1 files changed, 53 insertions, 6 deletions
diff --git a/arch/mips/include/asm/compiler.h b/arch/mips/include/asm/compiler.h
index 71f5c5cfc58a..2b06090a78b2 100644
--- a/arch/mips/include/asm/compiler.h
+++ b/arch/mips/include/asm/compiler.h
@@ -8,12 +8,59 @@
 #ifndef _ASM_COMPILER_H
 #define _ASM_COMPILER_H
 
-#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
-#define GCC_IMM_ASM() "n"
-#define GCC_REG_ACCUM "$0"
+/*
+ * 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")
+
+#define GCC_OFF_SMALL_ASM() "ZC"
+
+#ifdef CONFIG_CPU_MIPSR6
+#define MIPS_ISA_LEVEL "mips64r6"
+#define MIPS_ISA_ARCH_LEVEL MIPS_ISA_LEVEL
+#define MIPS_ISA_LEVEL_RAW mips64r6
+#define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW
+#elif defined(CONFIG_CPU_MIPSR5)
+#define MIPS_ISA_LEVEL "mips64r5"
+#define MIPS_ISA_ARCH_LEVEL MIPS_ISA_LEVEL
+#define MIPS_ISA_LEVEL_RAW mips64r5
+#define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW
 #else
-#define GCC_IMM_ASM() "rn"
-#define GCC_REG_ACCUM "accum"
-#endif
+/* MIPS64 is a superset of MIPS32 */
+#define MIPS_ISA_LEVEL "mips64r2"
+#define MIPS_ISA_ARCH_LEVEL "arch=r4000"
+#define MIPS_ISA_LEVEL_RAW mips64r2
+#define MIPS_ISA_ARCH_LEVEL_RAW MIPS_ISA_LEVEL_RAW
+#endif /* CONFIG_CPU_MIPSR6 */
 
 #endif /* _ASM_COMPILER_H */