1 /* SPDX-License-Identifier: GPL-2.0 */ 2 /* 3 * Copyright (C) 2004, 2007 Maciej W. Rozycki 4 * 5 * This file is subject to the terms and conditions of the GNU General Public 6 * License. See the file "COPYING" in the main directory of this archive 7 * for more details. 8 */ 9 #ifndef _ASM_COMPILER_H 10 #define _ASM_COMPILER_H 11 12 /* 13 * With GCC 4.5 onwards we can use __builtin_unreachable to indicate to the 14 * compiler that a particular code path will never be hit. This allows it to be 15 * optimised out of the generated binary. 16 * 17 * Unfortunately at least GCC 4.6.3 through 7.3.0 inclusive suffer from a bug 18 * that can lead to instructions from beyond an unreachable statement being 19 * incorrectly reordered into earlier delay slots if the unreachable statement 20 * is the only content of a case in a switch statement. This can lead to 21 * seemingly random behaviour, such as invalid memory accesses from incorrectly 22 * reordered loads or stores. See this potential GCC fix for details: 23 * 24 * https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html 25 * 26 * It is unclear whether GCC 8 onwards suffer from the same issue - nothing 27 * relevant is mentioned in GCC 8 release notes and nothing obviously relevant 28 * stands out in GCC commit logs, but these newer GCC versions generate very 29 * different code for the testcase which doesn't exhibit the bug. 30 * 31 * GCC also handles stack allocation suboptimally when calling noreturn 32 * functions or calling __builtin_unreachable(): 33 * 34 * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365 35 * 36 * We work around both of these issues by placing a volatile asm statement, 37 * which GCC is prevented from reordering past, prior to __builtin_unreachable 38 * calls. 39 * 40 * The .insn statement is required to ensure that any branches to the 41 * statement, which sadly must be kept due to the asm statement, are known to 42 * be branches to code and satisfy linker requirements for microMIPS kernels. 43 */ 44 #undef barrier_before_unreachable 45 #define barrier_before_unreachable() asm volatile(".insn") 46 47 #if !defined(CONFIG_CC_IS_GCC) || \ 48 (__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9) 49 # define GCC_OFF_SMALL_ASM() "ZC" 50 #elif defined(CONFIG_CPU_MICROMIPS) 51 # error "microMIPS compilation unsupported with GCC older than 4.9" 52 #else 53 # define GCC_OFF_SMALL_ASM() "R" 54 #endif 55 56 #ifdef CONFIG_CPU_MIPSR6 57 #define MIPS_ISA_LEVEL "mips64r6" 58 #define MIPS_ISA_ARCH_LEVEL MIPS_ISA_LEVEL 59 #define MIPS_ISA_LEVEL_RAW mips64r6 60 #define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW 61 #else 62 /* MIPS64 is a superset of MIPS32 */ 63 #define MIPS_ISA_LEVEL "mips64r2" 64 #define MIPS_ISA_ARCH_LEVEL "arch=r4000" 65 #define MIPS_ISA_LEVEL_RAW mips64r2 66 #define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW 67 #endif /* CONFIG_CPU_MIPSR6 */ 68 69 #endif /* _ASM_COMPILER_H */ 70