1 /*
2 * Copyright (C) 2004, 2007 Maciej W. Rozycki
3 *
4 * This file is subject to the terms and conditions of the GNU General Public
5 * License. See the file "COPYING" in the main directory of this archive
6 * for more details.
7 */
8 #ifndef _ASM_COMPILER_H
9 #define _ASM_COMPILER_H
10
11 /*
12 * With GCC 4.5 onwards we can use __builtin_unreachable to indicate to the
13 * compiler that a particular code path will never be hit. This allows it to be
14 * optimised out of the generated binary.
15 *
16 * Unfortunately at least GCC 4.6.3 through 7.3.0 inclusive suffer from a bug
17 * that can lead to instructions from beyond an unreachable statement being
18 * incorrectly reordered into earlier delay slots if the unreachable statement
19 * is the only content of a case in a switch statement. This can lead to
20 * seemingly random behaviour, such as invalid memory accesses from incorrectly
21 * reordered loads or stores. See this potential GCC fix for details:
22 *
23 * https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html
24 *
25 * It is unclear whether GCC 8 onwards suffer from the same issue - nothing
26 * relevant is mentioned in GCC 8 release notes and nothing obviously relevant
27 * stands out in GCC commit logs, but these newer GCC versions generate very
28 * different code for the testcase which doesn't exhibit the bug.
29 *
30 * GCC also handles stack allocation suboptimally when calling noreturn
31 * functions or calling __builtin_unreachable():
32 *
33 * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
34 *
35 * We work around both of these issues by placing a volatile asm statement,
36 * which GCC is prevented from reordering past, prior to __builtin_unreachable
37 * calls.
38 *
39 * The .insn statement is required to ensure that any branches to the
40 * statement, which sadly must be kept due to the asm statement, are known to
41 * be branches to code and satisfy linker requirements for microMIPS kernels.
42 */
43 #undef barrier_before_unreachable
44 #define barrier_before_unreachable() asm volatile(".insn")
45
46 #if !defined(CONFIG_CC_IS_GCC) || \
47 (__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)
48 # define GCC_OFF_SMALL_ASM() "ZC"
49 #elif defined(CONFIG_CPU_MICROMIPS)
50 # error "microMIPS compilation unsupported with GCC older than 4.9"
51 #else
52 # define GCC_OFF_SMALL_ASM() "R"
53 #endif
54
55 #ifdef CONFIG_CPU_MIPSR6
56 #define MIPS_ISA_LEVEL "mips64r6"
57 #define MIPS_ISA_ARCH_LEVEL MIPS_ISA_LEVEL
58 #define MIPS_ISA_LEVEL_RAW mips64r6
59 #define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW
60 #else
61 /* MIPS64 is a superset of MIPS32 */
62 #define MIPS_ISA_LEVEL "mips64r2"
63 #define MIPS_ISA_ARCH_LEVEL "arch=r4000"
64 #define MIPS_ISA_LEVEL_RAW mips64r2
65 #define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW
66 #endif /* CONFIG_CPU_MIPSR6 */
67
68 #endif /* _ASM_COMPILER_H */