
     24 
     25 	.text
     26 	.syntax unified
     27 
     28 //
     29 // The table contains signed byte sized elements which are 1/2 the distance
     30 // from lr to the target label.
     31 //
     32 	.align 2
     33 DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch8)
     34 	ldrb    ip, [lr, #-1]           // get first byte in table
     35 	cmp     r0, ip                  // signed compare with index
     36 	ldrsbcc r0, [lr, r0]            // get indexed byte out of table
     37 	ldrsbhs r0, [lr, ip]            // if out of range, use last entry in table
     38 	add     ip, lr, r0, lsl #1      // compute label = lr + element*2
     39 	bx      ip                      // jump to computed label
     40 
     41 	// tell linker it can break up file at label boundaries
     42 	.subsections_via_symbols
     43 
     44