src/compiler-rt/lib/builtins/arm/switch16.S

   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 2-byte sized elements which are 1/2 the distance
  30 // from lr to the target label.
  31 //
  32         .p2align 2
  33 DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch16)
  34         ldrh    ip, [lr, #-1]           // get first 16-bit word in table
  35         cmp     r0, ip                  // compare with index
  36         add     r0, lr, r0, lsl #1      // compute address of element in table
  37         add     ip, lr, ip, lsl #1      // compute address of last element in table
  38         ite lo
  39         ldrshlo r0, [r0, #1]            // load 16-bit element if r0 is in range
  40         ldrshhs r0, [ip, #1]            // load 16-bit element if r0 out of range
  41         add     ip, lr, r0, lsl #1      // compute label = lr + element*2
  42         bx      ip                      // jump to computed label
  43 END_COMPILERRT_FUNCTION(__switch16)
  44