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 */ 69
