| 1 | //===-- switch.S - Implement switch* --------------------------------------===// |
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| 2 | // |
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| 3 | // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
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| 4 | // See https://llvm.org/LICENSE.txt for license information. |
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| 5 | // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
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| 6 | // |
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| 7 | //===----------------------------------------------------------------------===// |
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| 8 | |
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| 9 | #include "../assembly.h" |
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| 10 | |
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| 11 | // |
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| 12 | // When compiling switch statements in thumb mode, the compiler |
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| 13 | // can use these __switch* helper functions The compiler emits a blx to |
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| 14 | // the __switch* function followed by a table of displacements for each |
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| 15 | // case statement. On entry, R0 is the index into the table. The __switch* |
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| 16 | // function uses the return address in lr to find the start of the table. |
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| 17 | // The first entry in the table is the count of the entries in the table. |
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| 18 | // It then uses R0 to index into the table and get the displacement of the |
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| 19 | // address to jump to. If R0 is greater than the size of the table, it jumps |
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| 20 | // to the last entry in the table. Each displacement in the table is actually |
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| 21 | // the distance from lr to the label, thus making the tables PIC. |
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| 22 | |
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| 23 | |
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| 24 | .text |
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| 25 | .syntax unified |
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| 26 | |
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| 27 | // |
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| 28 | // The table contains signed 2-byte sized elements which are 1/2 the distance |
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| 29 | // from lr to the target label. |
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| 30 | // |
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| 31 | .p2align 2 |
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| 32 | DEFINE_COMPILERRT_PRIVATE_FUNCTION(__switch16) |
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| 33 | ldrh ip, [lr, #-1] // get first 16-bit word in table |
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| 34 | cmp r0, ip // compare with index |
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| 35 | add r0, lr, r0, lsl #1 // compute address of element in table |
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| 36 | add ip, lr, ip, lsl #1 // compute address of last element in table |
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| 37 | ite lo |
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| 38 | ldrshlo r0, [r0, #1] // load 16-bit element if r0 is in range |
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| 39 | ldrshhs r0, [ip, #1] // load 16-bit element if r0 out of range |
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| 40 | add ip, lr, r0, lsl #1 // compute label = lr + element*2 |
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| 41 | bx ip // jump to computed label |
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| 42 | END_COMPILERRT_FUNCTION(__switch16) |
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| 43 | |
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| 44 | NO_EXEC_STACK_DIRECTIVE |
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| 45 | |
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| 46 | |
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