@@ -8,6 +8,40 @@
#ifndef _ASM_COMPILER_H
#define _ASM_COMPILER_H
+/*
+ * With GCC 4.5 onwards 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.
+ */
+#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"
Some versions of GCC for the MIPS architecture suffer from a bug which can lead to instructions from beyond an unreachable statement being incorrectly reordered into earlier branch 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, and link failures on microMIPS builds. See this potential GCC fix for details: https://gcc.gnu.org/ml/gcc-patches/2015-09/msg00360.html This bug can be worked around by placing a volatile asm statement, which GCC is prevented from reordering past, prior to the __builtin_unreachable call. This was actually done already for other reasons by commit 173a3efd3edb ("bug.h: work around GCC PR82365 in BUG()"), but without the MIPS specific .insn, which broke microMIPS builds on newer GCC 7.2 toolchains with errors like the following: arch/mips/mm/dma-default.s:3265: Error: branch to a symbol in another ISA mode arch/mips/mm/dma-default.s:5027: Error: branch to a symbol in another ISA mode Add a MIPS-specific definition of barrier_before_unreachable() which includes the .insn directive in order to satisfy the assembler that branch targets are in fact code. The original bug affects at least a maltasmvp_defconfig kernel built from the v4.4 tag using GCC 4.9.2 (from a Codescape SDK 2015.06-05 toolchain), with the result being an address exception taken after log messages about the L1 caches (during probe of the L2 cache): Initmem setup node 0 [mem 0x0000000080000000-0x000000009fffffff] VPE topology {2,2} total 4 Primary instruction cache 64kB, VIPT, 4-way, linesize 32 bytes. Primary data cache 64kB, 4-way, PIPT, no aliases, linesize 32 bytes <AdEL exception here> This is early enough that the kernel exception vectors are not in use, so any further output depends upon the bootloader. This is reproducible in QEMU where no further output occurs - ie. the system hangs here. Given the nature of the bug it may potentially be hit with differing symptoms. Fixes: 173a3efd3edb ("bug.h: work around GCC PR82365 in BUG()") Signed-off-by: Paul Burton <paul.burton@mips.com> [jhogan@kernel.org: Forward port and use asm/compiler.h instead of asm/compiler-gcc.h] Signed-off-by: James Hogan <jhogan@kernel.org> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: linux-mips@linux-mips.org --- Changes in v7: - Elaborate on affected GCC versions in comment. Changes in v6: None Changes in v5: - Comment & commit message tweaks. Changes in v4: None Changes in v3: - Forward port to v4.17-rc and update commit message. - Drop stable tag for now. Changes in v2: - Remove generic-y entry. arch/mips/include/asm/compiler.h | 34 ++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+)