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70 lines
2.7 KiB
C
70 lines
2.7 KiB
C
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/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* Copyright (C) 2004, 2007 Maciej W. Rozycki
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*
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* This file is subject to the terms and conditions of the GNU General Public
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* License. See the file "COPYING" in the main directory of this archive
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* for more details.
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*/
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#ifndef _ASM_COMPILER_H
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#define _ASM_COMPILER_H
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/*
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* With GCC 4.5 onwards we can use __builtin_unreachable to indicate to the
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* compiler that a particular code path will never be hit. This allows it to be
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* optimised out of the generated binary.
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*
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* Unfortunately at least GCC 4.6.3 through 7.3.0 inclusive suffer from a bug
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* that can lead to instructions from beyond an unreachable statement being
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* incorrectly reordered into earlier delay slots if the unreachable statement
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* is the only content of a case in a switch statement. This can lead to
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* seemingly random behaviour, such as invalid memory accesses from incorrectly
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* reordered loads or stores. See this potential GCC fix for details:
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*
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* https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html
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*
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* It is unclear whether GCC 8 onwards suffer from the same issue - nothing
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* relevant is mentioned in GCC 8 release notes and nothing obviously relevant
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* stands out in GCC commit logs, but these newer GCC versions generate very
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* different code for the testcase which doesn't exhibit the bug.
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*
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* GCC also handles stack allocation suboptimally when calling noreturn
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* functions or calling __builtin_unreachable():
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*
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* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
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*
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* We work around both of these issues by placing a volatile asm statement,
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* which GCC is prevented from reordering past, prior to __builtin_unreachable
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* calls.
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*
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* The .insn statement is required to ensure that any branches to the
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* statement, which sadly must be kept due to the asm statement, are known to
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* be branches to code and satisfy linker requirements for microMIPS kernels.
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*/
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#undef barrier_before_unreachable
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#define barrier_before_unreachable() asm volatile(".insn")
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#if !defined(CONFIG_CC_IS_GCC) || \
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(__GNUC__ > 4) || (__GNUC__ == 4 && __GNUC_MINOR__ >= 9)
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# define GCC_OFF_SMALL_ASM() "ZC"
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#elif defined(CONFIG_CPU_MICROMIPS)
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# error "microMIPS compilation unsupported with GCC older than 4.9"
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#else
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# define GCC_OFF_SMALL_ASM() "R"
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#endif
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#ifdef CONFIG_CPU_MIPSR6
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#define MIPS_ISA_LEVEL "mips64r6"
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#define MIPS_ISA_ARCH_LEVEL MIPS_ISA_LEVEL
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#define MIPS_ISA_LEVEL_RAW mips64r6
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#define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW
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#else
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/* MIPS64 is a superset of MIPS32 */
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#define MIPS_ISA_LEVEL "mips64r2"
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#define MIPS_ISA_ARCH_LEVEL "arch=r4000"
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#define MIPS_ISA_LEVEL_RAW mips64r2
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#define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW
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#endif /* CONFIG_CPU_MIPSR6 */
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#endif /* _ASM_COMPILER_H */
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