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 4-byte sized elements which are the distance 30 // from lr to the target label. 31 // 32 .align 2 33 DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch32) 34 ldr ip, [lr, #-1] // get first 32-bit word in table 35 cmp r0, ip // compare with index 36 add r0, lr, r0, lsl #2 // compute address of element in table 37 ldrcc r0, [r0, #3] // load 32-bit element if r0 is in range 38 add ip, lr, ip, lsl #2 // compute address of last element in table 39 ldrcs r0, [ip, #3] // load 32-bit element if r0 out of range 40 add ip, lr, r0 // compute label = lr + element 41 bx ip // jump to computed label 42 43 44 // tell linker it can break up file at label boundaries 45 .subsections_via_symbols 46 47
