1 files changed, 35 insertions, 0 deletions

diff --git a/arch/mips/include/asm/compiler.h b/arch/mips/include/asm/compiler.h
index e081a265f422..cc2eb1b06050 100644
--- a/arch/mips/include/asm/compiler.h
+++ b/arch/mips/include/asm/compiler.h
@@ -8,6 +8,41 @@
 #ifndef _ASM_COMPILER_H
 #define _ASM_COMPILER_H
 
+/*
+ * With GCC 4.5 onwards we can use __builtin_unreachable to indicate to the
+ * compiler that a particular code path will never be hit. This allows it to be
+ * optimised out of the generated binary.
+ *
+ * Unfortunately at least GCC 4.6.3 through 7.3.0 inclusive suffer from a bug
+ * that can lead to instructions from beyond an unreachable statement being
+ * incorrectly reordered into earlier delay slots if the unreachable statement
+ * is the only content of a case in a switch statement. This can lead to
+ * seemingly random behaviour, such as invalid memory accesses from incorrectly
+ * reordered loads or stores. See this potential GCC fix for details:
+ *
+ *   https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html
+ *
+ * It is unclear whether GCC 8 onwards suffer from the same issue - nothing
+ * relevant is mentioned in GCC 8 release notes and nothing obviously relevant
+ * stands out in GCC commit logs, but these newer GCC versions generate very
+ * different code for the testcase which doesn't exhibit the bug.
+ *
+ * GCC also handles stack allocation suboptimally when calling noreturn
+ * functions or calling __builtin_unreachable():
+ *
+ *   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
+ *
+ * We work around both of these issues by placing a volatile asm statement,
+ * which GCC is prevented from reordering past, prior to __builtin_unreachable
+ * calls.
+ *
+ * The .insn statement is required to ensure that any branches to the
+ * statement, which sadly must be kept due to the asm statement, are known to
+ * be branches to code and satisfy linker requirements for microMIPS kernels.
+ */
+#undef barrier_before_unreachable
+#define barrier_before_unreachable() asm volatile(".insn")
+
 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
 #define GCC_IMM_ASM() "n"
 #define GCC_REG_ACCUM "$0"